Group , Poster #123, Fault and Rupture Mechanics (FARM)

More than 40 years of potentially induced seismicity close to the San Andreas fault in San Ardo, central California

Thomas H. Goebel, & Manoochehr Shirzaei
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Poster Presentation

2021 SCEC Annual Meeting, Poster #123, SCEC Contribution #11054 VIEW PDF
Evidence for fluid injection-induced seismicity is rare in California hydrocarbon basins, despite wide-spread injection close to seismically active faults. We investigate a potential case of injection-induced earthquakes associated with San Ardo oilfield operations which began in the early 50’s. The largest potentially induced events occurred in 1955 (ML5.2) and 1985 (Mw4.5) within ∼6 km from the oilfield. We analyze SAR interferometric images acquired by Sentinel-1A/B satellites between 2016 and 2020, and find surface deformation of up to 1.5 cm/yr, indicating pressure-imbalance in parts of the oilfield. Fluid-injection in San Ardo is concentrated within highly-permeable rocks directly abov...e the granitic basement at depth of ∼800 m. Seismicity predominantly occurs along basement-faults at 6 to 13 km depths. Seismicity and wastewater disposal wells are spatially-correlated to the north of the oilfield. Temporal correlations are observed over more than 40 years with correlation coefficients up to 0.71 for seismicity within 24 km distance from the oilfield. Such large distances have not previously been observed in California but are similar to the large spatial footprint of injection in Oklahoma. The San Ardo seismicity shows anomalous clustering with earthquakes consistently occurring at close spatial proximity but long inter-event times. Similar clustering has previously been reported in California geothermal fields and may be indicative of seismicity due to long-term, spatially-persistent external forcing. The complexity of seismic behavior at San Ardo suggests that multiple processes, such as elastic stress transfer and aseismic slip transients, contribute to the potentially induced earthquakes. The present observations show that fluid-injection operations occur close to seismically-active faults in California. Yet, seismicity is predominantly observed on smaller unmapped faults with little observational evidence that large faults are sensitive to induced stress changes.
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