Interaction between magmatic and faulting processes in the Long Valley caldera, California

Rapid inflation at Long Valley caldera, California beginning in the summer of 1997 was accompanied by a 10-fold increase in seismicity rates in the South Moat seismic zone (SMSZ), a presumed right-lateral shear zone about 5 miles south of Long Valley's resurgent dome. Focal mechanism solutions show right lateral events as well as non-double-couple events associated with the SMSZ. Geodetic data are suggestive of right-lateral slip in the SMSZ following inflation. These observations have led to speculation that this is a 'leaky' fault zone that is driven into right-lateral slip by tectonic loads and opening by the infusion of fluids. It remains unclear to what extent the earthquakes, and movement on the fault, were the result of the inflation of the magma chamber. To better understand the relationship between inflation and seismicity, it is necessary to precisely determine the location and geometry of the magma chamber and the location and sense of fault slip. Using data collected during the 1997-1998 inflation event, we conduct Monte Carlo inversions of geodetic data to determine possible slip on a fault in the SMSZ and source parameters of the magma chamber. Using persistent scatterer InSAR data, initial inversion results place the magma chamber at a depth of 6.4 km, which is consistent with previous studies. Preliminary inversions of InSAR and EDM data do not strongly require fault slip in the SMSZ in order to fit the data and, in fact, modeled movement on the fault does not correlate with the observed sense of slip during the inflation event. We also use a boundary element model to compute the stress changes on the fault induced by the inflating magma chamber. Earthquake mechanisms show a dilatational, right-lateral movement on the fault during the inflation event, while the model predicts increased clamping of the fault and promotion of left-lateral movement over much of the fault. Further joint inversions of InSAR, EDM, and GPS will yield more details about the shape and location of the magma chamber, possible fault slip in the SMSZ, and the need for a deeper magma chamber to resolve the opening of the SMSZ.