A Geology and Geodesy Based Model of Dynamic Earthquake Rupture on the Rodgers Creek‐Hayward‐Calaveras Fault System, California. Issue 3 (16th March 2021) - Harris, R. A.
Abstract: The Hayward fault in California's San Francisco Bay area produces large earthquakes, with the last occurring in 1868. We examine how physics‐based dynamic rupture modeling can be used to numerically simulate large earthquakes on not only the Hayward fault, but also its connected companions to the north and south, the Rodgers Creek and Calaveras faults. Equipped with a wealth of images of this fault system, including those of its 3D geology and 3D geometry, in addition to inferences about its interseismic creep‐rate pattern and rock‐friction behavior, we use a finite‐element computer code to perform 3D dynamic earthquake rupture simulations. We find that the rock properties affect the locations and amount of slip produced in our simulated large earthquakes. Crucial factors that control rupture behavior in our modeling are the earthquake nucleation locations, the fault geometry, and the data that reveal where the fault system is creeping or locked. Our findings suggest that large Rodgers Creek‐Hayward‐Calaveras‐Northern Calaveras (RC‐H‐C‐NC) fault‐system earthquakes may result from dynamic rupture that starts in a locked part of the fault system, but is then stopped by the creeping parts, leading to high‐magnitude‐6 earthquakes; or, from dynamic rupture that starts in a locked part of the fault system, then cascades through some of the creeping parts, leading to magnitude‐7 earthquakes. Plain Language Summary: The San Francisco Bay area has experienced large earthquakes in the past and is likely to experience large earthquakes in the future. Although the San Andreas fault is the best‐known earthquake generator, other San Francisco Bay area faults, including some that slowly move (creep) all of the time are also known to be hazardous. We use computer modeling to simulate large earthquakes on the Hayward fault, and its connected companions to the north and south, the Rodgers Creek and Calaveras faults. In our computer simulations, we include information about the rock properties, rock‐friction, and fault geometry, in addition to inferences about where the faults creep. We find that the earthquake nucleation location, the fault geometry, and the pattern of fault creep, are most important. Our findings suggest that large earthquakes on these faults may result from a fast‐slipping rupture that starts in a locked part of the faults, but is then stopped by the slowly slipping parts of the faults, leading to high‐magnitude‐6 earthquakes, or, from fast‐slipping rupture that starts in a locked part of the faults, then cascades through some of the creeping‐fault parts, leading to magnitude‐7 earthquakes. Key Points: We simulated large earthquakes on the Rodgers Creek‐Hayward‐Calaveras fault system, California Our spontaneous rupture (dynamic rupture) simulations included the 3D fault geometry, 3D rock properties, and creeping‐fault patches The resulting earthquakes are most affected by which fault they start on, the fault geometry, and the pattern of interseismic fault creep