CHANGES IN WELL WATER LEVELS AND TRIGGERED EARTHQUAKES AFTER PASSAGE OF TELESEISMIC WAVES IN LONG VALLEY CALDERA, 2000-2017

Large and distant earthquakes have long been documented to cause changes in water levels in wells. They also, occasionally, appear to trigger seismicity, particularly during the passage of long wavelength (10-30 second) teleseismic waves. Long Valley Caldera, in eastern California, has some of the best-documented occurrences of both well level changes and dynamically triggered earthquakes. By comparing simultaneous changes in water levels and triggered events, we probe the coupling between teleseismic waves, pore pressure, and triggered seismicity at Long Valley Caldera.

We use new, high sampling rate (up to 0.2 Hz) water level and temperature measurements from Core Hole 10-b, southeast of the caldera’s resurgent dome. We compare these records with waveforms from several broadband stations within the caldera for hundreds of global and local earthquakes. We find that water levels start oscillating with the arrival of p-waves. When Rayleigh waves reach the caldera, the amplitude of this oscillation increases and the baseline of the water level drops by up to 30 cm. The water level drop persists for up to a few hours after the seismic waves have passed. The oscillation of water levels reflect dilatational strains while the drop in water level implies changes in the surrounding rock’s hydrologic properties due to teleseismic waves. We also find evidence for small, triggered earthquakes by filtering the waveforms from several stations in and around Long Valley. Our findings are consistent with a model for triggered seismicity in which pore fluid pressure and permeability change due to passing teleseismic waves.