TRANSIENT DEFORMATION DURING TRIGGERED SEISMICITY FROM THE 28 JUNE 1992 M(W)=7.3 LANDERS EARTHQUAKE AT LONG-VALLEY VOLCANIC CALDERA, CALIFORNIA

Continuous records from a borehole strainmeter and a long baseline til tmeter in the Long Valley caldera provide critical insights into the o rigin of at least one episode of minor seismicity in volcanic regions triggered by the 28 June 1992, M(L) 7.3 Landers, California, earthquak e. A strain transient reaching a peak of 0.25 microstrain occurred in the few days following the Landers event and decayed over the next 20 days. A tilt perturbation during the same time reached a peak amplitud e of 0.2 microradians. These signals correspond approximately in time to the primary seismic moment release across a 50 km(2) region of the south part of the caldera at depths between 2 and 10 km. Corresponding strain transients in 5-km geodetic lines across the south caldera are not apparent above the 95% confidence limits of about 0.4 microstrain in daily sampled data during this same period. These data rule out mo dels involving single localized inflation sources within the upper cru st beneath the caldera, including that responsible for the current rap id inflation of the resurgent dome. They also preclude models involvin g aseismic slip on single strike-slip or normal faults in the caldera. A single source in the form of a relaxing magma body at a depth of 50 km beneath the caldera can account for the deformation data, but whet her the small stress changes are sufficient to drive the triggered sei smicity is not clear. An alternate possibility involves distributed de formational sources triggered by the passage of the 10 microstrain pea k amplitude surface waves from the earthquake. This distributed deform ational source could result either from rupturing of overpressured flu id or gas chambers commonly encountered in volcanic regions or from ad vective gas overpressure during release of gas bubbles in hydrothermal or magmatic fluids.