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.;[52] 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.
Fig. 39.—Record of tide-gauge at Genoa. (Issel.)[ToList]
MISCELLANEOUS PHENOMENA.
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 8½ 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.