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The Cajon Pass Earthquake Gate: the effect of topography on dynamic rupture models

Christodoulos Kyriakopoulos, Baoning Wu, & David D. Oglesby

Published August 15, 2019, SCEC Contribution #9836, 2019 SCEC Annual Meeting Poster #152

One of the most distinctive topographic features in Southern California is the asymmetric disposition of the topography around Cajon Pass. Southeast of the Pass, the San Bernardino range is on the North side of the fault, whereas a short distance to the Northwest the topographic relief (San Gabriel Mountains) is to the South of the fault. This particular feature of the topographic relief motivates us to investigate its effects on the dynamics of potential earthquakes across the pass. We run a variety of models by using different pre-stress conditions, different nucleation locations and different locking depths. Specifically, we use three different nucleation locations: north of the Cajon pass, south of the Cajon pass, and at the Cajon pass. In our experiments we use both models that include the topographic surface and models that have a flat free surface. In this way we can produce useful comparisons and isolate the effects of topography. Our initial observations from these experiments are focused on the final slip distribution and normal stress pattern. In general, our topographic models show increased final slip compared to the flat models. Most interestingly, we also observe a time dependent change in normal stress that appears to be dependent on the rupture propagation through the asymmetric topography of the pass. This effect reverses sign when the rupture passes from the San Gabriels to the San Bernardino mountains, making the Cajon pass the fulcrum of this effect. As a further step towards the physical sources of these effects, we generated a simplified synthetic model that mimics the asymmetric disposition of the topographic relief across the Cajon pass, but with no bends in the fault along strike (unlike the true San Andreas). Dynamic rupture simulations using this simplified model confirm our findings regrading the time dependence of the normal stress pattern and its inversion across the Cajon pass. These effects could be important in other areas of the world where large strike-slip faults cut next to asymmetric topography.

Key Words
Topography, Dynamic Rupture, Cajon Pass

Kyriakopoulos, C., Wu, B., & Oglesby, D. D. (2019, 08). The Cajon Pass Earthquake Gate: the effect of topography on dynamic rupture models. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)