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The importance of the surface or interface P wave in near-earthquake studies

DEPARTMENT OF EARTH AND PLANETARY SCIENCES MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS 02139

Abstract

A study of the surface P wave radiated by propagating two-dimensional dislocation sources is presented. Theoretical seismograms computed in the near-field of an earthquake fault located in a half-space show an unexpectedly strong surface P pulse. For the configurations studied, this wave is the prominent phase on the horizontal seismogram. It has little effect on the vertical motion, which may result in apparently uncorrelated horizontal and vertical seismograms. The SP pulse obtained on the horizontal component looks very similar to the shear pulse of the infinite medium solution. This fact, combined with the large reduction in amplitude of the S wave past the critical distance, may easily result in the misidentification of the surface P pulse as the direct shear wave. The presence of a surface layer with compressional velocity larger than the shear velocity of the source medium is not seen to affect significantly the strength of the surface P wave. In the case of a low-velocity sedimentary layer, the surface P wave is replaced by an interface P wave which can radiate efficiently to the surface provided that the layer thickness is small.

Footnotes

^* Present address: Laboratoire de Géophysique Interne, Université Scientifique et Médicale de Grenoble, Grenoble, France.

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