SCEC Award Number 24169
Proposal Category Collaborative Research Project (Multiple Investigators / Institutions)
Proposal Title What Controls the Spatial Variation in Seismicity Rates in California and Nevada?
Investigator(s)
Name Organization
Corné Kreemer University of Nevada, Reno Daniel Trugman University of Nevada, Reno Yehuda Ben-Zion University of Southern California
SCEC Milestones D1-1 SCEC Groups EFP, PBS, Geodesy
Report Due Date 03/15/2025 Date Report Submitted No report submitted
Project Abstract
 We investigated the relationship between geodetic strain rates and seismicity rates in the CA/NV plate boundary, with a specific focus on the San Andreas Fault (SAF) system. Our analysis was motivated by the previously observed lower-than-expected seismicity rate along the SAF. We also developed and used our own earthquake database, which is based on a uniform magnitude definition (based on the seismic potency) and which has been published (Trugman & Ben‐Zion 2024).

While we confirmed the earlier observation that the SAF system has fewer-than-expected events, and found that this cannot be explained by spatial variation in seismogenic thickness or a fundamentally different moment frequency distribution between the SAF and the rest of the plate boundary, we also found that the discrepancy disappears when we use the strain rate model by Xu et al. (2021) based on InSAR and GNSS. This latter model captured much higher strain rates across the SAF’s creeping segments, and should be the preferred model going forward.

We find that, while the average seismicity rate along the SAF is in line with what one would expect based on the average strain rate and the seismicity vs strain rate relationship elsewhere, there is significant discordance in this relationship when looking at various sub-segments in the SAF system. Particularly, the 1906 and 1857 ruptures have relatively few events while the southern SAF (approx. between 34-34.5°N) and the Elsinore and San Jacinto fault zones have relatively many. This corresponds to segments being in different stages of their seismic cycle.
Intellectual Merit This work has important implications for how strain rate based seismicity-based forecasts can be improved, and also helps illuminate which other factors besides strain rate control earthquake productivity. Our plate boundary approach aligns well with the new “statewide” scope of the next generation of SCEC science. We find that the translation from strain rate to seismicity rate might work for large enough areas but at the level of faults (segments), their unique seismic cycle may need to be considered.

We developed a new earthquake catalog with a uniform magnitude designation, based on the seismic potency. This is a novel approach.
Broader Impacts Unraveling the full correspondence between geodetic strain rates and seismicity has profound implications for seismicity and hazard forecasting in California and beyond. The project supported a new multidisciplinary collaboration between geodesy and seismology
Project Participants Corne Kreemer - University of Nevada, Reno
Daniel Trugman - University of Nevada, Reno
Yehuda Ben-Zion - University of Southern California
Nina Miller - University of Nevada, Reno. Paid for by this project but worked on another project
Exemplary Figure Figure 15
Caption:
Triangles show observed number of events vs. geodetic potency with their colors corresponding to the bins shown in Figure 12. The strain rate model used is the InSAR+GNSS model of Xu et al. (2021). Blue (line is predicted values when only considering the San Jacinto (light green), Elsinore (white), and southern SAF (light blue). For comparison in grey are observations and predictions for non-SAF areas.
Linked Publications

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