CAUSE OF EARTHQUAKES.

Auckland Star, Volume LVI, Issue 199, 24 August 1925, Page 5

CAUSE OF EARTHQUAKES.

INNER MOVEMENTS OF GLOBE. I I ZONKS OF DAXGKK. Tho recent earthquake in California! occurred in a region of the earth included j in the well-defined Pacific (Hast branch - of the great eart.hquake belt that circles the globe nortli of the equator, from ; •Japan, across A-ia and the Mediterranean to the West Indies and Central America, where it sends oIT brandies north along the Pacific coast through . California and south towards Chile. In this zone practically all big earthquakes occur, and mainly in the vicinity ' of coastal mountain ranges. Beneath the surface in these region- the earth is unstable. and numerous geological j "faults" any exceptional movement of which may precipitate dislocations of the adjoining .itrata and bring about an earthquake. It has been suggested that such movements are precipitated at times when the moon and sun are exerting their most effective combined tidal pull on the earth: that i-. 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 in this earthquake such was not the case. The moon was just past it* first quarter, and both it ancl 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 counteracting each other. And very similar conditions prevailed during the great earthquake in California which devastated San Francisco in April. 1900. The fact th.it earthquakes are much more frequent in winter than in summer has induced the theory that in tiie earthquake zone an increase in atmospheric pressure may act as the "last straw," and bring about a 9eismic disturbance. All over the world the barometer stands at a higher average level in winter than in summer, and for every rise of onetenth of an inch there is an added pressure on the earth of sonic 100,000 tons per square mile. Except the Pacific coast of South America, earthquakes arc rare in the southern hemisphere, but f>olll fa of the equator the frequency of earthquake- in the winter i« also demonstrated. In this connection it would be interesting to know the height of the barometer in California at the time of this last earthquake. Modern investigation into the cause of earthquakes tend* to show that subterrestrial movements from 50 to 200 mile- beneath the .surface arc always in progress, and tlmt these movements are the original cause of the surface disturbances. They are often propagated for great distances before they affect the unstable outer crust in the earthquake zone, and bring about an earthquake. Whether th« tidal pull of tin , sun and the moon and the increase.l atmospheric pressure in winter are secondary causes is still in doubt; but. if so, they can only be effective when flic subterranean strains have practically reached the breaking point. An improved form of the seismometer has been suggested which mis'nt register the approach of jhc 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 precede an earthquake.