Investigation of Anomalous Seismic Radiation of Long Valley Earthquakes

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

Abstract for proposal EAR0087147 (PH #8x) Investigation of Anomalous Seismic Radiation of Long Valley Earthquakes Douglas Dreger University of California, Berkeley, Seismological Laboratory Whether seismic events in the Long Valley Caldera (LVC) region of eastern California are due to tectonic or volcanic (fluid) controlled processes has been the focus of research since a swarm of four M6 events in May of 1980 demonstrated non-double-couple (NDC) seismic moment tensors. The NDC radiation of those events was attributed to a number of possible mechanisms including 1) tensile failure under high fluid pressure, 2) complex shear failure, and 3) bias due to unmodeled near-source velocity heterogeneity, however the controlling mechanism has remained unresolved. Recently broadband recordings of LVC seismicity have displayed anomalous radiation that may be characterized as a combination of deviatoric non-double-couple and volumetric dilation (NDCI) seismic sources (Dreger et al., 2000), which indicates that fluids are directly involved in the source process of some events. Whether the NDCI radiation is due to magmatic or aqueous injection, due to gaseous phases and advective over-pressure, or the proposed mechanisms for the 1980 events is the focus of this research. To address these questions and to investigate how widespread the anomalous events are we will conduct seismic waveform studies to determine seismic moment tensors and source process time histories of 118 ML>3.5 LVC events. Additionally, we will perform 3D numerical simulations to test whether near-source velocity structure can bias moment tensor inversion results. Finally, microseismicity associated with the studied events will be relocated to determine event specific fault structure, geometric fault complexity and possible temporal migration of seismicity as evidence of fluid flow during cascades of multiple NDCI events. Understanding the nature of seismic radiation in the LVC is an important step to unraveling the tectonics and magmatic/hydrothermal processes in the region, and to better characterize the related seismic and volcanic hazard.