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Motion of the surface of the earth in the compressional phase of a deep-focus earthquake, and the effects of a layered crust

DEPARTMENT OF GEOLOGY UNIVERSITY OF WISCONSIN

Abstract

In this paper the difficulties which attend any attempt to obtain the ground displacement which occurs during the passage of an earthquake wave have been illustrated. Reliable curves of ground displacement are difficult to obtain because the majority of seismographs, as operated, are poorly adapted for any purpose but the indication of the time of arrival of earthquake phases. This study has demonstrated, however, the practicability of an attempt to obtain ground displacement from seismograms, and has illustrated some of the uses which may be made of curves of ground displacement, when obtained.

The use of the dynamic magnification to obtain the amplitude of ground displacement corresponding to an impulse on a seismogram was shown to give highly erroneous results. For long-period seismographs, calculation of displacement by methods in which the effect of transients was retained appeared to give fairly accurate results.

The results of the study of the effects of a layered crust on the surface motion are to be considered only as preliminary, and indicative of the usefulness of a technique which hitherto has seldom been applied to seismic problems. Displacements were indicated in the horizontal components of ground displacement that were absent in the vertical, suggesting effects of crustal discontinuities. If the use of composite curves of surface displacement is valid, and the agreement between the observed and predicted curves of horizontal component of displacement is significant, the following are possible conclusions: oscillation in seismograms is a result of reflections and transformations of primary waves within the crust; the average thickness of the crust beneath the stations whose records were studied is 32 km.; the velocity change at the base of the crust takes place in a distance which is small compared to the wave length.

An appropriate research for individual stations possessing good seismographs in three components would be a study of the thickness of the crust underlying each station. For this purpose the wave velocities as determined from local earthquakes or explosions, and the arrival time of for a few deep-focus earthquakes of simple initial form, would suffice.

A knowledge of the curves of ground displacement would facilitate the solution of the following problems, among others: the effects of crustal discontinuities on the ground motion in the S phase; and the azimuthal variation of the form of the P wave, in relation to the process at the focus.

The writer wishes to thank Professor L. B. Slichter, who suggested the problem, and the directors of the many seismological stations whose coöperation in providing seismograms made the investigation possible.