We utilize fundamental rock mechanics principle to understand earthquake and faulting mechanics, as well as coseismic stress transfer in the crust. We use high-precision borehole strainmeters installed in southern California, USA, to monitor earthquake-driven crustal deformation associated with faulting. Rock deformation from two major earthquakes (M7.2 El-Mayor Cucapah (EMC) and M5.4 Collins Valley (CV)) were detected by the borehole strainmeters. We first calculate the principal strain tensors that may represent true crustal deformation around the region, and then transform them into stress tensor changes based on linear elasticity. The orientation of coseismic stress changes associated with EMC is consistent with the results from an independent stress indicator (earthquake focal mechanism solution). In contrast, the orientation of coseismic stress changes associated with CV is not clearly constrained, which may indicate anisotropic stress transfer. To understand the mechanism of coseismic stress transfer, we compare the Coulomb stress transfer model results with the measured results. The modelled stresses associated with EMC are fairly consistent with the stress indicated by the strainmeter data. However, the model results for CV are in completely opposite sense (or polarity) with the measured results. This implies a possibility that CV might not be triggered by a prevailing NW-striking fault in the region, but by its conjugate NE-striking fault.

1. Introduction

Since the change of the crustal stress associated with earthquakes is an important factor in assessing earthquake hazard, numerous studies on this have been actively conducted (King et al., 1994; Stein 1999). However, because of the unpredictability of earthquakes, it is difficult to directly measure the stress change associated with an earthquake. Currently, the crustal stress change due to earthquakes can be indirectly estimated through rock deformation modelling on fault motion.

Since 1970s, borehole strainmeters have been deployed to measure crustal deformation at high-resolution, and have provided chances to study crustal strain associated with earthquakes (Gladwin, 1984; Gladwin et al., 1987). Based on constitutive relationships between strain and stress, we can estimate coseismic stress change and even mechanism of earthquakes from the measured deformation data. In this study, we investigate the coseismic stress change derived from strainmeter data when two major earthquakes occurred in the Anza area, southern California, USA in 2010. Our study demonstrates that the high-precision strainmeters can be utilized to understand earthquake-related stress change and thus faulting mechanics.

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