Results

Within the chosen space and time constraints, 33 high quality events are identified that have solutions with three or more stations in their inversion. Of these 33 events, 28 are best characterized using a simple DC model, four by a DC+isotropic source model, and one by a full moment tensor model. The isotropic components of the five non-DC events were between 27-48% of the total moment released for each event. All non-DC events are located either in the south moat of the caldera or in the Sierra Nevada block.

The first event with a significant volumetric component occurred on August 11, 1993 in the Sierra Nevada block during the Red Slate Mountain earthquake swarm. Previously, the only non-DC events to occur in this area were a 1978 M5.8 event and two M6 1980 events (Julian and Sipkin, 1985). Event 93.08.11, however, did not occur along the same fault planes as these earlier events. Additionally, it is not known if the earlier non-DC components were due to fluid involvement or complex shear faulting.

The next four events with coseismic volume increases occurred in November 1997 in the south moat of the Long Valley caldera during a period of unrest at the peak of a large earthquake swarm. These south moat events had been previously identified as having significant volumetric components by Dreger et al. (2000) however, the current study investigates a wider range of possible source mechanisms. A previous study using a dense temporary seismic network operating during the summer of 1997 showed that most of 26 microearthquakes less than M3.1 were characterized by positive CLVD and isotropic components (Foulger et al., 2004). The difference in the total number of isotropic events between our two studies suggests that physical conditions which produce isotropic components are scale dependent, possibly in terms of the ability of individual high pressure reservoirs to sustain pressurization during the faulting process as the crack or fault grows larger.

We were not able to analyze the source process of earthquakes in or near the vicinity of the Mono-Inyo volcanic chain or Mammoth Mountain because events greater than M3.5 were not recorded during the time interval investigated by this study.

Pure DC events sometimes occurred close in space and time to events with significant non-DC components. For example, DC Event 97.11.22c occurred 10 minutes before non-DC Event 97.11.22d and was located just a few kilometers away from all four south moat non-DC events. In some cases, DC events determined by this study were located near previously identified fluid influenced microseismicity structures. For example, Events 97.12.31 and 98.01.05 occurred close in space to a microseismicity trend inferred to be a compensated tensile failure plane (Foulger et al., 2004). Thus, it appears that the factors necessary to produce isotropic components only coalesce and trigger non-DC events within a relatively small physical and temporal window in the Long Valley volcanic region.

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