Shear-wave vibrational directions and related fault movements in Southern California Earthquakes

Abstract

Vibrational directions of direct shear waves from a number of small local earthquakes in southern California, recorded at Pasadena and Riverside, are determined and related to corresponding faulting at the source. A theoretical relationship between wave vibrational directions and fault displacements is proposed. Directions of SV and SH motions from various fault types are adduced from this relationship. Observations of initial shear wave motions indicate generally consistent SH displacements, usually less consistent SV displacements, and ratios of SV/SH which usually vary widely. Polarization of SH waves is indicated; that of SV wave, suggested. The entire shear wave is probably approximately plane-polarized. The results of this study indicate that horizontal components of faulting in southern California usually follow the same general direction, whereas vertical fault components appear to vary in direction. Comparison of observed SV and SH motions with (1) theoretical shear motions and (2) Gutenberg's (1941) observations of compressional wave impulses, provides analysis of faulting at the source. Seismic data and regional surface geology indicate a fault pattern involving, primarily, northwesterly-trending right-handed transcurrent faults in some parts of southern California, approximately east-west-trending reverse or thrust faults in other parts, and the coexistence of the two in a few localities. Simplified stress distributions in agreement with the data are discussed briefly.

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