Part 12 (1/2)
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 pa.s.sed 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.
[Ill.u.s.tration: FIG. 35.--Nature of shock of Riviera earthquake.
(_Taramelli and Mercalli._)]
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 Sal on the sh.o.r.e 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 princ.i.p.al 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 princ.i.p.al 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.
[Ill.u.s.tration: FIG. 36.--Seismographic record of the Riviera earthquake at Moncalieri. (_Denza._)]
During the princ.i.p.al Riviera earthquake, the former pendulum furnished an indistinct record, while the other traced the diagram reproduced in Fig. 36. The movement, as here represented, began at about 6h. 21m.
50s. A.M. (mean time of Rome) with a series of small tremors, which lasted for about twelve seconds. Then followed some large oscillations, always in a nearly east-and-west direction, which at 6h.
22m. 21s. gave place to a second series of tremors similar to those at the beginning of the shock, but of greater amplitude. These continued for at least twelve seconds, at the end of which time the motion of the smoked paper ceased. The total duration of the movement at Moncalieri cannot therefore have been less than forty-three seconds.
Interesting as this record is, it is doubtful how far it represents accurately the movement of the ground. The Moncalieri instrument was erected before the modern type of seismograph was designed, in which some part remains steady, or very nearly steady, during the complicated movements of the ground that take place in an earthquake.
It will be noticed that the curve in Fig. 36 shows no sign of the division of the shock into two distinct parts, and this may perhaps be due to the swinging of the pendulum itself; in which case, the curve described by the pointer would be the resultant of the oscillations of the ground and the proper motion of the pendulum.
SOUND-PHENOMENA.
The sounds that preceded and accompanied the Riviera earthquake have attracted but little study, although they seem to have been widely observed. No attempt was made to define the limits of the area over which they were audible; but Professor Mercalli states that in the two outer zones (Fig. 33) the sound generally pa.s.sed un.o.bserved. It was, however, heard near Piacenza in Lombardy and Reggio in Emilia, places which are about 115 and 140 miles from the princ.i.p.al epicentre.
In the area in which the shock was most violent, the sound resembled that of trains and vehicles in motion; while, outside this area it generally appeared to be like the hissing of a violent wind. In only a few places was it compared to detonations, the crashes of artillery or distant thunder. Some observers describe the sound as appearing at first as if a strong wind were rising, and then as the roaring of a heavy railway-train pa.s.sing.
Nearly all the observers, who were awake at the beginning of the earthquake, agree in a.s.serting that the sound distinctly preceded any movement of the ground. From this, as in the case of the Andalusian earthquake, Professor Mercalli infers that the sound-vibrations travelled with the greater velocity; but, as will be shown in Chapter VIII., the general precedence of the sound admits of another and more probable explanation.
THE UNFELT EARTHQUAKE.
If the Andalusian earthquake first drew general attention to the distant spread of unfelt earth-waves, the Riviera earthquake showed that this was no isolated phenomenon. We know now that the propagation of such waves is only limited by the surface of the earth, but in 1887 some doubt was felt at first as to the nature of the disturbance, whether it was magnetic or mechanical in its origin.
In 1884, the only observatories at which magnetographs were disturbed were those of Lisbon, Parc Saint-Maur (near Paris), Greenwich, and Wilhelmshaven. In 1887, the magnetographs registered the Riviera earthquake at these and several other observatories, the distribution of which is shown in Fig. 37. In this sketch-map, the position of the princ.i.p.al epicentre is represented by the small cross, while the nearly circular line shows the boundary of the disturbed area.
[Ill.u.s.tration: FIG. 37.--Distribution of observatories at which magnetographs were disturbed by the Riviera earthquake.]
Three of the observatories, those of Nice, Lyons, and Perpignan, lie inside this area. At Nice (which is thirty-seven miles from the princ.i.p.al epicentre), M. Perrotin states that the magnetograph curves show nothing of any interest, except a notable magnetic perturbation on the vertical force curve, the time of which, however, is not stated.[49] At Lyons (211 miles), the declination, horizontal force and vertical force, magnets were all disturbed at 6h. 25m. 47s. A.M., and Perpignan (264 miles), all three magnets, but especially those for the declination and horizontal force, were set abruptly oscillating at 6h. 25m. 20s.