Ground‐rupturing earthquakes on the northern Big Bend of the San Andreas Fault, California, 800 A.D. to Present

• Published in: Journal of geophysical research. Solid earth 2017-03, Vol.122 (3), p.2193-2218
• Main Authors: Scharer, Katherine, Weldon, Ray, Biasi, Glenn, Streig, Ashley, Fumal, Thomas
• Format: Article
• Language: English
• Subjects: Frazier Mountain; paleoseismology; pull‐apart basin; rupture history
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2016JB013606

Paleoseismic data on the timing of ground‐rupturing earthquakes constrain the recurrence behavior of active faults and can provide insight on the rupture history of a fault if earthquakes dated at neighboring sites overlap in age and are considered correlative. This study presents the evidence and ages for 11 earthquakes that occurred along the Big Bend section of the southern San Andreas Fault at the Frazier Mountain paleoseismic site. The most recent earthquake to rupture the site was the Mw7.7–7.9 Fort Tejon earthquake of 1857. We use over 30 trench excavations to document the structural and sedimentological evolution of a small pull‐apart basin that has been repeatedly faulted and folded by ground‐rupturing earthquakes. A sedimentation rate of 0.4 cm/yr and abundant organic material for radiocarbon dating contribute to a record that is considered complete since 800 A.D. and includes 10 paleoearthquakes. Earthquakes have ruptured this location on average every ~100 years over the last 1200 years, but individual intervals range from ~22 to 186 years. The coefficient of variation of the length of time between earthquakes (0.7) indicates quasiperiodic behavior, similar to other sites along the southern San Andreas Fault. Comparison with the earthquake chronology at neighboring sites along the fault indicates that only one other 1857‐size earthquake could have occurred since 1350 A.D., and since 800 A.D., the Big Bend and Mojave sections have ruptured together at most 50% of the time in Mw ≥ 7.3 earthquakes. Key Points Eleven paleoearthquake records of ground‐rupturing earthquakes fill 200 km long gap in records along the southern San Andreas Fault Average interval length between paleoearthquakes in last 1200 years is ~100 years, and individual intervals range from ~22 to 186 years Paleoenvironmental data and radiocarbon dating show that Big Bend and Mojave sections rupture together ≥ 50% of the time in major earthquakes