Asymmetric Topography Causes Normal Stress Perturbations at the Rupture Front: The Case of the Cajon Pass

We use 3D dynamic rupture simulations to discover a previously un‐described effect of asymmetric topography on the earthquake process. With the Cajon Pass along the San Andreas Fault as an example, we find that asymmetric topography generates an alternating normal stress perturbation around the rupture front, near the free surface. When topography lies to the right of the propagating right‐lateral front, the normal stress perturbation is clamping ahead of the rupture front and unclamping behind. When topography alternates to the left, the perturbation reverses sign. The process is analogous to the normal stress variations on dip‐slip faults. While this effect does not strongly affect rupture propagation and slip in our current parametrization, it requires explanation and exploration. An understanding of the normal stress perturbation due to asymmetric topography will help prevent its misattribution to other sources and lead to a better understanding of the interplay of multiple processes during earthquakes. Plain Language Summary The San Gabriel and San Bernardino mountains dominate the landscape of southern California, as they both reach high elevations (+3,000 m) and extend laterally for nearly 150 km. Together they form an elongated topographic feature that separates the densely populated Los Angeles metropolitan area from the Mojave Desert. In addition to dominating the landscape, the mountains are also placed asymmetrically with respect to the southern San Andreas Fault (SAF). More specifically, south of the Cajon Pass (CP) the topography lies to the north of the SAF, while north of the CP the topography switches sides and lies to the south of the SAF. This work investigates how the asymmetric configuration of the mountains interacts with earthquake rupture along the SAF. We find that the asymmetric topography produces a pattern of clamping and unclamping of the fault near the ground surface, and this pattern moves along with the propagation of the rupture. In addition, the pattern of clamping and unclamping reverses its sign when the rupture passes through the CP. Our results contribute toward a better understanding of earthquake rupture near the earth's surface in SoCAL and worldwide. Key Points Asymmetric topography on either side of vertical strike‐slip faults induces shallow dynamic normal stress changes near the rupture front The topographically induced normal stress perturbation exhibits a pattern of clamping and unclamping ahead and behind the