Preparation Processes of Mw > 2.5 Earthquakes at The Geysers Geothermal Field, California
Abstract
Preparatory processes accompanying or leading to nucleation of large earthquakes have been observed at both laboratory and field scale, but the precise conditions favoring them are still largely unknown. Here, we generated high resolution seismicity catalogs framing the occurrence of 20 Mw > 2.5 earthquakes at The Geysers geothermal field in California with the purpose of investigating the potential existence of emergent failure processes. The seismicity catalogs were developed using a matched-filter-algorithm. Our approach includes determination of P- and S-phase onsets and their inversion for absolute hypocenter locations with corresponding uncertainties. For each sequence a seismicity catalog of the 11 days framing the mainshock is created, containing events located within 2 km of its source region. Overall, we are able to detect and locate 3 times more events than the most detailed local catalog, decreasing the magnitude of completeness by about one order of magnitude. Our 20 selected sequences adequately sample the entire reservoir depth range, temporal periods with high/low injection rates and different tectonic settings within the field. We find a significant inverse correlation between the b-value from the different earthquake sequences and their mainshock depth. Sequences located above and below the reservoir display the highest and the lowest b-values, respectively. Within the reservoir, the b-value varies depending on the injection activity. Periods of high injection coincide with a low b-value, and vice versa. These observations most likely reflect 1) changes in differential stress with depth and 2) changing effective stresses depending on geothermal activity. We also perform a systematic search for seismicity localization using a multi-step cross-correlation analysis. We find no evidence for increased correlation between the occurring seismicity and the mainshock for any of the 20 sequences, indicating that each main nucleation spot was seismically silent prior to the main rupture. However, we do observe a number of highly inter-correlated earthquakes for sequences below the reservoir and during high injection activity. Under these conditions, the seismicity surrounding the future mainshock source region is more concentrated and might be evidence for a cascading nucleation process.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.S51B..02B
- Keywords:
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- 7215 Earthquake source observations;
- SEISMOLOGYDE: 7223 Earthquake interaction;
- forecasting;
- and prediction;
- SEISMOLOGYDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS