About the Presenter
Katie Freeman is a Senior Scientific Engineering Associate with EESA. Her primary focus is microearthquake monitoring and analysis for enhanced geothermal systems, but she also contributes to earthquake hazards projects and software development. She earned her M.S. in Geophysics at Stanford University, and her B.A./B.S. at the University of California, Los Angeles. Her M.S. work focused on varied spatial scale stress orientations in The Geysers geothermal field, and she has subsequently worked to improve the products of LBL’s Rapid Earthquake Monitoring and Assessment Software in order to learn more about induced seismicity in the geothermal setting.
In this talk, I will discuss key features of important M4+ earthquake sequences, both local and in the immediate region, recorded by the LBNL-Geysers monitoring array. The observations related to these events were made possible by a dense local monitoring network designed to record earthquakes with magnitudes less than 3.5.
The 2014 Napa earthquake
In the 6 hours after the magnitude 6.0 Napa earthquake of August 2014, a dramatic spike in triggers and located events was observed in The Geysers geothermal field. 194 microearthquakes were located throughout the field during this time period. Both the number and magnitude of Geysers events decayed rapidly with time after this initial increase. No coordinated migration of the events across the field was observed in those six hours. The events were well distributed around the field, most of them occurring at depths less than 4km b.s.l. Average daily triggers and events decreased slightly in the 90 days that followed. These observations are compared to smaller regionals, none of which were followed by changes to the numbers of triggers or events in The Geysers.
The 2016 magnitude 5.0 Geysers earthquake
On December 14, 2016, a magnitude 5.0 earthquake occurred in The Geysers. This event is the largest on record for The Geysers. It comes after a one-year quiescence in magnitude 4 events in the field following an M4 that occurred just after the 2014 Napa earthquake. The M5 event clipped at 5 stations, but some of the nearest stations to the event were not among them. Time series for the event showed emergent first arrivals as well as two similar, overlapping packets offset by approximately 3 seconds. The aftershock sequence is generally oriented NNW, with the majority of events to the north of the epicenter.
Magnitude 4 events in The Geysers
Twenty one events with estimated magnitude 4 or greater have occurred in The Geysers since monitoring began in 2003. Eight of these events were followed by a significant spike in fieldwide triggers and located events, and five were followed by a small to moderate spike. Surprisingly, another seven of the M4+ events showed no subsequent increased triggers and events, and one M4 event was preceded by a dip.