1941: Seismology

The only major earthquake during the year ending Nov. 30, 1941, was under the Atlantic Ocean north of the Maderia Islands on Nov. 25, but since it was far from land and unaccompanied by a seismic seawave no damage was caused. There were 43 widely recorded earthquakes many of which were submarine and 10 of which caused destruction or loss of life on a considerable scale.

In the United States there were no highly important shocks but those causing some damage include the following: New Hampshire, Dec. 20 and 24, 1940; and California, Santa Barbara, June 30, Los Angeles region Oct. 21, 22 and Nov. 14.

The United States and the regions under its jurisdiction are fairly well supplied with seismographs except that they are not uniformly distributed. There are about 112 in all, of which the principal purposes are: 34, distant earthquakes; 23, nearby earthquakes; 55, strong earthquakes, in the region of destruction. All except the strong-motion instruments record continuously, and with few exceptions photographic registration is used.

The strong-motion instruments are automatic; that is, the earthquake starts the recording which lasts only during the shock. The record of the Santa Barbara earthquake of June 30 supplied especially useful information, and the analysis of the records of the May 18, 1940, Imperial Valley earthquake gave the first accurate knowledge of the motions near a fault on which surface slipping occurred.

A map of the area affected by the earthquake is based on information from those who felt and observed the shock. Collection of thousands of reports from such witnesses is well organized. In regions of frequent earthquakes, cooperators are kept supplied with forms listing the desired information to be filled in and mailed after each earthquake; elsewhere the forms are sent out immediately after the earthquake occurs.

Organizations taking part in these various activities include: National Government through the Coast and Geodetic Survey, Weather Bureau and other services; the Jesuit Seismological Association; the Northeastern Seismological Association; colleges and universities, especially the University of California, California Institute of Technology and University of Alaska; and Science Service, which collects information making immediate earthquake location possible. In a number of western states a special representative coordinates the collection of information for a particular state.

The purposes of these investigations are to find where and at what depths earthquakes occurred; the extent of area affected; the nature of service affected; better knowledge of the earth's crust and interior, including the relation of earthquakes to slipping along fault planes; the determination of the seismicity of different parts of the earth; the determination of actual earth motions, especially in the central region of a strong earthquake.

The measurement of strong earth motion and studies of earthquake damage aid in the safe design of buildings, bridges, dams, water towers and other structures. The periods of buildings have continued to be measured to aid in the solution of these problems. Application has been made by a number of universities and engineering organizations of the information obtained, and at Stanford University and the California Institute of Technology shaking platforms have been made to simulate earth motions in the models of structures. Attention is being given to the importance of building codes.

Increase of stress in the earth's crust, possibly associated with earthquake occurrence, may be detectable by geodetic measurements and such measurements have been made during the year in the Lake Mead region of Nevada and Arizona. Measurements to determine tilt of the crust have continued at Berkeley, Calif., but without cooperative results in the absence of nearby earthquakes.

New and improved seismographs for the recording of the vertical component of the earth's motion have been developed during the year. In the past the cost and limited variety of such instruments have deprived many stations of this important adjunct.

Seismological investigation of which so much has been done in Europe has been handicapped to an unknown degree by the war, but the full effect will not be known for some years. The International Seismological Association has functioned partially in England and unoccupied France under great difficulties. Immediate exchange of earthquake information between the United States and Canada, Mexico, Guatemala and El Salvador has facilitated the location of earthquakes. See also GEOLOGY.