History of earthquakes along the creeping section of the San Andreas fault, California, USA

Creeping faults are difficult to assess for seismic hazard because they may participate in rupture even though they likely cannot nucleate large earthquakes. The creeping central section of the San Andreas fault in California (USA) has not participated in a historical large earthquake; however, earthquake ruptures nucleating in the locked northern and southern sections may propagate through the creeping section. We used biomarker thermal maturity and K/Ar dating on samples from the San Andreas Fault Observatory at Depth to look for evidence of earthquakes. Biomarkers show evidence of many earthquakes with displacements >1.5 m in and near a 3.5-m-wide patch of the fault. We show that K/Ar ages decrease with thermal maturity, and partial resetting occurs during coseismic heating. Therefore, measured ages provide a maximum constraint on earthquake age, and the youngest earthquakes here are younger than 3 Ma. Our results demonstrate that creeping faults may host large earthquakes over longer time scales.

Automated summary

Assessing seismic hazard on creeping faults is challenging as they can participate in rupture without nucleating large earthquakes. By studying the San Andreas fault, it was found that the creeping section showed evidence of significant displacements during earthquakes. The measured ages provide a maximum constraint on earthquake age, with the youngest earthquakes occurring within the last 3 million years.