In 1858, Mallet proposed a method which bears some resemblance to the above, but depending only on the intensity of the longitudinal waves. Major Dutton claims for his method that the effects of the longitudinal and transverse waves are not separated, that it takes account of the "total energy irrespective of direction or kind of vibration."
Objections to Dutton's Method.—I have described this method somewhat fully, though it seems to me open to more serious objections than Mallet's first method which it is intended to replace.
We have, in the first place, no reason for supposing that the focus is either a point or a sphere, or that the initial impulse is uniform in all directions. If the earthquake were caused by fault-slipping, both assumptions would be untrue, and it is for those who employ the method to prove their validity.
But of greater consequence is the fact that, if the method were correct, all earthquakes originating at the same depth must have index-circles of equal radii. If the depth of the focus were, say, ten miles, then the index-circle must have a radius of about six miles, whether the initial disturbance be of extreme violence or so weak that it is not felt at the surface at all, much less so far as six miles from the epicentre. The law of the inverse square is of course only true for a perfectly elastic and continuous medium, and the real curve of intensity is not that of the continuous line in Fig. 31, but something of the form represented by the dotted line. In this case, the rate of change of intensity is greatest at some point C', nearer than C to the epicentre, and the application of Major Dutton's rule would give a point F', nearer the surface than F, for the focus. Thus, assuming that the method can be applied in practice—and the test involved is one so delicate that it would be difficult to apply except with refined measurements—then all that we can assert is that the calculated depth is certainly less than the true depth.
Dutton's Estimate of the Depth of the Seismic Foci.—In applying the method, the chief difficulty is to obtain a series of isoseismal lines corresponding to equidistant degrees of intensity. As already pointed out, those given in Fig. 29 are merely diagrammatic; but the index-circle of the Woodstock focus, represented by the dotted line, is made to pass through the places where the rate of change of intensity was found to be greatest. The radius of this circle being very nearly seven miles, it follows that the resulting depth of the Woodstock focal point would be about twelve miles. Major Dutton regards this estimate as probably correct within two miles.
In the neighbourhood of the Rantowles epicentre, the isoseismals in both Figs. 28 and 29 are elongated in form. The index-circuit, as it would be called in such a case, cannot be drawn completely, but its radius parallel to the shorter axis of the curves is about 4-1/2 miles, and the resulting depth of the Rantowles focal point would be nearly eight miles.
VELOCITY OF THE EARTH-WAVES.
The recognition of the double epicentre is, from a geological point of view, the most important fact established by the investigation of the Charleston earthquake. But of equal interest, from a physical point of view, is the estimate of the velocity of the earth-waves, which is probably more accurate than that determined for any previous shock. Owing to the existence of the standard time system in the United States, the exact time is transmitted once a day to every town and village within reach of a telegraph line; and the effect of small errors in the observations is considerably lessened by the great distance traversed by the earth-waves, sixty good reports coming from places more than 500 miles from the epicentre, and ten from places more than 800 miles distant.
The total number of time-records collected is 316, but of these 130 had to be rejected, either because they were obviously too early or too late, or because they were only given to the nearest five-minutes' interval. There remain 186 observations which are divided by Major Dutton into four classes according to their probable value.
In an earthquake of such great duration (about 70 seconds at Charleston), it is necessary in the first place to select some special phase of the movement as that to which the records mainly refer, and then to determine as accurately as possible the time of occurrence of this phase at the origin.
There can be little doubt as to which phase should be chosen. The shock began with a series of tremors, which passed somewhat abruptly into the oscillations that formed the first and stronger maximum. These were clearly felt all over the disturbed area, and, as the beginning of the first maximum at places near the epicentre and the beginning of the shock at distant stations were probably due to the same vibrations, this particular phase may be fairly selected as that to which the time-measurements refer.
The time of this phase at the origin can only be ascertained from the time at which it reached Charleston, and our knowledge of this depends chiefly on the evidence of stopped clocks. How unreliable this may be is well known. Clocks may indeed be stopped at almost any phase of the movement; and, whenever stopped clocks can be compared with really good personal observations, they almost invariably show a later time. At Charleston three good clocks were stopped by the vibrations from the Woodstock focus, two of them being in close agreement (p. 121); and, allowing for a few oscillations before their final arrest, Major Dutton places the time of arrival of the selected phase at Charleston at 9h. 51m. 12s. P.M. The evidence of these clocks is also supported by that of other observations, which show that 9.51 was certainly the nearest minute to the time of arrival, and favour a somewhat later instant much more strongly than one a little earlier.
Now, the distance of Charleston from the Woodstock epicentre is sixteen miles, and from the corresponding focus (with the calculated value of its depth) twenty miles. A first estimate of the velocity gives a value of a little more than three miles a second, and the time at the Woodstock focus may therefore be taken as 9h. 51m. 6s. with a probable error of a few seconds.
Proceeding to the observations at a distance, we find them, even after all rejections, to be very different in value. They were therefore divided into groups consisting of observations which are as nearly as possible homogeneous.
The first group contains five records from places between 452 and 645 miles from the Woodstock epicentre. They give the time to within 15 seconds, obtained from an accurately kept clock, or from a clock or watch that was compared with such within a few hours of the earthquake. The resulting velocity is 3.236 plus or minus .105 miles (or 5205 plus or minus 168 meters) per second.
In the second group there are eleven observations (between distances of 438 and 770 miles) which satisfy the same conditions as those in the first group, except that the time is only given to the nearest minute or half-minute. The velocity obtained from them is 3.226 plus or minus .147 miles (or 5192 plus or minus 236 metres) per second.
The third group included all but the above records and those obtained from stopped clocks. They are 125 in number (between distances of 80 and 924 miles), but it is uncertain whether they correspond to the selected phase of the movement, and the errors of the clocks and watches used were unknown. They give a mean velocity of 3.013 plus or minus .027 miles (or 4848 plus or minus 43 metres) per second.
In the fourth group, we have the evidence of 45 stopped clocks (at places between 20 and 855 miles), which apparently give a velocity of 2.638 plus or minus .105 miles (or 4245 plus or minus .168 metres) per second. At six places, however, the times indicated by stopped clocks can be compared with good personal observations; and these show that the time of traverse from the origin obtained from the former is on an average 1.28 times the time of traverse obtained from the latter. If a similar correction be made for all the stopped clocks, the corrected velocity of the earth-waves would be from 3.17 to 3.23 miles (or 5100 to 5200 metres) per second.
In obtaining the mean value of the velocity from all the observations, those of the fourth group are omitted, and the weights of the first three groups are taken inversely as the squares of the probable errors—that is, as 2: 1: 4. The resulting mean velocity is 3.221 plus or minus .050 miles (or 5184 plus or minus 80 metres) per second; and, though it does not follow that all other estimates are erroneous (for the velocity may vary with the strength of the earthquake and with other conditions), it is probable that this result is more nearly accurate than any other previously obtained.
Fissures and Sand-Craters.—In point of size, there was nothing remarkable about the fissures in the ground produced by the Charleston earthquake. The largest were only a few hundred yards long, and, except near the river-banks, they rarely exceeded an inch in width. They seem, however, to have been unusually abundant; for, within an area of nearly 600 square miles surrounding the two epicentres, they were almost universal, and over a much wider area they still occurred in great numbers, though with somewhat less continuity.
From many of these fissures water was ejected, carrying with it large quantities of sand and silt, and so abundantly that every stream-bed, even though generally dry in summer, was flooded. By the passage of the water, some part of the fissures was often enlarged into a round hole of considerable size, ending in a craterlet at the surface. Certain belts within the fissured area contained large numbers of these craterlets, of all sizes up to twenty feet or more in diameter. One near Ten-Mile Hill was twenty-one feet across. In this district, they were apparently larger and more numerous than elsewhere; many acres of ground being covered with sand, which, close to the orifices, was two feet or more in depth.
Here and there, the water was ejected with considerable violence, as was manifest from the heights to which it spurted. The testimony of witnesses on this point is of course doubtful, for the earthquake occurred after nightfall, but in a few places the branches and leaves of trees overhanging the orifices were smirched with sand and mud up to a height of fifteen or twenty feet.
Effects on Human Beings.—It is interesting to notice the behaviour of different races under the influence of a violent earthquake, and perhaps no greater contrast could be observed than between the calmness exhibited by the Japanese in the presence of disaster and the wild fear merging into helpless panic that characterised the residents, and especially the negroes, of Charleston. "As we dashed down the stairway," says a writer already quoted (p. 108), "and out into the street, from every quarter arose the shrieks, the cries of pain and fear, the prayers and wailings of terrified women and children, commingled with the hoarse shouts of excited men.... On every side were hurrying forms of men and women, bareheaded, partially dressed, some almost nude, and all nearly crazed with fear and excitement.... A few steps away, under the gas-lamp, a woman lies prone and motionless on the pavement, with upturned face and outstretched limbs, and the crowd which has now gathered in the street passes her by, none pausing to see whether she is alive or dead ... no one knows which way to turn, or where to offer aid; many voices are speaking at once, but few heed what is said."
Between the selfish rush for safety here described and the calm interest of the most distant observers, Major Dutton records nearly every possible variety of mental effects, the actions resulting from which may be roughly classified as follows:
A. No persons leave their rooms.
B. Some persons leave their houses.
C. Most persons run into the streets, which are full of excited people.
D. People rush wildly for open spaces and remain all night out-of-doors.
In the map of the isoseismal lines (Fig. 25), the dotted curves bound the areas in which the effects corresponding to the three highest degrees of the above scale were observed. The curve for the first degree (A) coincides of course with the isoseismal line of intensity 2.
It will be seen that there is a certain rough correspondence between these curves and the isoseismal lines. The curve D and the isoseismal 8 are close together; in other words, people thought it wiser to camp out-of-doors for the night if the shock was strong enough to damage buildings slightly. The curve C and the isoseismal 6 are similarly connected; that is, if the movement made pictures swing, etc., people rushed into the streets. On the whole, the curve B and the isoseismal 3 roughly coincide, or, if the shock was not strong enough to make doors and windows rattle, some persons left their houses and public meetings were dispersed.
Feeling of Nausea.—A feeling of nausea was experienced by many persons at the time of the earthquake, somewhat rarely it appears in the neighbourhood of the epicentre and even outside the isoseismal 7, but more frequently beyond these limits, and perceptible as far as the broken line in Fig. 25. The most distant places at which it was noticed are Blue Mountain Creek (New York) and Dubuque (Iowa), which are respectively 823 and 886 miles from Charleston.
As Summerville lies six miles to the north-west of the Woodstock epicentre and Charleston 16 miles to the south-east, it is probable that many of the after-shocks were unfelt and a still greater number unrecorded. In Charleston, seven shocks, all much slighter than the principal shock, were felt during the night of August 31—September 1, and thirty before the end of September. Of these, the shock of September 3rd, at 11 P.M., was the strongest, but those which occurred on September 1st, 2nd, 21st, and 27th were also described as severe, and the remainder as moderate or slight. For weeks after the great shock, curious sensations were distinctly perceptible during the still hours of the night "as though the crust of the earth were resting on a gelatinous mass in constant motion." The last shock felt in Charleston seems to have been one recorded on March 18th, 1887.
At Summerville, many shocks occurred that were scarcely perceptible in Charleston, and those noticed in both places were usually stronger, and the motion more nearly vertical, at Summerville. "The peculiar characteristic of all of them was the deep, solemn, powerful boom, like the report of a heavy cannon, usually accompanied by a quick, short jar. Sometimes it was prolonged into a heavy roar or rumble, as if many reports were delivered in a volley. The number of them was never recorded." On September 3rd, Mr. W.J. McGee, of the United States Geological Survey, arrived at Summerville. During the evening of that day, detonations were heard at intervals, averaging perhaps half-an-hour, accompanied occasionally by very slight spasmodic tremors of an instant's duration. They were much like peals of thunder at a distance of half-a-mile or more, though rather more muffled. "It was my impression," Mr. McGee remarks, "that the sound was sometimes about as grave as the ear can perceive, resembling somewhat the tremulous roar sometimes accompanying combustion in locomotives." These sounds continued, but with diminishing frequency, throughout the remainder of the year and as late as July 1st, 1887.
ORIGIN OF THE EARTHQUAKE.
Major Dutton's valuable monograph is a record of the earthquake-phenomena. He offers no theory as to the cause of the shock, and is therefore in no way responsible for the account given in the remaining part of this chapter.
That there were two seismic foci he has shown, I think, conclusively; and my object is now to trace out briefly the probable nature of the movements that produced the double shock.
Referring to Figs. 28 and 29, it will be seen that, according to both Mr. Sloan and Major Dutton, the isoseismals surrounding the Rantowles epicentre are distorted along a line which runs from a few degrees east of north to a few degrees west of south. Their oval form is in all probability connected with a focus elongated in about the same direction. Near the Woodstock epicentre, the isoseismals are drawn differently in the two maps, and in neither case do they offer any sure guide as to the form of the seismic focus. If, however, we follow Mr. Sloan's interpretation of the evidence, and suppose the earthquake to have been fault-formed, then it is probable that in this region the fault bends round slightly towards the east.
The only other evidence on this point is that afforded by the regions of defective intensity, real or apparent, along the three railway-lines diverging from Charleston. One of these occurred near Mount Holly Station on the North-Eastern Railway (B, Figs. 28 and 29), another for four miles starting from the 11-1/2-mile point on the South Carolina Railway (A), and a third along the Charleston and Savannah Railway (C) over a distance of four miles from the Ashley River. In the first two cases, Major Dutton suggests that the less amount of damage was due to the nature of the soil traversed by the railway; but it is on the softer ground that the effects of an earthquake-shock are generally the more disastrous. On the whole, it seems to me probable that the three tracts referred to are really regions of less intensity, and it is worthy of notice that they lie along a nearly straight line.
To show the bearing of these remarks, let CD (Fig. 32) represent the section of a fault and EF that of the surface of the earth, and suppose the rock-mass A to slip slightly and suddenly downwards. Then the particles of A at the surface of the fault will, by impulsive friction, be drawn sharply upwards, and those of B correspondingly downwards; so that the earth-waves in the two rock-masses will start in opposite phases of vibration. Along the line of fault, every particle of rock, being urged upwards and downwards almost equally, will remain practically at rest. Thus, regions of defective intensity may arise from partial interference by the spreading of either earth-wave in the adjoining rock-mass.
If this be the correct explanation, the path of the originating fault may be taken as that indicated by the broken line in Fig. 28, a line which is nearly parallel to the chief branches of the isoseismal curves. As both epicentres lie on the west side of this line, the fault must hade or slope in this direction. The distortion of the Woodstock isoseismals towards the north-west confirms the latter inference, for the intensity of the shock is greater on the side towards which the fault hades.
From the comparative absence of earthquakes in South Carolina, we may infer that the fault is one subject to displacements at wide intervals of time. The gradually increasing stress along its surface was relieved at one or two points in or near the Woodstock focus on August 27th and 28th, and perhaps during the preceding months. But the first great slip took place suddenly in that focus, and spread gradually southwards—for there was no interruption in the movement—until about half-a-minute later it reached the Rantowles focus, where the second great slip occurred. Eight or ten minutes afterwards there was another slip—in what part of the fault is uncertain—and this was followed at irregular intervals by many small movements gradually diminishing in frequency and in focal area. Within a year from the first disturbance, the fault-system attained once more its usual condition of rest.
1. DUTTON, C.E.—"The Charleston Earthquake of August 31st, 1886." Amer. Geol. Survey, Ninth Annual Report, pp. 209-528.
2. Nature, vol. xxxv., 1887, pp. 31-33; vol. lxiii., 1901, pp. 165-166.
 The authorities for this statement are Mallet's Catalogue of Recorded Earthquakes (Brit. Assoc. Rep., 1852, pp. 1-176; 1853, pp. 117-212; 1854, pp. 1-326), which closes with the year 1842, and Fuchs' Statistik der Erdbeben von 1865-1885. According to Mallet, there was an earthquake in S. Carolina in November 1776, and the New Madrid earthquake of December 16th, 1811, was felt at Charleston. Fuchs records two earthquakes at Charleston on May 12th, 1870, and December 12th, 1876; and two in S. Carolina on December 12th and 13th, 1879.
 1. Recorded by a single seismograph, or by some seismographs of the same pattern, but not by several seismographs of different kinds, the shock felt by an experienced observer.
2. Recorded by seismographs of different kinds; felt by a small number of persons at rest.
3. Felt by several persons at rest; strong enough for the duration or direction to be appreciable.
4. Felt by several persons in motion; disturbance of movable objects, doors, windows; creaking of floors.
5. Felt generally by every one; disturbance of furniture and beds; ringing of some bells.
6. General awaking of those asleep; general ringing of bells; oscillation of chandeliers, stopping of clocks; visible disturbance of trees and shrubs; some startled persons leave their dwellings.
7. Overthrow of movable objects, fall of plaster, ringing of church bells, general panic, without damage to buildings.
8. Fall of chimneys, cracks in the walls of buildings.
9. Partial or total destruction of some buildings.
10. Great disasters, ruins, disturbance of strata, fissures in the earth's crust, rock-falls from mountains.
 In order to simplify these figures, the rivers, most of the inlets, and other details are omitted. Small figures are added along the railway lines to denote the distance in miles from the stations in Charleston.
 If this were so, the decrease in intensity would be only apparent; but it may have been real, and a possible explanation on this supposition is given later on (p. 135).
 If c be the depth of the focus, a the intensity at unit distance from the focus, and y the intensity on the surface at distance x from the epicentre, then
The inclination of the curve at any point is given by
and this is a maximum when
d^2y/dx^2 or (3*x^2-c^2)/(c^2+x^2)^3
is zero, which is satisfied when c=x*sqrt(3)
 British Association Report, 1858, pp. 101-103.
 The above time would have to be increased by one second if the depth of the focus were very small, and diminished by one second if it were as great as 23 miles; the difference in either case being less than the probable error.
 The method employed is as follows: Let t0 be the computed time (9h. 51m. 6s.) at the focus, x seconds the error in this estimate, t the reported time at a given place, D its distance from the focus in miles, and y the number of seconds required to travel one mile; then, assuming that y does not vary with the distance, we have x+Dy=t+t0. An equation of this form is obtained from each observation, and the method of least squares is then applied to determine the most probable values of x and y.
 This seems to me the more probable course. It is possible, however, that the fault-line may pass from Mount Holly Station to the east of the Woodstock epicentre as shown in Fig. 28, and then to the west of the Rantowles epicentre, the fault changing its direction of hade in the intermediate district.
THE RIVIERA EARTHQUAKE OF FEBRUARY 23RD, 1887.
Few earthquakes have aroused a more widespread interest than those which struck the thronged cities of the Riviera on February 23rd, 1887. The first and greatest of the shocks occurred at about 6.20 A.M., the second nine minutes later, and the third, intermediate in strength, at about 8.51 A.M. All three shocks were of destructive violence, the damage wrought by them extending along the coast and for a short distance inland from Nice to beyond Savona. Most of the injury to property and nearly all the loss of life were, however, concentrated on the eastern side of the frontier; and it therefore fell to the lot of the Italian Government to provide for the scientific investigation of the earthquakes, as well as to meet the wants of those deprived of home and support. Professors Taramelli and Mercalli, who two years before had studied the earthquakes in Andalusia, were again nominated, the former to examine the geology of the central regions, and the latter to report on the seismic phenomena. Their joint memoir forms one of the most complete accounts that we possess of any earthquake, and is the chief authority for the description given in this chapter. Another valuable monograph is that prepared by Professor A. Issel, of Genoa, who received an independent appointment from the same Ministry. A third official commission was also sent to estimate the amount of damage caused by the earthquakes in the Italian towns and villages. In France, the destruction of property was much less serious, and attention was confined chiefly to the records of the shock provided by magnetographs and other instruments in distant observatories. In Switzerland, the effects remarked were merely those due to the evanescent vibrations of a remote earthquake; but many interesting records were collected by the permanent seismological commission established in that country.
DAMAGE CAUSED BY THE EARTHQUAKES.
Owing to variations in the nature, foundation, and site of buildings, there is always great diversity in the destructive effects of an earthquake. In one and the same town, most of the houses may be razed to the ground, while in their midst may be found some that are shattered but still standing, and others perhaps that are practically unharmed. The stronger after-shocks often complete the ruin of the partially damaged houses; though in such cases the real loss is as a rule comparatively small.
The close succession of the two strong after-shocks of February 23rd made it impossible as a rule to separate their effects from those due to the first shock; but it has been roughly estimated that about one-quarter of the total damage was caused by the two after-shocks together. To them also must be referred in part the comparatively small number of wounded, many persons buried beneath the ruins having no doubt perished from subsequent falls before they could be extricated.
Taking all three shocks together, the total loss to property, according to Professor Mercalli, must be valued at about 22 million francs in Italy alone. For the province of the Alpes Maritimes in France, full details are wanting, but the loss there cannot fall far short of three million francs. The total amount of damage must therefore be placed at about a million pounds. From the figures given by the official commissions, it appears that the earthquakes were most disastrous at Diano Marina and Diano Castello; while other places, such as Oneglia, Bussana, Baiardo, Pompeiana, and Vallecrosia, suffered only a little less severely. At Mentone about 155 houses, and at Nice about 61 houses, were rendered uninhabitable, and many others were badly injured.
In Italy, 633 persons were killed, 432 seriously wounded, and 104 slightly wounded; in France, 7 persons were killed and 30 seriously wounded, the number of persons slightly wounded being unknown. The majority of the deaths occurred in two or three places. Thus, at Diano Marina, 190 persons were killed and 102 wounded; at Baiardo, 220 were killed and 60 wounded; at Bussana, there were 53 killed and 27 wounded. The death-rates were, however, comparatively small, amounting for the above places to not more than 8-1/2, 14, and 6-1/2 per cent., respectively; figures which only slightly exceed those obtained for places in the meizoseismal area of the Andalusian earthquake.
Though the damage can hardly be regarded as excessive, it was nevertheless largely due to the peculiar architecture prevalent in the Riviera. Arches in the walls are common even in the upper storeys, and, in Oneglia and Diano Marina, if not also in other places, the floors are nearly always brick arches abutting against the walls and without other lateral support. Professor Mercalli believes that, in private houses, more than 90 per cent. of the dead bodies were crushed beneath these fallen arches. The height of the buildings is also great in proportion to the foundation and to the thickness of the walls; and the main walls are interrupted by numerous apertures, from the corners of which nearly all the fissures sprang. In some of the coast towns, the houses are built of rounded stones gathered from the beach, or of rubble with stones of all shapes and sizes, bound by cement of the poorest quality. Lastly, as much of the damage due to previous earthquakes had been badly repaired, it is evident that the destructiveness of the Riviera earthquakes must to a great extent be referred to preventable causes.
The occurrence of the principal shock shortly after six on the morning of Ash Wednesday must also have increased the death-rate; for many persons, after a night of amusement, had lain down for a short time and were sleeping heavily; while others had already risen and were collected in the churches; the circumstances in either case rendering escape more difficult.
Taking account, however, of this accidental increase in the number of victims, Professor Mercalli considers that the earthquake of 1887 was the most disastrous of all those which have visited either the Riviera or northern Italy in the last three centuries; though, during the nineteenth century, there were three Italian earthquakes of far greater destructive power, but all confined to the southern part of the peninsula—namely, the Neapolitan earthquakes of 1805 and 1857, and the Ischian earthquake of 1883.
PREPARATION FOR THE EARTHQUAKES.
It is difficult, as usual, to specify the exact moment when the first earthquake of the 1887 series took place; but it is evident that the preparation for the great shock was very brief. At Oneglia, it is alleged that faint shocks and sounds were observed many times, chiefly at night, during the month preceding February 23rd; though they were not at the time supposed to be of seismic origin. A slight shock is also reported from Diano at about midnight on February 21-22.
The first undoubted shock occurred on February 22nd, at about 8.30 P.M., or ten hours before the principal earthquake. Though very slight, it was felt throughout the Riviera and in part of Piedmont. Another shock, also weak, took place at about 11 P.M.; and a third, sensible only in the eastern part of the Ligurian Apennines, on February 23rd, at about 1 A.M.; at which time the tide-gauge at Genoa recorded some abnormal oscillations. An hour later, a more important, though by no means a strong, shock occurred; this was perceptible all over the Riviera, in Piedmont, and in Corsica; in other words, it disturbed a region agreeing closely with the central area of the disastrous shock. At about 5 A.M., a fifth shock, somewhat weaker than the preceding, was felt over the same area, concurrently, or nearly so, with another abnormal oscillation of the tide-gauge at Genoa; while a sixth shock was noticed at several places a few minutes before the great shock.
During the night of February 22-23, nervous persons in many towns and villages were agitated without apparent reason. Birds and animals, more sensitive than human beings to faint tremors, were more distinctly affected, especially for some minutes before the earthquake. Horses refused food, were restless or tried to escape from their stables, dogs howled, birds flew about and uttered cries of alarm. As these symptoms were noticed at more than one hundred and thirty places within the Italian part of the central area, there can be little doubt that they were caused by microseismic movements for the most part insensible to man.
ISOSEISMAL LINES AND DISTURBED AREA.
The only complete map of the isoseismal lines is that drawn by Professor Mercalli. In this map, reproduced in Fig. 33, the continuous curves represent the principal isoseismal lines; the dotted curves define the disturbed areas of two of the stronger after-shocks.
The meizoseismal area, bounded by the curve marked 1 in Fig. 33, is also shown on a larger scale in Fig. 34. At the places denoted by small circles in the latter figure, the principal shock was "disastrous," some of the houses in each being either totally or partially ruined. At those marked by a small cross, the shock was "almost ruinous"; in other words, numerous houses were damaged, but in no case was the injury of a serious character. The meizoseismal area is thus a narrow band, skirting the Riviera coast from Mentone to Albissola, a distance of 106 miles, and extending inland for not more than from nine to twelve miles. The greatest intensity, corresponding to the ruin of many houses with considerable loss of life, was reached at only a few places between Bussano and Diano Marina, all lying within a littoral band about twenty miles in length and three to three and a half miles in width. If, however, the epicentre had lain on land, the area would have been much greater, Professor Mercalli estimates about four times greater, than its actual amount.
The curve marked 2 (Fig. 33) bounds the "almost ruinous" zone; its expansion towards the north and contraction towards the west, north-west, and east, being its most noteworthy features. The next zone, that of slight damage, is contained between the isoseismals 2 and 3, the latter curve probably grazing the north end of Corsica. Beyond this lies the "strong" zone, in which the shock was generally felt without causing any damage to buildings. Its boundary (marked 4) passes near Marseilles, Como, and Parma, and includes nearly the whole of Corsica; towards the north-west, in the valley of Aosta, it curves in towards the isoseismal 3.
In the outermost zone of all the shock was "slight," and towards the margin was only just perceptible. The boundary, which of course defines that of the disturbed area, reaches as far north as Basle and Dijon, to Perpignan on the west, Trento, Venice, and Pordenone on the east, and to the south as far as Tivoli (near Rome) and the northern end of Sardinia. In eastern Switzerland, it shows a marked curve inwards; possibly, as Professor Mercalli suggests, from the vibrations having to cross the northern Apennines in a direction nearly at right angles to their axis. Except for this bay, however, the curve differs little from a circle, the centre of which lies in the sea, a little to the south of Oneglia, close to the position assigned by other evidence to the epicentre. The radius of this circle being about 264 miles, it follows that the disturbed area must have contained about 219,000 square miles—by no means a large amount for so strong an earthquake.
POSITION OF THE EPICENTRES.
It is evident, from the form of the meizoseismal area shown in Fig. 33, that a mere fringe of it lies upon land, and that the epicentre must be situated some distance out at sea. Other facts may be mentioned which point to the same conclusion. There were, for instance, no purely vertical movements observed, even in the districts where the damage done by the shock was greatest. Nor were any large landslips to be seen in those areas; there were no lasting changes in the underground water-system; and in general, as Professor Mercalli remarks, all the superficial distortions of the ground which are so characteristic of the epicentral area of a great earthquake were conspicuous by their absence. There is evidence, again, of some disturbance of the sea-bed in the death and flight of fishes from great depths and in the seismic sea-waves recorded by the tide-gauges at Genoa and Nice. These phenomena will be described in a later section, but reference should be made here to an interesting observation at Oneglia on the occurrence of some of the stronger after-shocks. Persons on the coast, it is said, saw the sea curling and moving, and immediately afterwards the shock was felt.
In determining the position of the epicentre, Professor Mercalli had recourse as usual to observations on the direction of the shock, especially those derived from the oscillation of lamps or other suspended objects, the projection or fall of bodies free to move, fractures, etc., in damaged houses, and the stopping of pendulum clocks. Such observations were made at 120 places—72 in the western Riviera and the Alpes Maritimes, and 48 at Piedmont, Lombardy, and Tuscany.
At many of these places the movement was extremely complicated. In nearly all parts of the area most strongly shaken, for instance, the direction of the shock changed more than once; and it was therefore necessary to select whenever possible the principal direction of the shock at each place. In some towns, such as Oneglia, Mentone, Antibes, Cuneo, etc., the shock had two dominant directions, and these appeared to be sensibly at right angles to one another; an inclination which, as Professor Mercalli suggests, may be due in part to the approximation of the real directions to those of the principal walls of the houses in which the observations were made.
Most of the lines of direction, when plotted on the map, converge towards an area lying between the meridians of Oneglia and San Remo, and between nine and fifteen miles from the coast. For places near the epicentre, the most trustworthy, in Mercalli's opinion, are those made at Oneglia, Mentone, Taggia, Bordighera, Castel Vittorio, Nice, and Genoa; and the points in which these lines Intersect one another are Indicated by small crosses on the map of the meizoseismal area (Fig. 34). All of them lie at sea at distances between six and fifteen miles to the south of Oneglia. The most probable position of the principal epicentre is that marked by the small circle A, which is situated about fifteen miles south of Oneglia.
There are, however, several lines of direction which can have no connection with this epicentre. Besides the east and west lines at Nice, Mentone, and Antibes, there are others at the same places which run north and south or nearly so. Professor Mercalli believes that they were due to vibrations coming from a second focus lying to the south of Nice, and there are also several lines of direction at more distant places which converge towards the neighbourhood of the corresponding epicentre.
This conclusion receives unexpected support from some of the best time-records. At the railway-stations of Loano and Pietra Ligure, the times of occurrence were given as 6h. 20m. 5s. and 6h. 20m. respectively—estimates which are probably accurate to within a few seconds; for, at the moment of the shock, the officer who brought the exact time along the railway-line from Genoa was at Loana, and had just passed through Pietra Ligure. On the other hand, the estimates for Mentone and Nice—namely, 6h. 18m. 35s. and 6h. 19m. 43s., if not equally exact, cannot err by many seconds, certainly not by so much as one minute. Since the distances of Loana and Pietra Ligure from the principal epicentre are 31 and 32 miles, and those of Mentone and Nice 28 and 37 miles, it is therefore clear that the vibrations which arrived first at Nice and Mentone must have come from a local focus, where the impulse preceded that at the principal focus by several seconds.
DEPTH OF THE PRINCIPAL FOCUS.
Inaccurate as are all the methods of determining the depth of focus, it seems probable, as Professor Issel argues, that the principal Riviera focus was situated at a considerable distance from the surface. In no part of the meizoseismal area was the shock a really violent one; yet its intensity must have faded very slowly outwards, for it was strong enough to stop clocks at places in Switzerland and elsewhere not less than 250 miles from the origin.
Professor Mercalli regards Mallet's method with greater favour than most seismologists. He points to the gradual increase in the angle of emergence from the outer zones disturbed by the Riviera earthquake towards the meizoseismal area, where several good observations were made from fissures in walls parallel to the dominant direction of the shock. The angles of emergence which he considers as most trustworthy are those of 35 at Taggia, 40 at Oneglia, and about 30 at Bordighera. The corresponding depths for the focus are 10.4, 10.4, and 11.6 miles, giving an average of about 10-3/4 miles.
There are no similar observations forthcoming for the depth of the secondary focus near Nice and Mentone; but Professor Mercalli observes that it must have been shallower than the other, for the vertical component of the vibrations from this focus was much less sensible than that of the motion coming from the principal focus.
NATURE OF THE SHOCK.
The Double Shock.—In the valuable collection of records made by Professors Taramelli and Mercalli there appears at first sight to be the utmost diversity in the evidence with regard to the nature of the shock. Thus, in the province of P. Maurizio alone, the shock was described as subsultory first and then undulatory or vorticose at 25 places, undulatory and then subsultory at 22, undulatory and then subsultory and again undulatory or vorticose at 13, and subsultory first, then undulatory, and finally subsultory and vorticose at two places. It is clear that the shock was of considerable duration, not less than half-a-minute as a rule, and that there were several phases in the movement; and it would seem that one or more of these phases may have passed unnoticed owing to the alarm occasioned by the shock, and to the fact that most of the observers were asleep when the earthquake began. Defects of memory must also have an influence not to be neglected, for, even with the simple shocks felt in the British Isles, persons in the same or neighbouring places differ greatly in their testimony.
But, if we confine ourselves to the accounts of careful persons alone, the discrepancies to a large extent disappear. Indeed, all over the ruinous area (Fig. 33) the shock maintained a nearly uniform character. At Oneglia, for instance, there were two well-marked phases, the first of which began with a brief subsultory movement, followed by more horizontal undulations of longer period; a pause, lasting but for an instant, was succeeded by vibrations which, though not vertical, were highly inclined to the horizon; they continued throughout the second phase, but, towards the end, new undulations were superposed, and these, coming from different directions, resulted in an apparently vorticose movement. Professor Mercalli represents the motion diagrammatically by the curve a in Fig. 35. At Diano Marina, as will be seen from the curve b, the shock again consisted of two phases, each beginning with a few subsultory vibrations and ending with horizontal undulations of much longer period. In the first phase, the undulations were marked by a dominant direction, but, towards the close of the second phase, there was no determinate direction, and the impression was again that of a vorticose shock. At Savona, the movement, which is represented by the curve c, must have lasted from twenty-five to thirty seconds. It also consisted of two phases, with subsultory vibrations and undulations in the same order; and it was noticed that the second part of the shock was much stronger than the first. According to some observers, the concluding movements were vorticose.
In the zone surrounding the ruinous area, the vertical component of the motion was observed to diminish with the intensity; but, in other respects as well as in duration, the shock retained the same general form. At Genoa, Turin, Acqui, Alessandria, Antibes, and other places, two distinct phases were perceived, occasionally separated by a brief pause, the first being invariably the weaker. At some places, the observers speak of two shocks at about 6.20 A.M., separated by an interval of a few seconds; and this division was noticeable as far as Sal on the shore of Lake Garda and Vicenza in Venetia. Only in Switzerland and other districts near the boundary of the disturbed area did the weaker part of the shock become insensible, the other consisting of horizontal oscillations, remarkable for their slowness and regularity, and lasting for as much as twenty or thirty seconds.
We may thus conclude, with Professor Mercalli, that the earthquake resulted from the almost immediate succession of two distinct shocks, in each of which the nearly vertical vibrations were more marked at the beginning, while the slower undulations predominated towards the close, those of the second phase generally becoming vorticose through the superposition of movements coming from different directions. The second part of the shock in all of the more carefully written accounts is described as the stronger, especially as regards the subsultory vibrations in the meizoseismal area; except in the immediate neighbourhood of Nice, where the second phase was generally regarded as the weaker, or at any rate as not stronger than the first.
Origin of the Double Shock.—These observations show, not only that the principal earthquake consisted of two distinct shocks, but also that the shocks originated in different foci. For, if the vibrations of both had started from one focus, the second shock would have been everywhere the stronger; instead of which there was a small area near Nice where the intensity of the first was the greater. This points clearly to the existence of another focus situated not far from Nice; and it is evident that the greater intensity of the first part in that district was due solely to the proximity of this focus, for, still farther to the west, at Antibes, the second part was again the stronger.
There is thus a striking agreement in the inferences drawn from observations on the direction, time of occurrence, and nature of the shock. In the face of such concurring testimony, little doubt can remain as to the existence of two foci, one to the south of Oneglia and the other to the south of Nice, the initial impulse at the latter being decidedly the weaker, and preceding that at the eastern focus by an interval of some seconds, long enough at any rate for the resulting vibrations to reach the Oneglia focus and to spread beyond it before the vibrations from that focus started on their outward journey.
Seismographic Records.—In 1887, the Riviera and the districts adjoining it were unprovided with accurately constructed seismographs. The observatories at Alessandria, Milan, Monza, Parma, Florence, and other places in Italy contained seismoscopes and other pendulums, and these all registered the fact that an earthquake had occurred, and in many cases traced a series of elliptical or elongated curves. A record of the shock was also given by a Cecchi seismograph at Perpignan in France, but the distance from the epicentre was too great to allow details to be shown. The most valuable record was that obtained from a Cecchi seismograph at the observatory of Moncalieri, near Turin, about ninety miles north of the principal epicentre.
In this seismograph, the pendulums are provided with pointers, the tips of which touch vertical sheets of paper attached to the sides of an upright rectangular box. When an earthquake occurs, this box is made to descend slowly with a uniform velocity, while the moving pointers trace curves upon the smoked paper. The north-and-south component of the horizontal motion is inscribed on the sheet of paper facing west, and the east-and-west component on the paper facing south.
During the principal Riviera earthquake, the former pendulum furnished an indistinct record, while the other traced the diagram reproduced in Fig. 36. The movement, as here represented, began at about 6h. 21m. 50s. A.M. (mean time of Rome) with a series of small tremors, which lasted for about twelve seconds. Then followed some large oscillations, always in a nearly east-and-west direction, which at 6h. 22m. 21s. gave place to a second series of tremors similar to those at the beginning of the shock, but of greater amplitude. These continued for at least twelve seconds, at the end of which time the motion of the smoked paper ceased. The total duration of the movement at Moncalieri cannot therefore have been less than forty-three seconds.
Interesting as this record is, it is doubtful how far it represents accurately the movement of the ground. The Moncalieri instrument was erected before the modern type of seismograph was designed, in which some part remains steady, or very nearly steady, during the complicated movements of the ground that take place in an earthquake. It will be noticed that the curve in Fig. 36 shows no sign of the division of the shock into two distinct parts, and this may perhaps be due to the swinging of the pendulum itself; in which case, the curve described by the pointer would be the resultant of the oscillations of the ground and the proper motion of the pendulum.
The sounds that preceded and accompanied the Riviera earthquake have attracted but little study, although they seem to have been widely observed. No attempt was made to define the limits of the area over which they were audible; but Professor Mercalli states that in the two outer zones (Fig. 33) the sound generally passed unobserved. It was, however, heard near Piacenza in Lombardy and Reggio in Emilia, places which are about 115 and 140 miles from the principal epicentre.
In the area in which the shock was most violent, the sound resembled that of trains and vehicles in motion; while, outside this area it generally appeared to be like the hissing of a violent wind. In only a few places was it compared to detonations, the crashes of artillery or distant thunder. Some observers describe the sound as appearing at first as if a strong wind were rising, and then as the roaring of a heavy railway-train passing.
Nearly all the observers, who were awake at the beginning of the earthquake, agree in asserting that the sound distinctly preceded any movement of the ground. From this, as in the case of the Andalusian earthquake, Professor Mercalli infers that the sound-vibrations travelled with the greater velocity; but, as will be shown in Chapter VIII., the general precedence of the sound admits of another and more probable explanation.
THE UNFELT EARTHQUAKE.
If the Andalusian earthquake first drew general attention to the distant spread of unfelt earth-waves, the Riviera earthquake showed that this was no isolated phenomenon. We know now that the propagation of such waves is only limited by the surface of the earth, but in 1887 some doubt was felt at first as to the nature of the disturbance, whether it was magnetic or mechanical in its origin.
In 1884, the only observatories at which magnetographs were disturbed were those of Lisbon, Parc Saint-Maur (near Paris), Greenwich, and Wilhelmshaven. In 1887, the magnetographs registered the Riviera earthquake at these and several other observatories, the distribution of which is shown in Fig. 37. In this sketch-map, the position of the principal epicentre is represented by the small cross, while the nearly circular line shows the boundary of the disturbed area.
Three of the observatories, those of Nice, Lyons, and Perpignan, lie inside this area. At Nice (which is thirty-seven miles from the principal epicentre), M. Perrotin states that the magnetograph curves show nothing of any interest, except a notable magnetic perturbation on the vertical force curve, the time of which, however, is not stated. At Lyons (211 miles), the declination, horizontal force and vertical force, magnets were all disturbed at 6h. 25m. 47s. A.M., and Perpignan (264 miles), all three magnets, but especially those for the declination and horizontal force, were set abruptly oscillating at 6h. 25m. 20s.
Elsewhere in France, the disturbances were noticed at the observatories of Parc Saint-Maur and Montsouris, near Paris (about 447 miles), and at Nantes (538 miles). At Parc Saint-Maur, all three curves show a very clear trace of the earthquake at 6h. 25m. 35s., the oscillations lasting several minutes, and at Montsouris they also began at the same time. At Nantes, the perturbations were so slight that they escaped notice on a first examination.
In Austria, disturbances were observed at Pola (295 miles) and Vienna (506 miles), beginning at 6h. 28m. 35s. and 6h. 30m. 35s., respectively. They reached Brussels (522 miles) at 6h. 29m. 27s., and Utrecht (600 miles) at 6h. 28m. 38s. At Wilhelmshaven (690 miles), only the vertical force magnet was affected, the oscillations beginning at 6h. 30m. 35s., and lasting for fourteen minutes. At 6h. 27m. 55s., the declination and horizontal force magnets of Greenwich observatory (642 miles) were set vibrating, but no similar disturbances were revealed by the vertical force curve or by the two earth-current registers. At Kew (652 miles), the horizontal force magnetograph was moved by the earthquake at about 6h. 29m. 55s. The curves at Stonyhurst and Falmouth show no sign of any disturbance, nor do those at Pawlovsk in Russia, or Seville. At Lisbon (951 miles), however, the three curves indicate disturbances at 6h. 32m. 35s., but so feeble are they that they would have escaped discovery if the occurrence of the earthquake had been unknown.
The effects registered on the magnetograms are quite different from those which correspond to ordinary magnetic perturbations; but they are not unlike those produced by the action of the momentary currents which are used for making the hour-marks, except that the earthquake-oscillations lasted several minutes (see Fig. 21). In each case, then, the magnetic bars must have received a succession of several or many impulses.
Now, the effect of these impulses on each magnet must depend on the relations which exist between the period of oscillation of the magnet, the rate of damping of such oscillations, and the interval between the successive impulses. Also, the apparent commencement of the phenomena may be delayed if two impulses of contrary sense should follow one another before the bar is perceptibly displaced. It is therefore to be expected, as M. Mascart points out, that the disturbances of the three instruments need not be of the same order of magnitude, that with different forms of apparatus the effects may be very variable, and that the deflection of one instrument may precede that of another at one and the same place.
In all the magnetographs, the record is made on photographic paper, which travels so slowly that the time of a movement can only be ascertained to the nearest minute. As the disturbances on the French curves were apparently almost simultaneous, and as no two of the others differed in time of occurrence by more than five minutes, there is thus some colour for M. Mascart's contention that the magnetic apparatus registered, not the movements of the ground, but the passage of electric currents produced in the ground at a certain epoch of the earthquake.
On the other hand, it is important to notice that, in the central part of the disturbed area, at Nice, two, if not all three, of the magnetographs were unaffected at the time of the earthquake.
At first sight, this fact seems equally opposed to a mechanical explanation of the disturbance. But, when the vibrations are very rapid, as they are in the neighbourhood of the epicentre, the magnetic bars, owing to their mode of suspension, have not sufficient time to be sensibly deflected in the brief interval between successive phases of the impulse. The magnetograms of the Montsouris observatory show, for instance, hardly any perceptible trace of disturbance during the passage of railway trains along two adjacent lines. The farther, however, the earth-waves travel from the origin, the longer becomes the period of their vibrations. In Switzerland, they were remarkable for their slowness, even to the unaided senses. Thus, at places more or less remote from the Riviera, the magnets would receive impulses at intervals approximating to their own periods of vibration, and they would then oscillate freely for some time.
Again, notwithstanding some variations, it will be remarked that on the whole the retardation of the initial epoch of the disturbances increases with the distance from the epicentre. It thus seems clear, I think, that the cause of the disturbances must be sought in the shock itself; although their initial epochs at different places are too roughly defined for ascertaining the velocity with which the earth-waves travelled.
EFFECTS OF THE EARTHQUAKE AT SEA.
The Riviera earthquake, owing to its submarine origin, was marked by certain phenomena that were absent from the other earthquakes described in this volume.
Nature of the Earthquake at Sea.—At the time of the earthquake, several vessels were close to the epicentral area. One, about three miles off Diano Marina, was shaken twice at about 6.20 A.M., and so violently that it seemed as if the masts would be broken off. Another, about ten miles south of P. Maurizio, also experienced two shocks, a few minutes apart, as if each time it had struck the bottom. These observations are chiefly interesting in showing that the double shock was felt at sea as well as on land. As transverse vibrations are not propagated through water, it follows that the second part of the shock cannot, as some maintain, have been composed of transverse vibrations.
Destruction of Fishes.—During the days immediately following the earthquake, a large number of deep-sea fishes were found dead or half-dead either in shallow water or stranded on the beach, especially in the neighbourhood of Nice. Among them were numerous specimens, mostly dead and floating, of Alepocephalus rostratus, a typical deep-sea form, several of Pomatomus telescopium, Scopelus elongatus, and S. humboldti, and many of Dentex macrophthalmus and Spinax niger. The death and flight of these fishes must have been due to a sudden shock, almost like that caused by the explosion of dynamite, and communicated simultaneously to the whole surface of their bodies.
Seismic Sea-Waves.—Immediately after the earthquake, the sea retired a short distance, variously estimated at from ten to thirty metres, laying bare some rocks that were usually immersed. At P. Maurizio, the surface was lowered by a little more than a metre; and after a few minutes it rose to nearly a metre above its original level, returning to it after a series of continually-decreasing oscillations. At San Remo, a fall of about the same amount took place, the sea returning after five minutes, and a ship anchored in the harbour broke from her moorings. Again, at Antibes, the sea was suddenly lowered by about a metre, so that ships afloat in the harbour were aground for some instants, and then returned with some impetuosity to its original level.
The evidence of eye-witnesses is confirmed by that of the tide-gauges at Nice and Genoa, the curves of which are reproduced in Figs. 38 and 39. At Nice, the first arrest of the curve in its usual course occurred at 6.30 A.M.; the sea-level sank somewhat abruptly, and after a few marked oscillations gradually returned to its normal position at 7.50 A.M. At Genoa, the shock caused the writing-pen of the tide-gauge to dent the paper on which the record is made, and soon afterwards the curve shows a series of irregular oscillations, about eight taking place every hour, and gradually decreasing until they ceased to be perceptible about two hours after the principal earthquake.
Connection between Geological Structure and the Intensity of the Shock.—As with the Andalusian earthquake, faulty construction and defective materials were responsible for much of the damage caused by the Riviera earthquake. But, if we may judge from the sharp local variations in its amount, the nature of the surface-rocks must have exerted a still more potent influence. At Cervo, for example, the injury to property amounted to less than 3 per head of the population; at Diano Marina, only two or three miles to the west, it rose to 22 per head. The death-rate at Cervo was about one-tenth, and at Diano Marina about 8-1/2 per cent. Again, at Mentone, the damage must have been considerable, for about 155 houses were rendered uninhabitable; while Monte Carlo, only a few miles farther west, escaped almost unharmed. Now, Mentone and Diano Marina are for the most part built on clay or alluvial deposits, and Monte Carlo on a foundation of limestone.
Even within the limits of a single town, variations no less striking were perceptible. In Mentone, the greatest damage occurred to houses of two storeys built on alluvial soil in the low-lying parts near the sea and in the valleys. The effect of the foundation in this part was well shown in the case of two equally well-built houses not more than 300 yards apart. One in the valley, with doubtful foundations, was very much shattered; the other, built on rock, was uninjured. The large hotels, especially those on high ground, suffered least, few of them having their main walls seriously damaged. These buildings rise to heights of from four to six storeys, and of necessity have a firm and solid foundation.
Professors Taramelli and Mercalli have made a careful study of the subject of this section. The general conclusions at which they arrive are that the intensity of the shock was greatest at places built on pliocene conglomerates, beds of clay superposed on compact old rocks, patches of alluvium, miocene formations of some thickness formed of repeated alternations of strata of incoherent marls and limestones or compact sandstones, beds of chalk, or somewhat rotten dolomite.
The shock was also more destructive on the summits of isolated hills and ridges and on the steep slopes of mountains. The influence of the form of the ground was, however, subordinate to that exerted by the nature of the subsoil. Thus, at Mentone, as we have seen, and also at Nice and Genoa, houses built on rock in elevated positions suffered much less than those situated on the plains below that are composed of sand and recent alluvium.
Observations of the Earthquake in Railway-Tunnels.—Observations made in mines at various times and places have proved that an earthquake is felt less strongly in deep workings, if felt at all, than on the surface of the ground. In the railway-tunnels of the Riviera, as Professor Issel has shown, the same result was established during the earthquake of 1887.
On the line which runs northward from Genoa to Piedmont, a tunnel more than five miles in length pierces the hilly ground between Ponterosso and Ronco, the greatest thickness of rock above being about a thousand feet. At the time of the earthquake, the tunnel was not everywhere opened out to its full width, and men were at work in different sections. Outside, the shock was strong enough to damage buildings. Inside, at about 200 yards from the south end, only a feeble shock was felt; at 1,350 and 1,625 yards, some bricks were seen to fall from the facing, but the shock was not otherwise perceived, and only a few yards farther nothing unusual was noticed by the men at work.
Again, in an unfinished tunnel, about three-quarters of a mile long, between the harbour of Genoa and the eastern railway-station, the vibrations were very slightly felt. Even in the tunnels traversed by the coast railway from Genoa to Nice—that is, in those situated within the meizoseismal area—the shock was either very weak or not felt at all, and not one of the tunnels suffered the slightest injury.
To men at work inside a long tunnel, the conditions for observing earthquakes are somewhat imperfect, but these facts, nevertheless, bring out very clearly the inferior intensity of the shock at some depth below the surface.
While the unfelt earth-waves of the great earthquake were still wending their way over the zone that surrounds the disturbed area, the central regions were again shaken, at 6.29 A.M., by a shock strong enough to produce fresh ruins in the stricken towns along the coast. Nearly two and a half hours of quiet followed, broken only by a few subterranean rumblings in the central part of the meizoseismal area. Then, at 8.51 A.M., occurred another shock, short and sharp, and inferior in strength only to the principal earthquake. Both of these after-shocks were felt in Western Switzerland; indeed, they were perceptible nearly as far as the great shock; the second, however, a little farther than the first, for it alone was noticed at such places as Vicenza, Forl, and Florence. The shock at 6.29 was usually described as long and its vibrations as undulatory only; that at 8.51 as rather subsultory than undulatory and of very brief duration. The latter, however, was followed after an interval of a few seconds by another shock so weak that it generally passed unobserved. Both shocks were preceded by a rumbling sound.
During the next two days, tremors and earth-sounds were frequent in the Riviera; once an hour, on an average, the greater part of the meizoseismal area was shaken by vibrations more or less slight. But, between one shock and another, at Diano Marina and Alassio, and even as far as Nice, it only required attention from a careful observer to perceive an almost continual throbbing of the ground.
Only one of these shocks, that of February 24th, at 2.10 A.M., was strong enough to cause slight damage to buildings. It disturbed an area, not exceeded by any of the later shocks, the boundary of which, shown by the dotted line A in Fig. 33, extends to the north and east as far as Piacenza and Spezia, while to the west it includes Cannes. The centre of the curve so drawn lies on land, but, as the shock was not felt in Corsica, there is no evidence as to the southerly extension of the disturbed area; and it is probable, as Professor Mercalli suggests, that the shock originated in the eastern or Oneglia focus of the great earthquake.
After February 25th, slight shocks were felt during the next fortnight, at the rate of three or four a day, until March 11th, when the last after-shock resulting in slight damage occurred at about 3.12 P.M. The boundary of its disturbed area, represented in Fig. 33 by the dotted line B, passes a little to the east of Savona, and then through Alessandria, Moncalieri, and Marseilles. The shock, however, was not observed in Corsica, so that the exact position of the epicentre is unknown; but Professor Mercalli believes it to coincide with the western or Nice epicentre of the principal earthquake. At the moment of the shock, the sea was observed from Alassio to curl and to rise slightly, while the tide-gauge at Nice, which had traced a continuous curve earlier in the day, showed a characteristic notch about 3.7 P.M.
Of the remaining after-shocks, only two attained any notable degree of strength. One, on May 20th at about 8.15 A.M., disturbed an area nearly concentric with that of the great earthquake, and with a boundary coinciding nearly with the isoseismal 2 in Fig. 33. Again, on July 17th at 11.30 P.M., occurred a shock felt over an area nearly as large as that disturbed on February 24th at 2.10 A.M., and situated in the same part of the country.
Altogether, during the year following the Riviera earthquake, Professor Mercalli records 190 after-shocks, most of them slight or only just felt. With the exception of the first two (on February 23rd), none was observed outside the isoseismal 4 of the principal earthquake (Fig. 33); and, of the rest, only the four whose dates are given above disturbed an area of more than one-eighth of that of the great shock. Some of them, like the shock of March 11th, were stronger in the western part of the meizoseismal area; but the majority affected most the eastern portion and seem to be closely associated with the Oneglia focus.
From February 26th to April 20th, Professor Rumi made observations on the after-shocks by means of the Foucault pendulum erected at Genoa for demonstrating the rotation of the earth. In nearly every case, the oscillations took place along a north-east and south-west line, or in the same direction as the first great shock—a resemblance which supports the inference that many of the after-shocks originated within the Oneglia focus.
ORIGIN OF THE EARTHQUAKES.
Recent Movements in the Riviera.—The earliest movements that resulted in the great range of the Maritime Alps and the Ligurian Apennines date from pre-Carboniferous times, when the central crystalline massifs in part emerged. At the end of the Liassic epoch, the secondary formations of the district were uplifted, and it was at this time that the range assumed its characteristic curved form. Later still, at the close of the Eocene period, an elevation of more than 9000 feet took place, for upper Eocene beds are found at this height in the Maritime Alps.
Since that time, other important movements have occurred. Pliocene deposits have been found in the Riviera at an altitude of 1,800 feet. Recent soundings in the Gulf of Genoa have also shown that all the valleys of the Riviera between Nice and Genoa are continued far below the level of the sea to depths of not less than 3000 feet. Thus, at the end of the Pliocene or beginning of the Quaternary period, there was an elevation of nearly 5000 feet, accompanied or followed by the erosion of the valleys which, later on, during the Quaternary period, were submerged about 3000 feet. Even in still more recent times, probably in the Palolithic age, minor movements continued. Traces of recent elevation, varying in amount from a few feet to sixty feet or more, occur at the Balzi Rossi in the Alpes Maritimes, near Bergeggi, and in Genoa; while evidences of submergence are to be found near Monaco, at Beaulieu and at Diano Marina. It is important to notice that the great movements dating from the end of the Eocene period are almost confined to the Maritime Alps and the western portion of the Riviera. In the parts of Piedmont lying to the north of Cuneo and in the eastern Riviera, they produced hardly any sensible effect.
Seismic History of the Riviera.—The movements just referred to are those which, in course of time, have become sensible to the eye. They represent the sum of a long-continued series of displacements that may once have been on a large scale, but are now comparatively small. The earthquakes that occur in the Riviera show, however, that the final stage has not yet been reached. Their epicentres indicate the regions in which slips are still taking place, and the magnitude of these slips is roughly measured by the intensity of the resulting shocks.
The map in Fig. 40 is one of a series drawn by Professor Mercalli to represent the distribution of seismic activity in Piedmont and the Riviera. It corresponds to the period from 1801 to 1895. The whole area is divided into a number of seismic districts, each of which is distinguished by a particular degree of activity. In estimating this quantity, Professor Mercalli takes intensity as well as frequency into account. Thus, the lowest degree, represented by the lightest tint of shading, corresponds to one or two strong earthquakes with a few moderate or slight shocks; the eighth and highest to four or five ruinous or disastrous earthquakes followed by trains of after-shocks. The map shows very clearly that, during the last century, the seismic activity was greatest in the Maritime Alps and the western Riviera—that is, in the very districts in which the recent mountain-making movements have been most conspicuous.
In all these districts, Professor Mercalli distinguishes several well-marked seismic centres, to each of which he traces the origin of two or more earthquakes. In the districts with which we are at present concerned, those of the Alpes Maritimes and the western Riviera, the most important centres are situated near Oneglia (in the sea), near Taggia, in the valleys of the Vesubia and Tinea (near Nice), and in the sea to the south of Nice. To the first of these centres belongs the disastrous earthquake of February 23rd, 1887, as well as its after-shocks on February 24th, May 20th, July 17th, and September 30th of the same year, also the ruinous earthquakes of 1612 and 1854, and several others of a lesser degree of intensity. All of these were longitudinal earthquakes, the axes of their meizoseismal areas being parallel to the neighbouring mountain-ranges. A few miles to the west of Oneglia lies the Taggia centre, with which were connected the disastrous earthquake of 1831, the violent earthquake of 1874, and other strong or very strong shocks. These were for the most part transversal earthquakes, their axes being perpendicular to those of the Oneglia centre.
Some of the strongest earthquakes in this region originated in a centre lying to the north of Nice in the valleys of the Vesubia and Tinea. Among them may be mentioned the ruinous earthquakes of 1494, 1556, 1564, and 1644, and probably also the disastrous earthquake of 1227. A fourth centre, and one of considerable interest, is that which lies at sea, a short distance to the south of Nice, and nearly along the continuation of the valleys above-mentioned. This is the secondary centre of the earthquake of 1887, and probably also of that of December 29th, 1554. It is occasionally in action apart from the Oneglia centre, as on November 27th, 1771, June 19th, 1806, and December 21st, 1861; but such shocks, though rather strong, never reach a high degree of intensity.
Origin of the Earthquakes of 1887.—The most important feature in the principal earthquake of 1887 is its origination in two distinct foci, which are sometimes in action almost simultaneously, but more often separately. The earthquakes belonging to the two foci differ greatly in intensity and number, and the stronger part of the shock in 1887 originated in the focus associated with the more disastrous and more frequent earthquakes.
The existence of two foci would of course give rise to a meizoseismal area elongated in the direction of the line joining them. It is clear, however, that the Oneglia focus was also extended in the same direction; for, in the after-shock of February 24th, the isoseismals drawn by Professor Mercalli are parallel to this line; and this was also the case in the shock of March 11th. As both foci were under the sea, it is difficult to locate them with precision; but it seems very probable that they occupy portions of a submarine fault that runs parallel or nearly so to the Apennine axis between the meridians of Oneglia and Nice.
A brief period of preparation is a characteristic of the Riviera earthquakes. In 1887, two at least of the preliminary shocks on February 23rd (those of about 2 and 5 A.M.) originated in the Oneglia focus. At 6.20 A.M. the first and weaker movement took place in the western focus; and, a few seconds after the resulting vibrations reached the eastern focus, the second and greater slip took place there. The occurrence of seismic sea-waves is probably evidence of the formation of a small, though sensible, fault-scarp in the same region. To relieve the additional stresses thus brought into action along the fault-surface, numerous small slips took place in different parts, some as far to the west as the Nice focus, but the greater number probably within or close to the focus in the neighbourhood of Oneglia.
1. BERTELLI, T.—"Osservazioni fatte in occasione di una escursione sulle Riviera Ligure di ponente dopo i terremoti ivi seguiti nell' anno 1887." Boll. Mens. dell' Oss. di Moncalieri, vol. viii., 1888, Nos. 6, 7, 8.
2. CHARLON, E.—"Note sur le tremblement de terre du 23 fvrier 1887." Bull. del Vulc. Ital., anno xiv., 1887, pp. 18-23.
3. DENZA, F.—Alcune notizie sul terremoto del 23 febbraio 1887 (Turin).
4. ISSEL, A.—"Il terremoto del 1887 in Liguria." Boll. del R. Com. Geol. d'Italia, anno 1887, supplemento, pp. 1-207.
5. MERCALLI, G.—I terremoti della Liguria e del Piemonte. (Naples, 1897, 146 pp.)
6. ODDONE, E.—"I dati sismici della Liguria in rapporto alla frequenza ed alla periodicit." Boll. della Soc. Sismol. Ital., vol. ii., 1896, pp. 140-151.
7. OFFRET, A.—"Sur le tremblement de terre du 23 fvrier 1887. Discussion des heures observs dans la zone picentrale." Paris, Acad. Sci., Compt. Rend., vol. civ., 1887, pp. 1150-1153.
8. ——. "Tremblements de terre du 23 fvrier 1887. Heures de l'arrive des secousses en dehors de l'picentre." Ibid., pp. 1238-1242.
9. ROSSI, M.S. DE.—"Relazione sui terremoti del febbraio 1887." Bull. del Vulc. Ital., anno xiv., 1887, pp. 5-17.
10. ——. "Bibliografia: Sul terremoto ligure del 23 febbraio 1887." Ibid., pp. 60-62, 107-112, 115-128.
11. TARAMELLI, T., and G. MERCALLI.—"Il terremoto ligure del 23 febbraio 1887." Annali dell' Uff. Centr. di Meteor. e di Geodin., vol. viii., parte iv., 1888. (Roma, 298 pp.)
12. UZIELLI, G.—Le commozioni telluriche e il terremoto del 23 febbraio 1887 (Turin).
13. Nature, vol. xxxv., 1887, pp. 438, 462, 534-535; vol. xxxvi., 1887, pp. 4, 151-152.
14. Paris, Acad. Sci. Compt. Rend., vol. civ., 1887, pp. 556-557, 606-612, 634-635, 659-667, 744-745, 757-758, 759-760, 764-766, 822-823, 830-835, 884-890, 950-951, 1088-1089, 1243-1245, 1350-1352, 1416-1419; vol. cv., 1887, pp. 202-203; vol. cviii., 1889, p. 1189; vol. cix.; 1889, pp. 164-166, 272-274, 660.
 The above times and all others in this chapter are given in Rome mean time, which is 50m. earlier than Greenwich mean time.
 Professor Uzielli has also published a map of the isoseismal lines for the Italian part of the disturbed area.
 It seems doubtful whether this movement was connected with the earthquake. M. Offret does not include Nice in his list of observatories at which magnetographs were disturbed.
 This is the time given by M. Offret. According to M. Mascart, it should be 6h. 25m. 40s.
 In order to test the truth of this explanation, M. Moureaux suspended a bar of copper at the Parc Saint-Maur observatory by two threads in the same way as the horizontal force-magnet. The direction of this bar was also registered photographically, and it remained unmoved during the Verny earthquake of July 12th, 1889, and the Dardanelles earthquake of October 25th, 1889, while one or more of the magnets were disturbed. The experiment, however, was ineffective; for, in order that the magnet may rest in a horizontal position, its centre of gravity must be at unequal distances from the two points of support.
 The hour-marks in Fig. 38 refer to Paris mean time, and those in Fig. 39 to Genoa mean time.
 In the seventeenth century, the maximum seismic activity was manifested in the neighbourhood of Nice, and in the eighteenth century in Piedmont.
THE JAPANESE EARTHQUAKE OF OCTOBER 28TH, 1891.
Although several years have elapsed since the occurrence of the greatest of Japanese earthquakes, the final report that will embody the labours of all its investigators is yet to be written. Several important contributions to it, however, have already been made. Professor Koto, in an admirable memoir, has traced the course of the great fault-scarp and discussed the origin of the earthquake; Professor Omori, with equal care and thoroughness, has investigated the unrivalled series of after-shocks; Mr. Conder studied the damaged buildings from an architect's point of view; Professor Tanakadate and Dr. Nagaoka devoted themselves to a re-determination of the magnetic elements of the central district, while, by the compilation of his great catalogue of Japanese earthquakes during the years 1885-92, Professor Milne has provided the materials for a further analysis of the minor shocks that preceded and followed the principal earthquake.
The part of Japan over which the earthquake was sensibly felt is shown in Fig. 41. The small black area in the centre is that in which the shock was most severe and the principal damage to life and property occurred. The other bands, more or less darkly shaded according to the greater or less intensity of the shock, will be referred to afterwards. Fig. 45 represents the meizoseismal area on a larger scale; and, as the greater part of it lies within the two provinces of Mino and Owari, the earthquake is generally known among the Japanese themselves as the Mino-Owari earthquake of 1891.
THE MEIZOSEISMAL AREA.
More than half of the meizoseismal area occupies a low flat plain of not less than 400 square miles in extent. On all sides but the south, the plain, which is a continuation of the depression forming the Sea of Is, is surrounded by mountain ranges, those to the west, north, and north-east being built up mainly of Palozoic rocks, and those on the east side of granite. A network of rivers and canals converts what might otherwise have been unproductive ground into one of the most fertile districts in Japan. A great garden, as it has been aptly termed, the whole plain is covered with rice-fields, and supports a population of about 787 to the square mile—a density which is exceeded in only six counties of England. As a rule, the soil is a loose, incoherent, fine sand, with but little clayey matter; and it is, no doubt, to its sandy nature that the disastrous effects of the earthquake were largely due. In the northern half of the district, the meizoseismal area is much narrower, and here it crosses a great mountain-range running from south-west to north-east and separating the river-systems of the Japan sea from those of the Pacific. To the north, the meizoseismal area terminates in another plain, in the centre of which lies the city of Fukui, where the destructiveness of the earthquake was only inferior to that experienced in the provinces of Mino and Owari. There is also a detached portion of the area lying to the east of Lake Biwa, but it is uncertain whether the exceptional intensity there was due to the nature of the ground or to the occurrence of a secondary or sympathetic earthquake in its immediate neighbourhood.
The general plan of the geological structure of the central district is represented in Fig. 42. The thick line, partly continuous and partly broken, shows the course of the great fault, to the growth of which the earthquake chiefly owed its origin; while the thin continuous lines represent the changing direction of strike of the Palozoic rocks which surround the Mino-Owari plain, and the arrowheads the direction of the dip. It will be seen that the direction of the strike forms an S-shaped curve, and it is clear that the present torsion-structure of the district could not have been produced without the formation of many fractures at right angles and parallel to the lines of strike. Professor Koto points out that the regular and parallel valleys of the rivers Tokuno-yama, Neo, Mugi, and Itatori, indicated by broken lines in Fig. 42, have probably been excavated along a series of transverse fractures running from north-west to south-east; while fractures which are parallel to the line of strike may be responsible for the zigzag course of the valleys.
DAMAGE CAUSED BY THE EARTHQUAKE.
The great earthquake occurred at 6.37 A.M., practically without warning, and in a few seconds thousands of houses were levelled with the ground. Within the whole meizoseismal area there was hardly a building left undamaged. The road from Nagoya to Gifu, more than twenty miles in length, and formerly bordered by an almost continuous succession of villages, was converted into a narrow lane between two long drawn-out banks of dbris. "In some streets," says Professor Milne, "it appeared as if the houses had been pushed down from the end, and they had fallen like a row of cards." Or, again, a mass of heaped-up rubbish might be passed, "where sticks and earth and tiles were so thoroughly mixed that traces of streets or indications of building had been entirely lost." At Gifu, Ogaki, Kasamatsu, and other towns, fires broke out after the earthquake. In Kasamatsu the destruction was absolutely complete; nothing was left but a heap of plaster, mud, tiles, and charred timbers. At Ogaki, not more than thirty out of 8000 houses remained standing, and these were all much damaged. Within the whole district, according to the official returns, 197,530 buildings were entirely destroyed, 78,296 half destroyed, and 5,934 shattered and burnt; while 7,279 persons were killed, and 17,393 were wounded.
Next to buildings, the embankments which border the rivers and canals suffered the most serious damage, no less than 317 miles of such works having to be repaired. Railway-lines were twisted or bent in many places, the total length demolished being more than ten miles. In cuttings, twenty feet or more in depth, both rails and sleepers were unmoved; it was on the plains that the effects of the earthquake were most marked. The ground appeared as if piled up into bolster-like ridges between the sleepers, and in many places the sleepers had moved end-ways. When the line crossed a small depression in the general level of the plain, the whole of the track was bowed, as if the ground were permanently compressed at such places. "Effects of compression," says Professor Milne, "were most marked on some of the embankments, which gradually raise the line to the level of the bridges. On some of these, the track was bent in and out until it resembled a serpent wriggling up a slope.... Close to the bridges the embankments had generally disappeared, and the rails and sleepers were hanging in the air in huge catenaries."
ISOSEISMAL LINES AND DISTURBED AREA.
The land area disturbed by the earthquake and the different isoseismal lines are shown in Fig. 41. The "most severely shaken" district, that in which the destruction of buildings and engineering works was nearly complete, contains an area of 4,286 square miles, or about two-thirds that of Yorkshire. This is indicated on the map by the black portion. Outside this lies the "very severely shaken" district, 17,325 square miles in area, extending from Kobe on the west to Shizuoka on the east, in which ordinary buildings were destroyed, walls fractured, embankments and roads damaged, and bridges broken down. The third or "severely shaken" district contains 20,183 square miles; and in this some walls were cracked, pendulum clocks stopped, and furniture, crockery, etc., overthrown. Tokio and Yokohama lie just within this area. In the fourth region the shock was "weak," the motion being distinctly felt, but not causing people to run out-of-doors; and in the fifth it was "slight," or just sufficient to be felt. These two regions together include an area of 51,976 square miles.
Thus, the land area disturbed amounts altogether to 93,770 square miles—i.e., to a little more than the area of Great Britain. According to Professor Omori, the mean radius of propagation was about 323 miles, and the total disturbed area must therefore have been about 330,000 square miles, or nearly four times the area of Great Britain. Considering the extraordinary intensity of the shock in the central district, this can hardly be regarded as an over-estimate.
The isoseismal lines shown in Fig. 41 are not to be regarded as drawn with great accuracy; for there is no marked separation between the tests corresponding to the different degrees of the scale of intensity. The seismographs at Gifu and Nagoya were thrown down within the first few seconds, and failed to record the principal motion. But a great number of well-formed stone lanterns and tombstones were overturned, and, from the dimensions of these, Professor Omori calculated the maximum horizontal acceleration necessary for overturning them at fifty-nine places within the meizoseismal area. At five of these it exceeded 4000 millimetres per second per second, an acceleration equal to about five-twelfths of that due to gravity. Making use of these observations, Professor Omori has drawn two isoseismal lines within the central district, which are shown in Fig. 44. At every point of the curve marked 2, the maximum acceleration was 2000 millimetres per second per second, and of that marked 1, 800 millimetres per second per second. The dotted line within the curve marked 2 represents the boundary of the meizoseismal area, which, it will be observed, differs slightly from that given by Professor Koto (see Fig. 45). The difference, however, is apparently due to the standard of intensity adopted, Professor Koto's boundary agreeing rather closely with the curve marked 2 in Fig. 44.