Constraints on continued episodic inflation at Long Valley Caldera, based on seismic and geodetic observations

Abstract

Long Valley Caldera, a large and potentially explosive silicic system, has experienced highly anomalous continued inflation since late 1970s. We characterize an episode of rapid episodic uplift occurring between 2002 and 2003 following similar episodes of 1979–1980, 1983, 1989–1990, and 1997–1998. This most recent episode was the first to be observed by a dense array of 13 continuous Global Positioning System (GPS) stations. Similar to previously observed episodes of deformation, uplift is quasi‐radially symmetric and is mostly explained by a compact pressure source located ∼3 km west of the resurgent dome. The maximum uplift during the 2002–2003 episode is ∼35 ± 8 mm, about 1/3 the magnitude but with a similar time‐dependent behavior as the 1997–1998 episode. The horizontal source location is well constrained at −118.930°, 37.678°, for a small spherical source, and indistinguishable from the location of a vertically dipping prolate spheroidal source. A trade‐off between depth and volume change is observed for both spherical and prolate models, with depth between 7.5 and 13.5 km and a volume change of 0.01–0.03 km3 at 95% confidence. For prolate spheroidal models, depth and volume change are additionally affected by the source axis ratio (b/a), which is greater than 0.55. Though the background seismicity remained low during the 2002–2003 episode, we identified a significant spike in activity during the maximum rate of uplift, similar to observations in both the much larger episodes in 1989–1990, and 1997–1998. More interestingly, we additionally find that all three episodes begin immediately after a short period of seismic quiescence, with background seismicity falling to levels below background levels following the prior uplift event. With the dense GPS coverage, we also identify increased opening of the Mono‐Inyo volcanic chain after the 2002–2003 episode suggesting potential interaction of magmatic fluids between the two systems.