Non-double-couple earthquakes in the Long Valley volcanic region

To better understand the connection between earthquake production and geothermal/magmatic systems, we studied the extent of fluid-influenced faulting in the Long Valley volcanic region. We focused on a 100-km-wide circular area centered at the Long Valley caldera that also encompassed the Mono-Inyo craters to the north and the Sierra Nevada mountain block to the south. We performed a comprehensive search for events greater than M 3.5 since 1993 with significant coseismic volume changes in their source region. Using three-component broadband digital waveforms at regional distances, we solved for four different source models: double couple (DC), deviatoric DC + isotropic, and full moment tensor. Using the F-test as a statistical aid, we determined which of the four models was most appropriate for each event. We then conducted stability tests to determine the robustness of the focal mechanism solutions and isotropic components. Our results show that fluid-influenced earthquakes in the magnitude range studied are quite rare in the Long Valley volcanic region. Of 33 high-quality events, 28 are best characterized by a simple DC source model, 4 by a DC + isotropic source model, and 1 by a full moment tensor model.