Part 11 (2/2)
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.[48] In this map, reproduced in Fig. 33, the continuous curves represent the princ.i.p.al 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 princ.i.p.al 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.
[Ill.u.s.tration: FIG. 33.--Isoseismal lines of the Riviera earthquake. (_Mercalli._)]
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) pa.s.ses near Ma.r.s.eilles, 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 a.s.signed 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 princ.i.p.al 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 princ.i.p.al 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 princ.i.p.al epicentre is that marked by the small circle A, which is situated about fifteen miles south of Oneglia.
[Ill.u.s.tration: FIG. 34.--Meizoseismal area of the Riviera earthquake. (_Taramelli and Mercalli._)]
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 pa.s.sed 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 princ.i.p.al 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 princ.i.p.al focus by several seconds.
DEPTH OF THE PRINc.i.p.aL FOCUS.
Inaccurate as are all the methods of determining the depth of focus, it seems probable, as Professor Issel argues, that the princ.i.p.al 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 princ.i.p.al focus.
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