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The Landers (southern California) destructive (M = 7.3) earthquake of June 28, 1992 had epicentral coordinates of (34.2 • N, 116.44 • W) and was a major seismic event of the period from 1985 through 2002. It was examined in many papers, mostly published in a special issue of Bulletin of Seismological Society of America (BSSA) of June 1994 (vol. 84, no. 3). The majority of the works were devoted to phenomena related to the earthquake itself and its effects. The present paper presents the results derived from the analysis of rigidity variations at the preparation stage of the main shock. This analysis was based on a method developed by the authors for the monitoring of rigidity variations in the seismically active layer of the crust. The method reduces to the possibility of discriminating between the brittle and viscous failure modes using P wave first arrival patterns.

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