_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.
On the line which runs northward from Genoa to Piedmont, a tunnel more than five miles in length pierces the hilly ground between Ponterosso and Ronco, the greatest thickness of rock above being about a thousand feet. At the time of the earthquake, the tunnel was not everywhere opened out to its full width, and men were at work in different sections. Outside, the shock was strong enough to damage buildings.
Inside, at about 200 yards from the south end, only a feeble shock was felt; at 1,350 and 1,625 yards, some bricks were seen to fall from the facing, but the shock was not otherwise perceived, and only a few yards farther nothing unusual was noticed by the men at work.
Again, in an unfinished tunnel, about three-quarters of a mile long, between the harbour of Genoa and the eastern railway-station, the vibrations were very slightly felt. Even in the tunnels traversed by the coast railway from Genoa to Nice--that is, in those situated within the meizoseismal area--the shock was either very weak or not felt at all, and not one of the tunnels suffered the slightest injury.
To men at work inside a long tunnel, the conditions for observing earthquakes are somewhat imperfect, but these facts, nevertheless, bring out very clearly the inferior intensity of the shock at some depth below the surface.
AFTER-SHOCKS.
While the unfelt earth-waves of the great earthquake were still wending their way over the zone that surrounds the disturbed area, the central regions were again shaken, at 6.29 A.M., by a shock strong enough to produce fresh ruins in the stricken towns along the coast.
Nearly two and a half hours of quiet followed, broken only by a few subterranean rumblings in the central part of the meizoseismal area.
Then, at 8.51 A.M., occurred another shock, short and sharp, and inferior in strength only to the princ.i.p.al earthquake. Both of these after-shocks were felt in Western Switzerland; indeed, they were perceptible nearly as far as the great shock; the second, however, a little farther than the first, for it alone was noticed at such places as Vicenza, Forl, and Florence. The shock at 6.29 was usually described as long and its vibrations as undulatory only; that at 8.51 as rather subsultory than undulatory and of very brief duration. The latter, however, was followed after an interval of a few seconds by another shock so weak that it generally pa.s.sed un.o.bserved. Both shocks were preceded by a rumbling sound.
During the next two days, tremors and earth-sounds were frequent in the Riviera; once an hour, on an average, the greater part of the meizoseismal area was shaken by vibrations more or less slight. But, between one shock and another, at Diano Marina and Ala.s.sio, and even as far as Nice, it only required attention from a careful observer to perceive an almost continual throbbing of the ground.
Only one of these shocks, that of February 24th, at 2.10 A.M., was strong enough to cause slight damage to buildings. It disturbed an area, not exceeded by any of the later shocks, the boundary of which, shown by the dotted line A in Fig. 33, extends to the north and east as far as Piacenza and Spezia, while to the west it includes Cannes.
The centre of the curve so drawn lies on land, but, as the shock was not felt in Corsica, there is no evidence as to the southerly extension of the disturbed area; and it is probable, as Professor Mercalli suggests, that the shock originated in the eastern or Oneglia focus of the great earthquake.
After February 25th, slight shocks were felt during the next fortnight, at the rate of three or four a day, until March 11th, when the last after-shock resulting in slight damage occurred at about 3.12 P.M. The boundary of its disturbed area, represented in Fig. 33 by the dotted line B, pa.s.ses a little to the east of Savona, and then through Alessandria, Moncalieri, and Ma.r.s.eilles. The shock, however, was not observed in Corsica, so that the exact position of the epicentre is unknown; but Professor Mercalli believes it to coincide with the western or Nice epicentre of the princ.i.p.al earthquake. At the moment of the shock, the sea was observed from Ala.s.sio to curl and to rise slightly, while the tide-gauge at Nice, which had traced a continuous curve earlier in the day, showed a characteristic notch about 3.7 P.M.
Of the remaining after-shocks, only two attained any notable degree of strength. One, on May 20th at about 8.15 A.M., disturbed an area nearly concentric with that of the great earthquake, and with a boundary coinciding nearly with the isoseismal 2 in Fig. 33. Again, on July 17th at 11.30 P.M., occurred a shock felt over an area nearly as large as that disturbed on February 24th at 2.10 A.M., and situated in the same part of the country.
Altogether, during the year following the Riviera earthquake, Professor Mercalli records 190 after-shocks, most of them slight or only just felt. With the exception of the first two (on February 23rd), none was observed outside the isoseismal 4 of the princ.i.p.al earthquake (Fig. 33); and, of the rest, only the four whose dates are given above disturbed an area of more than one-eighth of that of the great shock. Some of them, like the shock of March 11th, were stronger in the western part of the meizoseismal area; but the majority affected most the eastern portion and seem to be closely a.s.sociated with the Oneglia focus.
From February 26th to April 20th, Professor Rumi made observations on the after-shocks by means of the Foucault pendulum erected at Genoa for demonstrating the rotation of the earth. In nearly every case, the oscillations took place along a north-east and south-west line, or in the same direction as the first great shock--a resemblance which supports the inference that many of the after-shocks originated within the Oneglia focus.
ORIGIN OF THE EARTHQUAKES.
_Recent Movements in the Riviera._--The earliest movements that resulted in the great range of the Maritime Alps and the Ligurian Apennines date from pre-Carboniferous times, when the central crystalline ma.s.sifs in part emerged. At the end of the Lia.s.sic epoch, the secondary formations of the district were uplifted, and it was at this time that the range a.s.sumed its characteristic curved form. Later still, at the close of the Eocene period, an elevation of more than 9000 feet took place, for upper Eocene beds are found at this height in the Maritime Alps.
Since that time, other important movements have occurred. Pliocene deposits have been found in the Riviera at an alt.i.tude of 1,800 feet.
Recent soundings in the Gulf of Genoa have also shown that all the valleys of the Riviera between Nice and Genoa are continued far below the level of the sea to depths of not less than 3000 feet. Thus, at the end of the Pliocene or beginning of the Quaternary period, there was an elevation of nearly 5000 feet, accompanied or followed by the erosion of the valleys which, later on, during the Quaternary period, were submerged about 3000 feet. Even in still more recent times, probably in the Palaeolithic age, minor movements continued. Traces of recent elevation, varying in amount from a few feet to sixty feet or more, occur at the Balzi Rossi in the Alpes Maritimes, near Bergeggi, and in Genoa; while evidences of submergence are to be found near Monaco, at Beaulieu and at Diano Marina. It is important to notice that the great movements dating from the end of the Eocene period are almost confined to the Maritime Alps and the western portion of the Riviera. In the parts of Piedmont lying to the north of Cuneo and in the eastern Riviera, they produced hardly any sensible effect.
_Seismic History of the Riviera._--The movements just referred to are those which, in course of time, have become sensible to the eye. They represent the sum of a long-continued series of displacements that may once have been on a large scale, but are now comparatively small. The earthquakes that occur in the Riviera show, however, that the final stage has not yet been reached. Their epicentres indicate the regions in which slips are still taking place, and the magnitude of these slips is roughly measured by the intensity of the resulting shocks.
The map in Fig. 40 is one of a series drawn by Professor Mercalli to represent the distribution of seismic activity in Piedmont and the Riviera. It corresponds to the period from 1801 to 1895. The whole area is divided into a number of seismic districts, each of which is distinguished by a particular degree of activity. In estimating this quant.i.ty, Professor Mercalli takes intensity as well as frequency into account. Thus, the lowest degree, represented by the lightest tint of shading, corresponds to one or two strong earthquakes with a few moderate or slight shocks; the eighth and highest to four or five ruinous or disastrous earthquakes followed by trains of after-shocks.
The map shows very clearly that, during the last century, the seismic activity was greatest in the Maritime Alps and the western Riviera--that is, in the very districts in which the recent mountain-making movements have been most conspicuous.[53]
[Ill.u.s.tration: FIG. 40.--Distribution of seismic activity in the Riviera. (_Mercalli._)]
In all these districts, Professor Mercalli distinguishes several well-marked seismic centres, to each of which he traces the origin of two or more earthquakes. In the districts with which we are at present concerned, those of the Alpes Maritimes and the western Riviera, the most important centres are situated near Oneglia (in the sea), near Taggia, in the valleys of the Vesubia and Tinea (near Nice), and in the sea to the south of Nice. To the first of these centres belongs the disastrous earthquake of February 23rd, 1887, as well as its after-shocks on February 24th, May 20th, July 17th, and September 30th of the same year, also the ruinous earthquakes of 1612 and 1854, and several others of a lesser degree of intensity. All of these were longitudinal earthquakes, the axes of their meizoseismal areas being parallel to the neighbouring mountain-ranges. A few miles to the west of Oneglia lies the Taggia centre, with which were connected the disastrous earthquake of 1831, the violent earthquake of 1874, and other strong or very strong shocks. These were for the most part transversal earthquakes, their axes being perpendicular to those of the Oneglia centre.
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 princ.i.p.al 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 a.s.sociated 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 fevrier 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.)
6. ODDONE, E.--"I dati sismici della Liguria in rapporto alla frequenza ed alla periodicita." _Boll. della Soc. Sismol.
Ital._, vol. ii., 1896, pp. 140-151.
7. OFFRET, A.--"Sur le tremblement de terre du 23 fevrier 1887.
Discussion des heures observes dans la zone epicentrale."
Paris, _Acad. Sci., Compt. Rend._, vol. civ., 1887, pp.
1150-1153.
8. ----. "Tremblements de terre du 23 fevrier 1887. Heures de l"arrivee des secousses en dehors de l"epicentre." _Ibid._, pp. 1238-1242.
9. ROSSI, M.S. DE.--"Relazione sui terremoti del febbraio 1887."
_Bull. del Vulc. Ital._, anno xiv., 1887, pp. 5-17.
10. ----. "Bibliografia: Sul terremoto ligure del 23 febbraio 1887."
_Ibid._, pp. 60-62, 107-112, 115-128.
11. TARAMELLI, T., and G. MERCALLI.--"Il terremoto ligure del 23 febbraio 1887." _Annali dell" Uff. Centr. di Meteor. e di Geodin._, vol. viii., parte iv., 1888. (Roma, 298 pp.)
12. UZIELLI, G.--_Le commozioni telluriche e il terremoto del 23 febbraio 1887_ (Turin).
13. _Nature_, vol. x.x.xv., 1887, pp. 438, 462, 534-535; vol. x.x.xvi., 1887, pp. 4, 151-152.
14. Paris, _Acad. Sci. Compt. Rend._, vol. civ., 1887, pp. 556-557, 606-612, 634-635, 659-667, 744-745, 757-758, 759-760, 764-766, 822-823, 830-835, 884-890, 950-951, 1088-1089, 1243-1245, 1350-1352, 1416-1419; vol. cv., 1887, pp. 202-203; vol.
cviii., 1889, p. 1189; vol. cix.; 1889, pp. 164-166, 272-274, 660.
FOOTNOTES:
[47] The above times and all others in this chapter are given in Rome mean time, which is 50m. earlier than Greenwich mean time.