Observations and mechanisms of distant and deep injection induced earthquakes in California and Oklahoma hydrocarbon basins

The complexity of induced seismicity mechanisms significantly hampers seismic hazard assessment around injection wells. The largest magnitude events are commonly thought to be controlled by the size of the injection-affected area, but what controls the size of this area and what is the role of the regional geologic setting? Here we explore observations of deep and distant induced earthquakes in Oklahoma and California. Despite wide-spread injection close to seismically active faults, fluid injection-induced seismicity is comparably rare in California hydrocarbon basins. We identified a potential case of injection-induced earthquakes associated with San Ardo oilfield operations, with the largest events occurring in 1955 (ML5.2) and 1985 (Mw4.5) within 6 km from the oilfield. We performed an interferometric analysis of SAR 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. Temporal correlations are observed over more than 40 years, with correlation coefficients of up to 0.71 for seismicity within 24 km of 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, analogous to induced earthquakes in geothermal reservoirs. The complexity of seismic behavior at San Ardo indicates that multiple processes, such as elastic stress transfer and aseismic slip transients, contribute to the potentially induced earthquakes. The observed power-law distance decay of induced events from the reservoir is in line with observations of stress decay from poroelastic models in which basement faults may be hydraulically isolated from the injection zone. Our model's resolved fluid/solid stress interactions suggest that shallow injection can 1) activate deep basement faults and 2) lead to spatially extensive induced earthquake sequences. Both of these observations may significantly elevate the seismic hazard associated with fluid injection operations.