Geodetic and seismic constraints on recent activity at Long Valley Caldera, California: evidence for viscoelastic rheology

Long Valley Caldera is an active volcanic region in east central California. Surface deformation on the resurgent dome within the caldera was an order of magnitude higher for the five-month period September 1997 through January 1998 compared to the previous three-year average. However, the location of the immediate (shallow) source of deformation remained essentially constant, 5-7 km beneath the dome, near the top of a region of probable magma accumulation defined by seismic data. Similarly, although the rate of seismic moment release increased dramatically, earthquake locations remained similar to earlier periods. The rate of deformation increased exponentially between April-May 1997 and late November 1997 with a time constant of similar to 55-65 days, after which it decreased exponentially with about the same time constant. We develop a model consistent with these observations and also consistent with independent constraints on sub-surface rheology from thermal, geochemical and laboratory data. Deformation at sites on the resurgent dome most sensitive to the shallow deformation source are well fit by a model with a single pressure source at 6 km depth which experienced a pressure pulse that began in late 1996. peaked in November 1997. close to the time of major seismic moment release, and essentially ended in mid-1999. The pressure source in our model is surrounded by a 1 km thick "shell" of Maxwell viscoelastic material (shell viscosity 10(16) Pa s) within an elastic half space, and has peak values that are much lower than corresponding purely elastic half splice models. The shell viscosity is characteristic of a weak, deformable solid, e.g. quartz-bearing country rock surrounding the magma chamber at temperatures in the range 500-600 degreesC, i.e. above the brittle-ductile transition, and/or largely crystallized rhyolite: near its solidus temperature of similar to 670 degreesC. material that probably exists near the top of the zoned magma chamber at Long Valley. (C) 2001 Elsevier Science B.V. All rights reserved.