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Measuring Gaps Between Geodetic, Geologic, and Seismic Moment Rates Across the Western U.S.: How to Determine a Budget for Earthquake Rates?

Description: 

Updated geodetic, geologic, and seismic datasets compiled for the 2023 U.S. National Seismic Hazard Model (NSHM23) provide new views of moment rates across the western U.S. We use these updated datasets to assess regional estimates of off-fault deformation by asking: is geodetic moment completely resolved by earthquakes? There is a long-held assumption that better characterization of fault networks will yield better agreement between geodetic and geologic moment rates. We find that this is not the case because newly added faults tend to be short and have low slip rates. Using updated datasets, this discrepancy appears persistent and not significantly reducible by adding faults, incorporating geologic slip rate uncertainty, or increasing fault plane area. Furthermore, the gap cannot be closed with seismicity moment rates alone. Rather, such a difference is emblematic of volumetric crustal strain, and is therefore only partially associated with diffuse deformation around faults, typically termed “off-fault deformation.” When we consider smaller areas for moment rate comparisons using a grid approach rather than broad tectonic regions, the gap between geodetic and geologic moment rates decreases as the influence of volumetric processes (i.e., distant off-fault deformation) is minimized. Some volumetric processes that may contribute to this quantity may include volcanic activity, hydrologic changes, rotations at the boundaries of fault systems, folding, post-seismic transients, interseismic locking on the Cascadia megathrust, shallow and deep creep on crustal faults, lower crustal flow, mid-crustal detachments, and some spatially variable combination of the above. These processes are not the same as diffuse deformation associated with a particular fault zone (e.g., hanging wall deformation and distributed cracking). Before geodetic off-fault deformation is incorporated into seismic hazard analyses or physics-based rupture simulations, we must understand if and how these broad-scale processes contribute to the earthquake engine.

Session: Learning Across Geological, Geophysical & Model-Derived Observations to Constrain Earthquake Behavior - III

Type: Oral

Date: 5/1/2024

Presentation Time: 03:00 PM (local time)

Presenting Author: Alexandra

Student Presenter: No

Invited Presentation: 

Authors