Revised date of the penultimate earthquake that ruptured the Carrizo Section of the South-central San Andreas Fault, California.

Sinan O. Akciz, Lisa Grant Ludwig, Ramon Arrowsmith, Tomas N. Capaldi, & Ed J. Rhodes

In Preparation September 18, 2022, SCEC Contribution #12645

A primary step toward assessing the time and size of future earthquakes is identifying earthquake recurrence patterns in the seismic record. Hazard analyses rely on geologic and geomorphic data when sufficiently long historical or instrumental seismic data sets are unavailable. The San Andreas Fault (SAF) is one of the most studied active faults in the world. However, the unequivocal interpretation of paleoseismic data to determine the timing and rupture extent does not extend beyond the historical 1857 and 1906 ruptures. The penultimate earthquake, for example, is chronologically the least well-constrained earthquake all along the SAF, including at sites with abundant organic materials. The main reason for this poor determination is because the past few hundred years have seen large fluctuations in atmospheric 14C concentration. In part, these have been the result of natural factors, including the climatic changes of the Little Ice Age and the Sporer and Maunder solar activity minima. The large fluctuations in atmospheric 14C that occurred before 1955, when atmospheric nuclear weapon testing began, mean that a single radiocarbon date may yield an imprecise calibrated age consisting of several possible age ranges. At sites along frequently rupturing faults with historical ruptures, such as the SAF and the North Anatolian Fault, determining the incision age of channels displaced only by the most recent earthquake can place a tighter maximum limit on the possible age range of the penultimate earthquake. In our study, we dated five sandy fill units with the post-IR-IRSL method on single feldspar grains of channel Sieh31 in the Carrizo Plain. The data indicate the incision age of Sieh31, offset ~6m during the 1857 earthquake, predates ~1750+-30 A.D. (1sigma). This new result trims the age constraint of the penultimate earthquake that ruptured the Carrizo section of the SAF determined at the nearby Bidart Fan site from 1640-1857 A.D. to 1640-1750 A.D., trimming the age constraint by nearly 100 years. This new approach can not only put a tighter age constraint on the penultimate surface rupturing earthquake along a fault, but it can also be used to improve the ages of older earthquakes and assess slip per earthquake history along a fault section is known.

Akciz, S. O., Grant Ludwig, L., Arrowsmith, R., Capaldi, T. N., & Rhodes, E. J. (2022). Revised date of the penultimate earthquake that ruptured the Carrizo Section of the South-central San Andreas Fault, California.. Bulletin of Seismological Society of America, (in preparation).