Group B, Poster #108, Fault and Rupture Mechanics (FARM)

Normal Fault Displacement History Provides Clues on Ruptures Along the Southern San Andreas Fault

Luis I. Bazán Flores, Christodoulos Kyriakopoulos, David D. Oglesby, Aron J. Meltzner, Thomas K. Rockwell, John M. Fletcher, & Daniel S. Brothers
Poster Image: 

Poster Presentation

2023 SCEC Annual Meeting, Poster #108, SCEC Contribution #12988 VIEW PDF
The generation of large earthquakes along the Southern San Andreas Fault (SSAF) is of major interest for the scientific community. Of particular interest is the initiation phase and the triggering of the SSAF by adjacent smaller faults that might participate and affect the evolution of larger multi-fault ruptures. The BSZ lies in the middle of the transtensional step-over between the SSAF and the Imperial Fault (IF), concentrating structural elements (including smaller faults) that could host these interactions. The BSZ's continuous micro-seismicity suggests that the SSAF projects southeast of its apparent terminus at Bombay Beach, under the Salton Sea, and is intersected by smaller fau...lts. Although most of the seismicity in the BSZ occurs along vertical NE-SW oriented strike-slip cross faults, there is clear evidence of slip along a network of SW dipping normal faults under the Salton Sea (Brothers et al., 2009; Brothers et al., 2011; Brothers et al., 2022). Therefore, normal faults should be considered candidates to initiate future large earthquakes on the SSAF or that might participate in SSAF earthquakes nucleating far from Bombay Beach. Our experiments are based on a model where the SSAF is intersected by a normal fault near Bombay Beach. We consider two configurations, one in which the SSAF terminates at Bombay Beach and another where it continues below the Salton Sea towards the city of Brawley. Our results indicate that the capability of the SSAF to trigger normal-fault slip depends mainly on the earthquake's nucleation location, the locking depth of both faults, and the magnitude and direction of normal fault pre-stress (e.g., oblique vs. normal). More specifically, our models reproduce vertical displacements on the normal fault comparable to those observed by Brothers et al., 2022, when using a variable locking depth model with the shallowest depth close to Bombay Beach, and when using a lower pre-stress value. The locking depth also appears to control the ability of normal fault earthquakes to propagate on the SSAF. Furthermore, SSAF ruptures approaching from the north promote slip on the normal fault, while slip is inhibited on the normal fault for ruptures initiated in the south. Lastly, in all our models, we observe that the normal fault undergoes a rake rotation, dynamically imposed by the SSAF, tending to become slightly more right-lateral than the slip that would occur on the normal fault in isolation.