Finite-Source Inversion of the 14 December 2016 M5 Geysers Geothermal Field Earthquake, Post-Earthquake Velocity Changes and Finite-Source Scaling

Date: 4/25/2019

Time: 06:00 PM

Room: Fifth Avenue

Seismicity from the EGS demonstration experiment at The Geysers, CA Prati-32 injection well has been investigated for the seismic moment tensor, in situ stress, and the scaling of micro-earthquake source parameters. One objective is to develop an approach for estimating a statistical representation of the fracture network generated from the injection of fluids into hot dry rock. We have performed finite-source inversion of seismic moment rate functions obtained for earthquakes spanning the magnitude range of Mw 1.0 to 4.5 at or near the EGS demonstration site to investigate the scaling of source dimension, slip, and stress drop. We find that micro-earthquakes are not simple ruptures and can have strong directivity effects, with multiple asperities of rupture. Rupture area is found to be consistent with the Wells and Coppersmith (1994) and Leonard (2010) scaling laws developed for much larger (Mw>5.5) earthquakes. Owing to the complex nature of the kinematic finite-source models we use the method of Ripperger and Mai (2004) to compute the static stress change. We find that peak stress drops can be large, however on average they are consistent with the aforementioned source scaling laws. On December 14, 2016 a Mw 5.0 earthquake occurred approximately 1.9 km SSW of the Prati-32 injection well. We develop finite-source models using both empirical and theoretical Green’s functions methods, combining seismic and geodetic data. We consider the results in the context of the average slip and rupture area scaling laws for the region and compare to the micro-earthquake (magnitude 1.0 to 4.5) results. In addition to the source characterization, we explore seismic velocity changes associated with local Geysers earthquakes. We find a systematic velocity reduction following the 2016 Mw 5.0 Geysers earthquake, which may be explained by a temporary increase of fracture density due to stress changes induced by the Mw 5.0 mainshock.

Presenting Author: Douglas S. Dreger

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