Revisiting Earthquakes in the Los Angeles, California, Basin During the Early Instrumental Period: Evidence for an Association With Oil Production

A total of seven independent M(L)4.0 earthquakes occurred in the Los Angeles, California, basin, during the early instrumental period between 1932 and 1952, the largest of which was the 1933 Long Beach earthquake. Revising available macroseismic and instrumental data for a total of 6 4.0M(L)5.1 events between 1938 and 1944, we conclude that early instrumental locations can be grossly inconsistent with detailed macroseismic data. We use available macroseismic data to revisit event locations. We further present evidence that most if not all of these moderate earthquakes may have been induced by oil production. We quantify the predicted stress change associated with production from eight oil fields in the southwestern Los Angeles basin and show that frictional failure would have been encouraged beneath and at the periphery of high-volume fields, with stress changes upward of 0.1MPa at 5-km depth. The results suggest that if earthquakes are induced by stress changes associated with production, the magnitudes of events might tend to be limited by the limited spatial extent of lobes of increased Coulomb failure stress. It further appears that the advent of fluid injection recovery methods (water-flooding) around 1960 mitigated induced earthquake risk considerably. Plain Language Summary We reconsider the locations and magnitudes of six moderate earthquakes that occurred in the Los Angeles region between 1935 and 1950. We show that, while these earthquakes were recorded by early seismometers, detailed damage observations provide a better indication of locations than do limited instrumental data. We further present evidence for an association between these earthquakes and oil production during the oil boom that began in the late 1930s. Using available industry data and established computation methods, we show that by the 1930s, oil production would have perturbed significantly the stress and faults in proximity to major fields at depths of 3-5km. We suggest that the extent and therefore magnitude of induced earthquakes would have been generally limited by the spatially complex patterns of stress change. We further suggest that induced earthquake risk was mitigated significantly by the advent of so-called secondary recovery methods, whereby water was reinjected into fields to compensate for oil extraction.