CAUSE OF EARTHQUAKES.

Shannon News, 2 October 1925, Page 3

CAUSE OF EARTHQUAKES.

INTERESTIN'G THEORIES EFFECT OF TIDAL PULL. What is the cause of earthquakes? This question has been discussed by a special correspondent in the London Observer. His theories are interesting to this country, which is within the earthquake belt. . The terrible earthquake in California occurred in a region of the earth, included in the well defined Pacific coast branch of the great earthquake belt that circles the globe north of the equator, from Japan, across Asia and the Mediterranean to’ the West Indies and Central America where it sends oft' branches north along the Pacific coast through California and south towards Chile.

In this zone practically all big earlhquakes occur, and mainly in the vicinity of coastal mountain ranges. Beneath the surface in these regions the earth is unstable, and numerous geological "faults - ’ exist, any exceptional movement of which may precipitate dislocations of the adjoining strata and bring about an earthquake. it has been suggested that such movements are precipitated qt times when the moon and sun are their most effective combined tidal pull on the earth; that is, when these bodies are nearest the earth and the moon new or full. Many disastrous earthquakes have occurred when the Sun and moon have been thus placed. But on the recent occasion such was not the case. The moon was just past its first quarter, and botli it and the sun almost at their maximum distance from the earth; so that the pull of each on the earth was almost at a minimum, and The two bodies counter-acting each other. And very similar conditions prevailed during the great earthquake in California which devastated San Francisco in April, 1906. The fact that earthquakes arc much more frequent in winter than in summer has induced the theory that m the earthquake zone an increase in atmospheric pressure may act as the

"last straw” and bring about a seismic disturbance. All over the world the barometer stands at a higher, average level in winter than in summer, and for every rise of one-tenth of an inch there is an added pressure on the earth of some 100,000 tons a square mile. Except, along the Pacific coast of South America earthquakes are rare in the southern hemisphere, but south of ithe equator the frequency of earthquakes in tile winter is also demonstrated. In this connection it would bo interesting to know the height of the barometer in California during the recent earthquake.

Modern investigations int 0 the cause of earthquakes tend to show that subterrestrial movements frqm 50 to 200 miles beneath the surface are always in progress, and that these movements are tlie original- cause ot the surface, disturbances. They arc often propagated for great distances before they affect the unstable outer crust in the earthquake zone, and bring an earthquake. Whether the tidal pull of the sun and moon -and the increased atmospheric pressure in winter are secondary causes is still m doubt; but if so, they can only be effective when the subterrannean strains have practically reached the breaking point. An improved form of ihe seismometer has been suggested which might register the approach of ihe critical period, and enable earthquake -warnings to be given several hours in advance by indicating the very small earth movements,, due to the distant and deep-seated disturbance, which always precedes an earthquake.