The effect of topography on dynamic earthquake rupture models across the Cajon Pass, California

In the vicinity of the Cajon pass in California, the San Andreas fault cuts through complex surface topography. More specifically, along the San Bernardino range, there is much higher relief on the North side of the fault, whereas a short distance away in the San Gabriel mountains the higher relief is to the South of the fault. Here we investigate the effects of the asymmetric disposition of the topography on the dynamics of potential earthquakes across the pass. We run models with different pre-stress conditions, different nucleation locations and different locking depths. More specifically, we use three different nucleation locations, north of the Cajon pass, south of the Cajon pass and at the Cajon pass. All the simulations with the topographic model are repeated with a model that implements a flat free surface, allowing us a direct comparison of the numerical results and an ability to 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 of Cajon Pass 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 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.