Relationships between seismic velocity, metamorphism, seismic and aseismic fault slip in the Salton Sea Geothermal Field region

Jeffrey J. McGuire, Rowena B. Lohman, Rufus D. Catchings, Michael J. Rymer, & Mark R. Goldman

In Preparation 2014, SCEC Contribution #1957

The Salton Sea Geothermal Field is one of the most geothermally and seismically active areas in California and presents an opportunity to study the effect of high-temperature metamorphism on the properties of seismogenic faults. The area includes numerous active tectonic faults that have recently been imaged with active-source seismic reflection and refraction. We utilize the active-source surveys, along with the abundant microseismicity data from a dense borehole seismic network, to image the 3D variations in seismic velocity in the upper 5 km of the crust. There are strong velocity variations, up to ~30%, that correlate spatially with the distribution of shallow heat-flow patterns. The combination of hydrothermal circulation and high-temperature contact metamorphism have significantly altered the shallow sandstone sedimentary layers within the geothermal field to denser, more feldspathic, rock with higher P-wave velocity, as is seen in the numerous exploration wells within the field. This alteration appears to have a first-order effect on the frictional stability of shallow faults. In 2005, a large earthquake swarm and deformation event occurred. Analysis of InSAR data and earthquake relocations indicate that the shallow aseismic fault creep that occurred in 2005 was localized on the Kalin Fault system that lies just outside the region of high- temperature metamorphism. In contrast, the earthquake swarm, which includes all of the M>4 earthquakes to have occurred within the SSGF in the last 15 years, ruptured the Main Central Fault (MCF) system that is localized in the heart of the geothermal anomaly. The background microseismicity induced by the geothermal operations is also concentrated in the high-temperature regions in the vicinity of operational wells. However, while this microseismicity occurs over a few-km scale region, much of it is clustered in earthquake swarms that last from hours to a few days and are localized near the MCF system.

McGuire, J. J., Lohman, R. B., Catchings, R. D., Rymer, M. J., & Goldman, M. R. (2014). Relationships between seismic velocity, metamorphism, seismic and aseismic fault slip in the Salton Sea Geothermal Field region. Bulletin of the Seismological Society of America, (in preparation).