In Italy, probable records of the earthquake were obtained at two observatories, but, owing to the approximate times given, their connection with it is not established. At Velletri, near Rome, Professor Galli's seismodynamograph registered a very slight movement at 10 P.M., and at Rome itself Professor de Rossi found a tromometer making unusual oscillations at 10.15 P.M.[31]
The most interesting records, however, are those furnished by the magnetographs at Lisbon, Parc Saint-Maur (near Paris), Greenwich, and Wilhelmshaven. At Lisbon, the records are extremely clear. The curves of the declination, horizontal force and vertical force magnets, as seen in Fig. 21, are abruptly broken at 8.33 P.M. (Lisbon time, or 9h. 9m. 45s., G.M.T.). The disturbances, which are greatest on the declination curve and least on the vertical force curve, lasted in all three for about 12 minutes, and are quite distinct from the ordinary magnetic perturbations. At Parc Saint-Maur, the magnetographs seem to be ill-adapted to act as seismographs, for only a slight mark was discovered on a re-examination of the curves, beginning at 9.24 P.M. (Paris time, or 9h. 14m. 39s., G.M.T.) At Greenwich, Mr. W. Ellis writes, there was "a small simultaneous disturbance of the declination and horizontal force magnets, occurring at 9h. 15m.... Both magnets were at this time set into slight vibration, the extent of vibration in the case of declination being about 2' of arc, and in horizontal force equivalent to .001 of the whole horizontal force nearly." Of the three instruments at Wilhelmshaven, only one showed any movement at the time of the earthquake. The declination magnet was undisturbed, the horizontal force curve was accidentally interrupted, but the vertical force curve indicated a very perceptible shock. Beginning at 9.52 P.M. (Wilhelmshaven mean time, or 9h. 29m. 29s., G.M.T.), the curve was broken for four minutes, for the rapid swinging of the needle could not be registered until the motion became fainter. Further disturbances also occurred at 9.59, 10, 10.2, and 10.5 P.M.[32]
POSITION OF THE EPICENTRE.
The innermost isoseismal being too large, and the time-records too inaccurate, to give the position of the epicentre, both Commissions resorted to observations of the direction, Professor Fouqué and his colleagues depending chiefly on the oscillation of hanging lamps, and Professors Taramelli and Mercalli on the fall or displacement of statues and other objects, and all avoiding as far as possible the evidence of fissures in buildings.
The Italian observers point out that, among the divergent directions visible at any place, there is generally one more distinctly marked than the others, and this, they consider, corresponds to the movement coming almost directly from the centre of disturbance. Plotting these directions (36 in number), they find that they converge as a rule within the triangle formed by joining Ventas de Zafarraya, Alhama, and Jatar, while a large number of them traverse the elliptical area, whose boundary is represented by the dotted line in Fig. 19. This area is about 9 miles long and 2½ miles wide, its longer axis runs nearly east and west, and its centre coincides with the western focus of the ellipse which forms the boundary of the meizoseismal area. It lies, moreover, close to Ventas de Zafarraya and Arenas del Rey, the two places where the seismic death-rate was highest, while its major axis almost coincides with the line joining them.
The evidence of hanging lamps collected by the French Commission was more consistent than that of the fallen objects. At every place, the plane in which the lamps oscillated was nearly constant, the deviations being generally attributable to irregularities in the mode of suspension. The azimuths again intersect within an elliptical area, which, according to the Commission, differs little from the central region of the earthquake (Fig. 20). It Is clear, however, from the map accompanying the French report, that the majority converge towards a narrow band extending east and west from near Arenas del Rey to near Ventas de Zafarraya, and therefore agreeing closely with the epicentral area as determined by Professors Taramelli and Mercalli.[33]
DEPTH OF THE SEISMIC FOCUS.
If the depth of the seismic focus amounts to several miles, one of the most serious objections to Mallet's method lies in the varying refractive power of the different strata traversed by the earth-waves (p. 28). At present we have no way of meeting this objection, and all calculations of the depth of the focus are therefore more or less doubtful. A difficulty in practice has also been urged, depending on the widely differing inclinations of the fractures at any place; but the Italian observers found that the errors from this source were greatly reduced by avoiding all fissures in poorly-built houses, or which start from windows or other apertures, and selecting only those which occur in homogeneous walls directed towards the epicentre. The best angles of emergence thus measured by them are thirteen in number, all made at places lying within 5 and 23 miles from the centre of the epicentral area, and, with two exceptions, inside the meizoseismal zone (Fig. 19). The depths corresponding to the different wave-paths vary from 5.3 to 23.0 miles, the mean depth of the focus given by all thirteen observations being 7.6 miles.
The only estimate made by the French Commission—and it is one that they rightly regarded with considerable doubt—was based on a method devised by Falb. As the sound generally precedes the shock, Falb assumes that it travels with a greater velocity. If the velocities of both series of waves are known, and if they start at the same instant and from the same region, the interval that elapses between the arrivals of the sound and shock should give the distance traversed by them and consequently the depth of the focus. It is unnecessary to mention more than two of the serious objections to this method. The duration of the preliminary sound should increase rapidly with the distance from the focus, and of this there is not the slightest evidence. Moreover, the sound-vibrations that are first heard do not necessarily come from the same part of the focus as those which cause the shock, but, as will be seen in Chapter VIII., probably from its nearer lateral margin. The French Commission, finding the average duration of the fore-sound near the epicentre to be 5 seconds, estimate the depth of the focus at about 7 miles—a result which agrees remarkably with that obtained from the angles of emergence, but which is not, on that account, entitled to credit.