A Study of Recent Earthquakes

by Charles Davison

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 10¾ 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.