CHANGES IN SHEAR-WAVE POLARIZATION IN THE CRUST OF KAMCHATKA BASED ON RECORDS OF LOCAL EARTHQUAKES

Abstract

The idea of using anisotropic parameters to study earthquake precursory processes relies on the results of theoretical research into the behavior of porous cracked media under stress. These results show that all media become anisotropic under stress, therefore because stress controls the precursory processes of earthquakes, the study of how the anisotropic parameters behave over time aids toward a better understanding of the precursory processes. Nevertheless, the structure of seismic zones is much more complex than that used in conventional theoretical models, so that experimental data generally help reveal merely the presence of seismic anisotropy and its time variations. We examined three-component records of 270 small local earthquakes that had occurred in southeastern Kamchatka (Gulf of Avacha). The period of observation was 21 months (March 1992 to November 1993). We studied and interpreted the direction of the horizontal component of the earliest shear wave first motion and a difference in the arrival times of two shear waves. We found that there was a dominant orientation of the earlier S-wave horizontal component before earthquakes of magnitudes greater than 4.5 and that this direction changed after the earthquake.