A model for deformation in Long Valley, California, 1980–1983

Geodetic data collected in Long Valley, California, from 1975 through 1983 define a pattern of uplift and strain which is evidently associated with a sequence of earthquakes occurring in May 1980 and subsequent swarm activity continuing until the present. We have constructed a model to explain the deformation observed since May 1980 in terms of inflation of two subsurface magma chambers, faulting in the south moat region of the caldera, and slip on the Hilton Creek fault. The most significant new feature of the model is the shallow magma chamber at 5 km depth, located a few hundred meters to the east of the Casa Diablo hot spring area. Inflation of this chamber causes stresses which show consistency with various aspects of the seismicity in the south moat of the caldera. Calculations of stress across vertical planes over the magma chamber can be used together with failure criteria to estimate the inflation volume at which the rock layers intervening between the chamber and the surface will fail by extensional fracture.