SCEC Award Number 21008 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Conditions that promote or inhibit deep and distant induced earthquakes in southern California hydrocarbon basins
Investigator(s)
Name Organization
Thomas Goebel University of Memphis
Other Participants Manoo Shirzaei, Virginia Tech.
Shirzaei will assist with training the graduate student from the University of Memphis in InSAR analysis.

1 Graduate student, University of Memphis
SCEC Priorities 3f, 3a, 1e SCEC Groups FARM, EFP, SDOT
Report Due Date 03/15/2022 Date Report Submitted 07/20/2022
Project Abstract
 Within the scope of the SCEC-funded project, we investigated 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 analyzed In 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 above 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.
Intellectual Merit 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.
Broader Impacts 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.
Exemplary Figure Figure 1: The San Ardo oilfield (green polygon) shows the most significant, relative surface uplift west of the San Andreas fault. A: InSAR line-of-sight velocity and local GPS measurements (black circles) with 3D displacement projected onto the line-of-site direction of the imaging satellite. The velocity map is dominated by the right-lateral shearing along the San Andreas Fault. Blue triangles show injection wells. B: The histogram shows the differences between InSAR and GPS with a standard deviation of 2 mm/yr. C: InSAR displacement time series at the center of the oilfield between 2016 and 2020. The yellow envelope indicates the 1-sigma error range.