Constraints on time-dependent volcanic source models at Long Valley Caldera from 1996 to 2009 using InSAR and geodetic measurements

Abstract

SUMMARY Continuous monitoring of Long Valley Caldera since the late 1970s, including data from seismic and geodetic networks has shown renewed episodic unrest with accelerated uplift separated by reduced uplift, no activity, or slow deflation. We examine the time-dependent behaviour at Long Valley Caldera from 1996 to 2009 by integrating InSAR and continuous GPS (CGPS) measurements. The ERS-1/2 radar data between 1996 and 2008 and reprocessed three-component CGPS data from the Long Valley GPS network from 1996 to 2009 were combined to invert for source geometry and volume change for the following deformation episodes: 1997–1998 uplift, 2002–2003 uplift, 2004–2007 slow subsidence, and 2007–2009 slow uplift. We employed non-linear Monte-Carlo random search approaches (random cost and simulated annealing) in our inversion and examined models including spherical and finite sources (dipping prolate spheroid). Our results show that the sources of all post-2000 events (i.e. 2002–2003, 2007–2009 uplift and 2004–2007 subsidence) locate at shallow depths of ∼6–8 km and have nearly identical surface locations, suggesting that these events may be caused by the same source in the mid-crust, possibly a mixture of hydrothermal and partial-melt magma. All three events, 2002–2003, 2004–2007, 2007–2009, are characterized by the low total volume change: ∼0.01, ∼–0.003, ∼0.006 km3, respectively, with corresponding volume change rates of ∼0.007, ∼–0.001, ∼0.002 km3 yr–1. The 1997–1998 inflationary event has a steeper source geometry and much greater volume change rate (∼0.03 km3 yr–1) than the other events, in agreement with previous studies. This suggests it is possibly driven by magma intrusion beneath the resurgent dome from a deeper source. If we regard post-2000 events as proxies for future eruption hazard, the inferred source dynamics (e.g. mid-crustal location and subdued volume change) and accompanied decrease in earthquake activity from these post-2000 events suggest that the probability for near-term eruption is low. Our study demonstrates that CGPS, along with InSAR, provide a valuable tool for monitoring time-dependent source processes at active volcanic regions.