1940: Seismology

There were two major earthquakes, or rather series of great shocks during the year. The first was in Turkey on Dec. 27, 1939, with aftershocks for several months; the second was in Rumania on Nov. 9, 1940, apparently originating about 100 miles beneath the surface and widely destructive. The other most noteworthy earthquakes of the year occurred May 24, Callao, Peru; July 15, Tibet; and Oct. 4, Iquique, Chile.

In the United States there was only one important earthquake, in Imperial Valley, California, on May 18, with considerable property loss. Other shocks in the East as well as the West were of moderate intensity.

There are 115 seismograph stations of various kinds in the United States and the regions under its jurisdiction. The number and the principal purpose are about as follows: 37, recording of distant earthquakes; 18, nearby light and moderate earthquakes; 55, strong earth motions; and 5, volcanic earthquakes of all types (Hawaii).

The teleseismic (distant earthquake) stations are fairly well distributed; the others, except as mentioned, are chiefly in the Pacific Coast and western mountain regions. The strong-motion instruments are automatic, that is, the earthquake starts the recording which lasts only during the shock. Several valuable records were obtained for the Imperial Valley earthquake. All other instruments are continuously recording and, with few exceptions, photographic recording is used in all types in the United States.

The complete description of an earthquake requires both an instrumental epicenter and information from thousands of observers and eyewitnesses. In regions of frequent earthquake, forms listing the desired information are placed in the hands of individuals in advance; elsewhere they are sent out immediately after the earthquake. Organizations taking part in these varying activities include: the Coast and Geodetic Survey and the Weather Bureau, with collaborating state representatives in the western mountain states; Jesuit Seismological Association; Northeastern Seismological Association; University of California and associated institutions; California Institute of Technology; and the University of Alaska.

The purpose of these investigations is to find where and at what depth earthquakes occurred; the extent of area affected and nature of surface effects; better knowledge of the earth's interior and crust, including the relation of earthquakes to faults, or slips at the surface; and the character of the earth motion in the central region of an earthquake.

The last-named information, together with studies by engineers of earthquake destruction, aids in the design of buildings, bridges, dams and other structures to resist earthquake. The periods of buildings and ground vibrations are measured to aid in the solution of the problems. The application of this information which is collected by the Coast and Geodetic Survey and collaborators has been made chiefly at the Massachusetts Institute of Technology, California Institute of Technology, and Stanford University, which also operate shaking platforms of various types on which models of structures are subjected to actual reproductions of the earthquake motion. A number of organizations are working on improved building codes.

Increase of stress in the earth's crust, possibly associated with earthquake occurrence, may be detected by geodetic measurements, and such work has been done in the Lake Mead region of Nevada and Arizona and in southern California. Tilt measurements have continued at Berkeley, California, but without conclusive results in the absence of nearby earthquakes. Studies of microseisms made at St. Louis University have resulted in better knowledge of their form and method of propagation.