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Introducing the Cajon Pass Earthquake Gate Area

Julian C. Lozos, Craig Nicholson, & Nate W. Onderdonk

Published August 15, 2017, SCEC Contribution #7865, 2017 SCEC Annual Meeting Talk on Tue 10:40 (PDF)

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Cajon Pass is the primary junction between the San Andreas (SAF) and San Jacinto (SJF) faults. Its location in western San Bernardino County – in the middle of the densely populated Inland Empire – means that even a local rupture within the Pass could have a considerable impact on cities and lifelines. Fault interactions within the Pass may also control whether rupture is able to continue toward Los Angeles and the western Transverse Ranges. Thus, understanding fault interactions and possible rupture behaviors in Cajon Pass affects our understanding of hazard throughout the southern San Andreas system.

A number of eastern Transverse Ranges faults of different types may also interact with the SAF and SJF within Cajon Pass. Even from a purely geometrical standpoint, the multi-stranded nature of this junction raises the question of whether it functions as an earthquake gate. However, there are also a number of geological and geophysical observations and models that support the idea of Cajon Pass as a behavioral boundary within the southern San Andreas system. Geology and geodesy alike show a decrease in slip rate on the SAF south of Cajon Pass, while the slip rate increases along the subparallel SJF. Seismicity highlights a rotation of the regional stress field to the immediate south of the SAF-SJF junction. Paleoseismology suggests that many SAF ruptures stop at Cajon Pass, and that the event record on the SAF north of the junction better correlates with the record on the SJF than it does with the record on the SAF to the south. Rupture modeling corroborates the idea that the SAF and SJF can rupture together in a single event. The distribution of precariously balanced rocks near the SAF and SJF may also imply that directivity plays a role in how rupture negotiates the junction.

Here, I will go into more depth on the previous research that motivates the concept of Cajon Pass as an Earthquake Gate. I will also introduce the key research questions raised by the Cajon Pass EGA leaders.

Key Words
Cajon Pass, San Andreas Fault, San Jacinto Fault, earthquake gate, EGA, seismology, geodesy, paleoseismology, rupture modeling

Lozos, J. C., Nicholson, C., & Onderdonk, N. W. (2017, 08). Introducing the Cajon Pass Earthquake Gate Area. Oral Presentation at 2017 SCEC Annual Meeting.

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