Abstract

Paleoseismologic data from the Sierra Madre fault, a major north-dipping reverse fault that extends for 75 km across the northern edge of the Los Angeles metropolitan region, indicate that the most recent surface rupture on the eastern part of the fault occurred more than 8000 years ago. Coupled with evidence for a minimum reverse-slip rate of 0.6–0.9 mm/yr on the strand that we trenched, the long elapsed interval since the most recent event suggests that the Sierra Madre fault breaks during very infrequent, large-magnitude (MW ≥ 7) earthquakes. Events of such large magnitude are much larger than the largest earthquakes that have occurred on any of the Los Angeles area urban faults during the ∼200-year-long historic period (e.g., the 1971 MW 6.7 San Fernando and 1994 MW 6.7 Northridge earthquakes) and must be considered in future seismic hazard analyses for southern California. Although more paleoseismologic data are needed to determine whether or not the Sierra Madre fault ruptures together with adjacent faults, available data already show that the Raymond fault, a west-southwest-trending left-lateral strike-slip fault that intersects the central Sierra Madre fault, has ruptured to the surface at least once, and possibly several times, since the most recent surface rupture on the eastern Sierra Madre fault. Moreover, if the San Andreas fault (SAF) and the Sierra Madre fault ever rupture together, then such events must be exceedingly rare, with at least 50–100 SAF MW ∼8 so-called Big Ones occurring between every possible combined SAF–Sierra Madre fault event.

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