A Study of Recent Earthquakes Part 6

_f._ Campagnano.

_g._ Vezza.

_h._ Imperatore.

_i._ C. St. Angelo.

_j._ Lo Toppo.

_k._ Marecocco.

_l._ Zale.

_m._ Rotaro.

_n._ Montagnone.

_p._ Bagno.

_q._ Tabor.

_r._ P. Castiglione.

_s._ Cremate.

_t._ Arso.

_u._ Porto d'Ischia.

[22] It is possible that Monte Campagnano may form an exception to this statement.

[23] Shocks were felt in the island in 1559 and 1659, but one at least was of external origin.

[24] Prof. Mercalli, from the five estimates of the angle of emergence which he considered most reliable, found the mean depth to be about 3,280 feet.

[25] Professor de Rossi estimated the mean duration as not much exceeding ten seconds. Dr. Johnston-Lavis, on the other hand, considers the general estimate of fifteen seconds as far too low. In one case, at Casamicciola, he ranks it as high as thirty-one seconds.

[26] Quoted from the useful translation of Covelli's memoir given by Dr. Johnston-Lavis.

[27] Baldacci supposes that the thermal springs and fumaroles of Forio, Stennecchia, Montecito, Casamicciola, and Castiglione lie along a tangential fracture starting from Forio and pa.s.sing by Casamicciola to near Punta di Castiglione. Mercalli, however, argues forcibly against this inference.

[28] Professor Mercalli adds, as a fourth point of contact between Ischian earthquakes and volcanic phenomena, the changes in the fumaroles and hot springs which preceded or accompanied or followed the earthquakes of 1828, 1881, and 1883.

[29] "Fontana," he says, "occupies the centre of the great crater of Epomeo..., and therefore lies immediately over the ancient chimney, which in all probability is filled by an old plug of consolidated trachyte, which must descend to the igneous reservoir. Any ma.s.s of igneous matter, that might determine the further rupture of a collateral fissure, would result in the conduction of any changes of pressure or vibrations, along the column of highly elastic trachyte; whilst the same earth-waves would be annulled or absorbed by the inelastic tufas surrounding it, so that the blow would be struck perpendicularly to the surface, and in a small area with well defined limits. The undulatory sensations, after the princ.i.p.al local shock, were those that arrived from the great centre of impulse beneath Casamenella."

[30] The above paragraph is a summary of the reasoning stated with admirable clearness by Dr. Johnston-Lavis. It should be mentioned that the late Professor Palmieri, relying on the extremely limited disturbed area, dissented from this view; but his difficulty is met by supposing the focus to be small as well as shallow, a supposition that is supported by the shortness of the meizoseismal band, as well as by the elongation of the isoseismal lines in the direction perpendicular to this band.

CHAPTER IV.

THE ANDALUSIAN EARTHQUAKE OF DECEMBER 25TH, 1884.

In most countries the princ.i.p.al seismic districts are of limited extent. Thus, in central j.a.pan, the east coast is frequently visited by earthquakes, while the west coast is relatively undisturbed. Of the earthquakes felt in the kingdom of Greece during the years 1893-98, 63 per cent. were observed in Zante, and were for the most part confined to that island. In the interior of the Iberian peninsula--in Leon and in New and Old Castile--destructive earthquakes are practically unknown; while the littoral regions of central and southern Portugal, Andalusia, and Catalonia are noted for their disastrous shocks.

During the eighteenth century seismic activity was chiefly concentrated in Portugal, and culminated in the great Lisbon earthquake of 1755. In the following century the seat of disturbance was transferred from the west to the south of the peninsula; Portugal remained throughout in comparative repose, while Almeria experienced destructive shocks in 1804, 1860, and 1863, and Murcia in 1828-29 and 1864, leading up to the Andalusian earthquakes of 1884-85, described in the present chapter.

The preparation for the princ.i.p.al earthquake of December 25th, 1884, was unusually indistinct. For a day or two before, shocks were felt here and there in Andalusia, but so weak were they that they pa.s.sed almost unperceived. During the night of December 24-25, one slight shock was noticed at Colmenar (Fig. 19) and another at Zafarraya. On the 25th, a faint movement of the ground was noticed at Malaga, and a few weak tremors at Periana; and shortly after came the great shock at about 8.50 P.M. mean time of Malaga, or about 9.8 P.M. Greenwich mean time.

This earthquake was investigated by no fewer than three official committees. The first in the field was nominated by the Spanish Government on January 7th, 1885, and consisted of four members, the President being Senor M.F. de Castro, the director of the Geological Survey of Spain. The report of this commission was presented to the Minister of Agriculture, etc., on March 12th. Early in February a French Commission, appointed by the Academy of Sciences, proceeded to the scene of the disaster. With Professor F. Fouque as chief, and MM.

Levy, Bertrand, Barrois, Offret, Kilian, Bergeron, and Breon as members, this committee resolved itself after a time into one for studying the geology of the central area; and, of their voluminous report of more than 700 quarto pages (published in 1889), only 55 are immediately concerned with the earthquake. At the beginning of April, Professors Taramelli and Mercalli, sent by the Italian Government, arrived in Andalusia; and their memoir, read a few months later before the Reale Accademia dei Lincei, forms by far the most valuable contribution to our knowledge of the earthquake.

DAMAGE CAUSED BY THE EARTHQUAKE.

The meizoseismal area (see Figs. 19 and 20) lies in a mountainous district, almost equidistant from the cities of Malaga and Granada. In this area, which contains nearly 900 square miles, the shock was disastrous to all but well-built houses. Whole villages were overthrown. In the surrounding zone many buildings escaped serious damage, and only a few were completely destroyed. It is estimated by the Spanish Commission that, in the province of Granada, 3,342 houses were totally, and 2,138 partially, ruined; in the province of Malaga, 1,057 houses were totally, and 4,178 partially, ruined; while in the two provinces together 6,463 houses were damaged; making a total of 17,178 buildings more or less seriously injured.

As usual in the South of Europe, bad construction and narrow streets were largely responsible for the loss of property, houses that were regularly built and made of good materials being only slightly injured. But, in this case, the great slope of the ground, the bad quality of the foundations, and the nature of the underlying rocks were contributing factors. Many buildings also had been damaged by previous shocks, and their ruin was only completed by the earthquake of 1884.

The total loss of life is variously estimated. According to the Spanish Commission, 690 persons were killed and 1,426 wounded in the province of Granada, while 55 were killed and 59 wounded in that of Malaga, making a total of 745 persons killed and 1,485 wounded. The Italian seismologists, having additional materials at their disposal, raise the total figures to 750 persons killed and 1,554 severely wounded. Careful inquiries were also made on this subject by the conductors of the newspaper _El Defensor de Granada_. In Granada alone, they reckon that 828 persons were killed and 1,164 wounded.

From the table given in the Italian report, it appears that 330 persons were killed at Alhama, 118 at Arenas del Rey, 102 at Albunuelas, 77 at Ventas de Zafarraya, and 40 at Periana; the percentage of mortality being 9 at Arenas del Rey, about the same at Ventas de Zafarraya, and 3 or 4 at Alhama, Albunuelas and Periana.

Comparing these latter figures with the death rates of 71 per cent. at Montemurro, caused by the Neapolitan earthquake, and of about 45 per cent. at Casamicciola, by the Ischian earthquake of 1883, it will be seen that the loss of life during the Andalusian earthquake was comparatively small--an exemption which is attributed by the Italian commissioners to the absence of inhabited places from the immediate neighbourhood of the epicentre, and to the fact that the destructive vibrations occurred towards the end of the shock, thus allowing opportunity for escape.

ISOSEISMAL LINES AND DISTURBED AREA.

Fig. 19 shows the princ.i.p.al isoseismal lines as drawn by the Italian commissioners. The meizoseismal area, which included all places at which the shock was disastrous, is bounded by an ellipse (marked 1 on the map) 40 miles long from east to west, 28 miles wide, and about 886 square miles in area. The next isoseismal (2) includes the places in which some buildings were ruined, but not as a rule completely, and in which there was no loss of life. Its bounding line is also elliptical, the longer axis being about 71 miles long and running nearly east and west. Towards the south this zone is interrupted by the sea. It will be noticed that these isoseismals are not concentric, the second extending much farther to the west and south-west than in the opposite direction. A third isoseismal (not shown in the map) encloses the district in which the shock was "very strong," or just capable of producing cracks in the walls of houses. It is similar in form to the second isoseismal, reaching as far as Estepone to the south-west, Osuna, Cordova, and Seville to the west, Jaen to the north, while towards the east it stops short of Almeria.

[Ill.u.s.tration: FIG. 19.--Isoseismal lines of Andalusian earthquake. (_Taramelli and Mercalli._)]

The French Commission have also published a map of the earthquake, and, though the work of an experienced seismologist like Professor Mercalli is probably more trustworthy, it is interesting to compare his isoseismal lines with those obtained by his French colleagues, which are reproduced in Fig. 20. The curves in this figure are drawn so as to include the places that were, respectively, ruined, seriously damaged, and slightly damaged, by the shock. They should therefore correspond with the lines in Fig. 19. It will be seen that they differ considerably in form, but at the same time they present certain points of agreement, such as the east and west elongation of the meizoseismal area, and the great extension of the two outer isoseismals towards the west and south-west The greatest difference is to be found in the eastern portion of the third isoseismal, which, according to the Italians, extends beyond the limits included in Fig. 20, and, according to the French, is bayed back by the great ma.s.ses of the Sierra Nevada.

Outside Andalusia the earthquake was sensibly felt to the north as far as Madrid and Segovia, to the west at Huelva, Carceres and Lisbon, and to the east at Valencia and Murcia. Towards the south, the greater part of the disturbed area was cut off by the Mediterranean, and there are no records forthcoming from the opposite coast of Africa. The total area disturbed by the earthquake is roughly estimated by the French Commission at about 154,000 square miles, and by the Italian Commission at about 174,000 square miles; but, as the shock was strong enough to stop clocks and ring bells at Madrid, it is evident that even the greater of these values is too small.

[Ill.u.s.tration: FIG. 20.--Isoseismal lines of Andalusian earthquake. (_Fouque, etc._)]

THE UNFELT EARTHQUAKE.

Far beyond the limits of the disturbed area, however, the long slow waves sped over the surface, disturbing magnetographs and other delicate instruments. More than a century before, the great Lisbon earthquake of 1755 had caused oscillations in Scottish lakes, and on other occasions the effects of remote earthquakes had been witnessed at isolated places. But, in 1884, the concurrent registration of the Andalusian earth-waves at distant observatories attracted general attention, and in part suggested the world-wide network of seismological stations, the foundation of which was laid before another decade had pa.s.sed.

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]

[Ill.u.s.tration: FIG. 21.--Magnetograph records of Andalusian earthquake at Lisbon. (_Fouque, etc._)]

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 Fouque 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 2-1/2 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]