A Study of Recent Earthquakes

by Charles Davison

The only observations of any real value in determining the velocity are those given by the stopped clock at the observatory of San Fernando (Cadiz) and by the magnetographs at Lisbon, Parc Saint-Maur, Greenwich, and Wilhelmshaven. Taking the times at Cadiz, Lisbon, Greenwich, and Wilhelmshaven at 9.18, 9.19, 9.25, and 9.29 P.M. respectively (Paris mean time) and the mean epicentral point as coinciding with Alhama, the French Commission estimates roughly the mean surface-velocity between Cadiz and Lisbon at 3.6 kms. per second, between Cadiz and Greenwich at 4.5 kms. per second, between Cadiz and Wilhelmshaven at 3.1 kms. per second, and between Greenwich and Wilhelmshaven at 1.6 kms. per second. Dr. Agamennone, however, notices that the distances from Alhama are not correctly measured, and substitutes for the above figures 4.83, 3.43, 2.82, and 1.75 kms. per second respectively.

These results apparently show a decrease in the velocity with the outward spread of the earth-waves, but, as Dr. Agamennone again points out, a comparatively small error in the time at Cadiz would neutralise the apparent decrease. It is not to be supposed that the astronomical clock at this observatory was wrong by more than a second or two, but the behaviour of clocks during an earthquake is so irregular—some stopping at once, others staggering on for some seconds before arrest—that the Cadiz time may differ from the true time by several seconds.

Besides this possible error, there is also considerable uncertainty in the records from the magnetic observatories, owing to the slow rate at which the photographic paper travels. At Parc Saint-Maur this rate is only 10 mm. per hour, and at the other observatories about 15 mm. per hour. Allowing, therefore, for an error of half-a-minute in the time-record at Cadiz, of one minute in those of Lisbon, Greenwich, and Wilhelmshaven, and of two minutes in that at Parc Saint-Maur, and taking the mean epicentral point as determined by the Italian observers, Dr. Agamennone, applying the method of least squares, finds the probable value of the velocity of propagation to be 3.15 kms. (or nearly 2 miles) per second, with a possible error of .19 kms. per second. This result agrees closely with the value found for the long slow undulations of more recent earthquakes.

MISCELLANEOUS PHENOMENA.

Connection between Geological Structure and the Intensity of the Shock.—While a great part of the injury to buildings must be attributed to their faulty construction, the connection between the nature of the underlying rock and the amount of damage was very clearly marked. Other conditions being the same, houses built on alluvial ground suffered most of all; and the destruction was also great in those standing on soft sedimentary rocks such as clays and friable limestones. On the other hand, when compact limestones or ancient schists formed the foundation-rock, the amount of damage was conspicuously less than in other cases.

The members of both the French and the Italian Commissions agree in ascribing the peculiar form and relative positions of the isoseismal lines to geological conditions. To the east of the epicentre, the schists and crystalline limestones form a deep, uniform, and compact mass; while, to the west, the old crystalline rocks are covered by jurassic, cretaceous, and eocene formations, constituting a less homogeneous and less elastic mass, in which the intensity of the shock would fade off much more rapidly, with the result that the epicentre occupies the western focus of the elliptical boundary of the meizoseismal area (Fig. 19).[35]

That mountain-ranges have an important influence on the form of isoseismal lines is evident from both maps (Figs. 19 and 20), but especially from that published by the French Commission (Fig. 20). The resistance offered by the Sierra Nevada to the propagation of the earth-waves is shown in the former map by the approximation of the first and second isoseismals at the east end, and in the latter by the great bay in the third isoseismal line. Whichever interpretation of the evidence is the more accurate, the action of the mountainous mass is clearly to lessen rapidly the intensity of the shock—an effect which is probably due to the abrupt changes in the direction and nature of the strata encountered normally by the earth-waves. On the opposite side of the epicentre, the waves meet the Sierra de Ronda obliquely. In traversing this range, the shock lost a great part of its strength, while it continued to be felt severely along its eastern foot, thus giving rise to the south-westerly extension of the third isoseismal in Fig. 20, and, though to a less extent, that of the second in Fig. 19.

Fissures, Landslips, etc.—The earthquake resulted in many superficial changes, such as fissures, landslips, and derangement of the underground water-system—all changes of the same order as the destruction of buildings—but, so far as known, in no fault-scarps or other external evidence of deep-seated movements.

Some of the fissures were of great length. One of the most remarkable occurred at Guevejar, a village built on the south-west slope of the Sierra de Cogollos. It was in the form of a horse-shoe, and was about two miles long, from ten to fifty feet wide, and of great depth. In its neighbourhood, innumerable small cracks appeared, some perpendicular and others parallel to the great fissure. The ground within, a bed of clay resting on limestone, also slid down towards the river. Houses near the centre of the fissured tract were shifted as much as thirty yards within the first month, and others near its extremity about ten feet; while the accumulation of the material at the south end of the fissure resulted in the formation of a small lake, of about 250 to 350 square yards in area and about 30 feet deep. All streams within the fissured zone disappeared, and the spring, which provided the drinking-water of the village, ceased to flow.