SCEC Award Number 20199 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Modeling the Rupture Dynamics of Strong Ground Acceleration (>1g) in Fault Stepovers
Name Organization
Julian Lozos California State University, Northridge Sinan Akciz California State University, Fullerton
Other Participants
SCEC Priorities 4a, 4c, 4d SCEC Groups FARM, GM, CS
Report Due Date 03/15/2021 Date Report Submitted 03/24/2021
Project Abstract
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 entirely 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. Together, the Ridgecrest and El Mayor-Cucapah examples suggest that some aspect of how earthquake rupture negotiates a strike-slip fault stepover can produce extremely localized strong ground acceleration. Here, we conduct 3D dynamic rupture simulations to investigate how the geometry 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 compressional and extensional stepovers, and for both subshear and supershear rupture velocities.
Intellectual Merit Seismic hazard is quantified in terms of ground acceleration. This study can deepen our understanding of what physical conditions generate high accelerations, what may cause them to localize around fault stepovers, and what pattern of strong shaking occurs within that local zone.

This study will also introduce displaced rocks like the ones associated with the Ridgecrest and El Mayor-Cucapah earthquakes as a form of quantifiable fragile geologic feature. Classic precariously balanced rocks have been used as constraints on model ground motions before, but smaller and subtler features like these displaced rocks have not previously been used in this way. We also hope that this study may inspire others to consider how to use more unorthodox fragile geologic feature data (or other unusual datasets) to quantify and constrain ground motion from historic or modeled earthquakes.
Broader Impacts From an educational standpoint, this project has introduced CSUN undergraduate student Holland Ladage to the concepts and process of dynamic rupture modeling, as well as to the SCEC community. She has run about half of the models herself, and has been very actively engaged in interpretation as well. She presented our preliminary work at the 2020 SCEC meeting, and she will be helping to write the manuscript once the modeling period is over.

From a societal standpoint, this work will help quantify how complex fault geometries affect strong ground motion. This in turn may help refine seismic hazard maps around stepovers in strike-slip faults. While this is very pertinent in southern California (the cities of San Jacinto and Hemet lie within a major stepover on the San Jacinto Fault, and the city of Lake Elsinore contains a stepover on the Elsinore Fault), these results will have applications to anywhere with discontinuous strike-slip faults.
Exemplary Figure This is still work in progress, and this is only an interim report. We are still running models and processing results, and we therefore do not have any representative figures yet.