SCEC Award Number 19223 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Is topography part of the “key” in the Cajon Pass earthquake gate?
Investigator(s)
Name Organization
Christodoulos Kyriakopoulos University of Memphis David Oglesby University of California, Riverside
Other Participants Baoning Wu, Graduate Student, UCR
SCEC Priorities 4a, 1e, 2e SCEC Groups FARM, SAFS, CS
Report Due Date 04/30/2020 Date Report Submitted 05/01/2020
Project Abstract
 The Cajon Pass (CP) is a key area for the generation of large earthquakes in southern California and more specifically is thought to be an “Earthquake Gate” (EG). The mechanisms for which the CP serves as an EG are net yet understood and is of great importance to identify first order effects that modulate and control the generation of region-wide ruptures. In this work we are investigating whether the asymmetric topographic relief surrounding the CP could have an effect to this “gate-like” behavior. To do that we are using full 3D dynamic rupture models and comparing results between models with topography and models with a flat free surface. In our preliminary results we observe a distinct pattern of normal stress change (not observed in the flat models) around the rupture front near the free surface. For example, in models with nucleation south of the CP (here the topography lies to the right of the fault), the rupture front is preceded by an increase in normal stress (clamping - blue color) and followed by a concentration of decreasing normal stress (unclamping – red color). When rupture passes the CP (now topography lies to the left of the fault) the normal stress pattern is inverted. The same pattern inversion is observed when rupture nucleates to the north of the CP and propagates southward. These preliminary results show that the CP marks a transition for rupture behavior and that this transition is (at least partially) related to the asymmetric disposition of the topographic relief.
Intellectual Merit Our work is relevant for at least three major areas of SCEC research. First, the collective effort to study and understand the Cajon Pass Earthquake Gate as a key area for the development of future large events in Southern California. Second, implementing and understanding the effects of realistic features in dynamic rupture models (4a, 4e). For example, in the specific case of southern California, the asymmetric disposition of the San Bernardino and San Gabriel Mountains around the Cajon Pass and the feedback between topography and the rupture front. Third, combining advanced numerical models and HPC to investigate and better understand earthquake rupture phenomena. This activity presented new numerical observations of topography induced rupture effects based on a "bi-topography" interface.
Broader Impacts The project developed original ideas around one of the possible mechanisms controlling the propagation of rupture across the Cajon Pass Earthquake Gate, southern California. In addition, our work could be very important for the interpretation of large historical ruptures on the southern San Andreas fault in the past thousands years.
Exemplary Figure Figure 3, page 4.

Figure 3. Normal stress variations in the SSAF during northward and southward rupture propagation. Panels (A) to (D) show experiments with nucleation south of the Cajon Pass – northward propagation. (A) and (B) show the normal stress perturbation at t=9 s while rupture is to the south of the Cajon Pass for the topo and flat models respectively. Here the pattern of normal stress at the rupture front is clamping (blue) – unclamping (red). (C) and (D) shows the normal stress perturbation at t=27 s while rupture is to the north of the Cajon Pass. Note the inversion in normal stress pattern at t=27 s, that is now unclamping (red) – clamping (blue). Panels (E) to (H) correspond to experiment with nucleation north of the Cajon Pass – southward propagation. (E) and (F) show the normal stress perturbation at t=14 s while rupture is to the north of the Cajon Pass for the topo and flat models respectively. Here the pattern of normal stress at the rupture front is clamping (blue) – unclamping (red). (G) and (H) shows the normal stress perturbation at t=28 s while rupture is to the south of the Cajon Pass. Note the inversion in normal stress pattern at t=28 s, that is now unclamping (red) – clamping (blue). Note: use the red and green (1) and (2) to locate the observation points in Figure 1. The black think arrows in panels (A), (B), (E) and (F) show the direction of rupture. The term N0 in the colorbar indicates the initial normal stress.