Stanford Geothermal Workshop
February 12-14, 2018

Irregular Focal Mechanisms Observed at Salton Sea Geothermal Field: Possible Influences of Anthropogenic Stress Perturbations

Aren CRANDALL-BEAR, Andrew BARBOUR, Martin SCHOENBALL

[USGS, USA]

At the Salton Sea Geothermal Field (SSGF), strain accumulation is released through seismic slip and aseismic deformation. Earthquake activity at the SSGF often occurs in swarm-like clusters, some with clear migration patterns. We have identified a slow earthquake sequence composed entirely of focal mechanisms representing an ambiguous style of faulting in-between normal and thrust faulting. In order to more accurately determine the style of faulting for these events, we revisit the original waveforms and refine estimates of P and S wave arrival times and displacement amplitudes. We calculate the acceptable focal plane solutions using S/P amplitude ratios, and determine the preferred fault plane. Constraints from local variations in stress, found by inverting the full earthquake catalog, make it difficult to explain the occurrence of such events using standard fault-mechanics and friction. Comparing these variations with the effects of local production and injection of geothermal fluids suggests that anthropogenic activity could affect the style of faulting through poroelastic deformation.

Topic: Geophysics
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