Paleoearthquakes within 100 km and 1000 years of modern Cajon Pass, California

Katherine M. Scharer

Submitted August 15, 2018, SCEC Contribution #8704, 2018 SCEC Annual Meeting Poster #263

Cajon Pass, located at the juncture of the San Andreas Fault (SAF) and the San Jacinto Fault (SJF), is an area of significant risk due to its dense network of energy, communication, and transportation infrastructure that connects the greater Los Angeles region with the rest of the nation. To examine the hazards posed by earthquakes in Cajon Pass, I compiled available paleoearthquake data from the region within a 100 km radius of the Pass. I examine hazards from fault displacement using on a rupture model based on age correlation of paleoearthquakes on the SAF and SJF. Within the restricted region examined, at least 13 ground-rupturing earthquakes have occurred on these faults last 1000 years. Rupture lengths in the model vary from the northern ~10 km of the Mw6.9 SJF earthquake in 1918 to maximum rupture lengths that could span the 200 km diameter such as a ca. 1330 AD event on the SAF and a ca. 980 AD earthquake that could have ruptured both the SAF and the SJF. Apart from the SAF and SAF, other faults pose shaking hazards to the Pass. The most active source within the 100 km radius is the Elsinore Fault (Glen Ivy section), with 6 earthquakes in the last 1000 years but calculated average displacement less than 50 cm given the slip rate on the fault. More typical for the region is the Newport Inglewood fault, or the Cucamonga and Sierra Madre thrust faults, which have repeat times on the order of 500-1000 years and displacements of multiple meters. Closer to Cajon Pass, the most notable data gap exists along the boundaries of the San Bernardino basin, where a ~40-km-long swath along both the SAF and SJF have sparse earthquake age and slip per event data, and no paleoearthquake data is available for the NE-trending extensional Crafton Hills Fault complex. Overall, the dataset for understanding earthquake hazards in Cajon Pass is quite rich; however, further work around the San Bernardino basin is needed to better characterize the earthquake hazard near the junction of the SAF and SJF.

Scharer, K. M. (2018, 08). Paleoearthquakes within 100 km and 1000 years of modern Cajon Pass, California. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)