Recognition of Paleoearthquakes on the Puente Hills Blind Thrust Fault, California

Borehole data from young sediments folded above the Puente Hills blind thrust fault beneath Los Angeles reveal that the folding extends to the surface as a discrete zone (≤145 meters wide). Buried fold scarps within an upward-narrowing zone of deformation, which extends from the upward termination of the thrust ramp at 3 kilometers depth to the surface, document the occurrence of at least four large (moment-magnitude 7.2 to 7.5) earthquakes on this fault during the past 11,000 years. Future events of this type pose a seismic hazard to metropolitan Los Angeles. Moreover, the methods developed in this study can be used to refine seismic hazard assessments of blind thrusts in other metropolitan regions.