Channel Incision Ages to the Rescue: An Improved Age for the Penultimate Earthquake That Ruptured the Carrizo Section of the South- Central San Andreas Fault

A primary step toward assessing the time and size of future earthquakes is identifying earth-quake recurrence patterns in the seismic record. The San Andreas fault (SAF) is one of the most studied active faults in the world. However, there is no unequivocal interpretation of paleoseismic data to determine the timing and rupture extent of the earthquakes that occurred prior to the historical 1857 and 1906 ruptures. The penultimate earthquake is the least well-dated earthquake along the Cholame, Carrizo, and Big Bend sections of the SAF. The main reason for this poor determination is because the past few hundred years have seen large natural fluctuations in atmospheric 14C concentration. These fluctuations mean that a single radiocarbon date may yield a 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 in Turkey, determining the incision age of channels displaced only by the most recent earthquake can place a tighter minimum limit on the possible age range of the penultimate earthquake. In our study, we dated five sandy fill units with the post -infra-red infrared-stimulated luminescence method on single feldspar grains of channel Sieh 31 in the Carrizo Plain. The data indicate the channel Sieh 31, offset -6 m during the 1857 earth-quake, incised before-1740 +/- 30 C.E. (1 sigma). 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 to 1631-1745 C.E., tightening the age constraint by nearly 80 yr. The revised mean recurrence interval for surface rupturing earthquakes along the Carrizo section of the south-central SAF is 117 yr (95% confidence interval 62-255 yr). This approach can improve paleoearthquake age and slip-per-earthquake constraints.