Join us for our annual meeting with CSULB Geology Department at The Chatroom on Monday, March 11, 2019 from 6 – 9 PM. We are delighted to host Nate Onderdonk, Ph.D who will present his topic “Evidence for simultaneous rupture of the San Jacinto and San Andreas faults from spatial and temporal patterns of pre-historic earthquakes and fault slip-rates.”
The San Andreas and San Jacinto faults are the primary plate-boundary structures in southern California and present a large earthquake hazard for the region. They approach each other in the Cajon Pass area between the San Gabriel and San Bernardino Mountains, where the northern end of the San Jacinto fault forms a 2-km-wide releasing step with the San Andreas fault. We used slip-rate measurements, slip-per-event data, and paleoseismic data from sites on the San Jacinto and San Andreas faults near their juncture to evaluate spatial and temporal patterns of surface rupture on these faults. The data suggest that; (1) the San Jacinto accommodates an equal amount of plate-boundary motion as the southern San Andreas fault (2) the San Andreas and San Jacinto faults have probably ruptured together multiple times in the past 2000 yr; (3) a joint rupture of the San Jacinto fault with the Mojave section of the San Andreas fault may be a more likely source of “The Big One” in southern California than rupture on the southern San Andreas fault alone; and (4) the lack of major surface rupture on the two faults south of Cajon Pass in the past 200 years is not unusual and neither fault is necessarily “overdue”.
Nate is a Professor of Geology at CSULB where he teaches geomorphology, structure, and field classes. He got a BS in Physics from Principia College, a Masters and PhD from UC Santa Barbara, and spent 2 years as a Post-Doc at the University of Oslo before joining the CSU system. His research interests have varied quite a bit and he has published work in the fields of structural geology, paleomagnetism, arctic geology, hydrothermal seeps, and active tectonics. His current research is focused on active tectonics in the western Transverse Ranges, interactions between the San Jacinto and San Andreas faults, and extensional tectonics in the Mojave Desert.