Evidence for large Holocene earthquakes on the Compton thrust fault, Los Angeles, California

Abstract

We demonstrate that the Compton blind thrust fault is active and has generated at least six large‐magnitude earthquakes (Mw 7.0–7.4) during the past 14,000 years. This large, concealed fault underlies the Los Angeles metropolitan area and thus poses one of the largest deterministic seismic risk in the United States. We employ a methodology that uses a combination of high‐resolution seismic reflection profiles and borehole excavations to link blind faulting at seismogenic depths directly to near‐surface fault‐related folding. Deformed Holocene strata record recent activity on the Compton thrust and are marked by discrete sequences that thicken repeatedly across a series of buried fold scarps. We interpret the intervals of growth as occurring after the formation of now‐buried paleofold scarps that formed during uplift events on the underlying Compton thrust ramp. Minimum uplift in each of the scarp‐forming events, which occurred at 0.7–1.75 ka (event 1), 0.7–3.4 ka or 1.9–3.4 (event 2), 5.6–7.2 ka (event 3), 5.4–8.4 ka (event 4), 10.3–12.5 ka (event 5), and 10.3–13.7 ka (event 6), ranged from ∼0.6 to ∼1.9 m, indicating minimum thrust displacements of ≥1.3 to 4.2 m. Such large displacements are consistent with the occurrence of large‐magnitude earthquakes (Mw ≥ 7). This multidisciplinary methodology provides a means of defining the recent seismic behavior, and therefore the hazard, for blind thrust faults that underlie other major metropolitan regions around the world.