Fig. 35.—Nature of shock of Riviera earthquake. (Taramelli and Mercalli.)[ToList]
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 Salò 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.
