Group A, Poster #117, Fault and Rupture Mechanics (FARM)

Modeling the rupture dynamics of strong ground acceleration in strike-slip fault stepovers

Julian C. Lozos, Sinan O. Akciz, & Holland Ladage
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Poster Presentation

2023 SCEC Annual Meeting, Poster #117, SCEC Contribution #13132 VIEW PDF
Following the July 2019 Ridgecrest earthquakes, multiple field investigators noted that pebble- to boulder-sized rocks had been displaced from their place in the desert pavement along the right-lateral strike-slip M7.1 rupture trace. This implies localized ground motions in excess of 1 g, in contrast to instrumentally recorded ground motions which peak at ~0.7 g. However, these features are not pervasive along the entire rupture; they are concentrated within extensional stepover region near the southern end of the M7.1 rupture. Similar observations of displaced rocks concentrated in stepovers exist for the predominantly right-lateral strike-slip 2010 M7.2 El Mayor-Cucapah earthquake. Togethe...r, the Ridgecrest and El Mayor-Cucapah examples suggest that some aspect of how earthquake rupture negotiates a strike-slip fault stepover produces extremely localized strong ground acceleration.

Here, we use the 3D finite element method to investigate how the geometry and connectivity of stepovers in strike-slip faults influences strong ground acceleration. In particular, we focus on how the amount of overlap between the two fault strands, and the width of the stepover, influences the location and intensity of the strongest ground motion, for both subshear and supershear rupture velocities. For subshear ruptures, we find that the presence of a stepover in general matters more than its dimensions; the strongest ground accelerations occur at the end of the first fault, but whether or not rupture jumps to the second fault strand controls just how strong the shaking is. For supershear ruptures, the stepover is effectively irrelevant, since the strongest particle accelerations occur at the point of the supershear transition on the first fault. Our specific choice of initial conditions does not produce accelerations above 1 g in any of our simulations, the location of our strongest ground motions in the subshear cases is consistent with the locations of displaced rocks and localized strong shaking in both the Ridgecrest and El Mayor-Cucapah earthquakes.