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.
REFERENCES.
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 février 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.)