THE CAUSES OF EARTHQUAKES

New Zealand Tablet, 28 January 1915, Page 47

THE CAUSES OF EARTHQUAKES

RESULTS OF RECENT INVESTIGATIONS. The disastrous earthquake in Italy, which occasioned such a' fearful loss of life, has again drawn attention to the causes that govern such dreadful visitations. In the olden days, among semi-civilised and barbarous people, very crude, not to say grotesque, views were held as to the origin of such upheavals. In Japan (writes the Rev. F. A. Tondorf, S.J., of Georgetown Observatory, in America) it was supposed that a restless, monstrous spider made the earth’s bowels its haunt. In Mongolia the earth shaker was conceived as a subterranean hog; in India, it is a mole. The Mussulman pictured it as an elephant, while in North America the tortoise rocked the crust. In spite of these wild attempts to explain earth-shakings, we note classic writers touching the scientific reasons. So Pythagoras and Seneca, ascribe quakes to the presence of masses of fire at the earth’s centre, Aristotle to pent-up gases. Lucretius fancies them the consequence of the underminings of rock-strata due to water agents, while De-

mocritus recognises them as the consequence of upheavals and subsidences of the earth’s crust. Cardano, an Italian mathematician and philosopher, popular about the middle of the sixteenth century, figures that all seismic energy is the resultant expression of the chemical action between the saltpetrous, bituminous, and sulphurous accumulations in the earth.

Poey and Kluge first connected earthquakes with sun spots, Perrey and Falb with lunar attraction, Buch and Humboldt stoutly advocated the volcanic theory even in case of those quakes whose focal point is faxremoved from the seat of volcanic activity ; Volga and Mohr have suggested that some of the small earthquakes felt in Germany may be referred to the falling-in of the roof of enormous subterranean cavities, formed by the long-continued solvent action of water on deposits of rock-salt, limestone, and gypsum. Nauman, writing in 1850, classified earthquakes as volcanic and tectonic : the former being due to volcanic explosions, the latter to movements in the rock-masses. The importance of this latter class has grown with time, and it is now most generally accepted that all the really heavy quakes are to be included under this type and that those of volcanic origin are of slight importance. In this connection we quote from an article entitled ‘ The Relation of Seismic Disturbances in the Philippines to the Geologic Structure,’ by Father Miguel Saderra Maso, Director of the Philippine Seismological Observatories ;

‘ Beyond a doubt, many seismic disturbances are due to causes other than volcanism. Many of the worst disasters we have experienced have nothing to do with volcanoes, and that volcanoes are near by is only a coincidence, or may be explained by the fact that the place where great disturbances in the earth’s crust occur is naturally a zone of weakness and where molten material would be expected to seek an outlet. At the time of the Messina earthquake Mount Etna, which can be seen from Messina, .was comparatively quiet. The great disturbance at Messina, as is generally known, was due to an adjustment along the lino of a great fault or fracture in the earth’s crust which is marked by the Straits of Messina.’

The great California earthquake of April, 1906, is another striking instance of a disturbance due to dislocation. The shifting was along the San Andreas fault, the slip measuring a distance of 270 miles, in which the two sides of the fracture had been displaced relatively to each other by an amount varying from a maximum bf 21-feet to an uncertain minimum, but which must I have disappeared entirely at the ends of . the fault. As a striking example of an earthquake due primarily to volcanism we'might cite the recent cataclysm of Taal, Philippine Islands. Yet it should be added that the volcano is located along a-line of crustal weakness, and at the time of the quake a very appreciable displacement occurred along one or more lines passing through the Taal volcano. * One of these lines,’ writes Father Mazo,

ran om the volcano to the coast through the town of Lein ex y, and the other from the Taal volcano to the barrio of Sinisian, making with the sea a triangular strip of several; square kilometres in area. ./ This whole block dropped a metre or more, so. that the sea washed inland for a distance ox a kilometre over the main highway along this 'coast/ - * - The practical certainty of the dislocation theory calls fox a more thorough study of geological formations, so that the more unstable areas may be indicated on the map. his accomplished, a more exact interpretation oi the warnings given by lesser shocks would be useful m places situated along a serious fault line. Had this been done in the past, Messina and San Francisco would have been better prepared to meet the shocks that were so disastrous for them.

Repeated attempts have been made to establish some connection between magnetic and seismic disturbances with a view to predicting the latter. Yamasaki reports that for three days prior to the great Japanese quake of August 31, 1896, in North Honshu, the magnetographs at Sendai, Tokio, and Nagoya were violently agitated. Nakamura, chronicles like magnetic phenomena preceding the great Japanese sea quake of June 15 of the same year. Professor Milne, the noted English seismologist, writes :— 1 These magnetic disturbances may of course be regarded as mere coincidences, but when we consider volcanic and seismic activities as evidences of physical and chemical changes, together with mechanical displacements of a magnetic magma (molten mass), it is reasonable to suppose that they should have at least a local influence upon magnetic needles/ Quite a few seismic observatories, favorably located, have, within recent years, added magnetographs to their equipment with a view to settling this question. In case a correlation is established it is just possible that a rule for earthquake forecasting will be at hand.