What Does the Discrepancy Between Seismicity Rate and Strain Rate Tell Us About the Seismogenic Properties of the San Andreas Fault System?

Corné Kreemer, Daniel T. Trugman, & Yehuda Ben-Zion

Published September 8, 2024, SCEC Contribution #13677, 2024 SCEC Annual Meeting Poster #060

Spatial variations in the rate of background seismicity are assumed to be controlled primarily by stress concentrations produced by deformation rates, approximated by geodetic strain rates. However, we recently identified an important unexplained conundrum: the highest straining parts of the Pacific-North America plate boundary in California-Nevada, particularly the San Andreas Fault System (SAFS), have significantly fewer events than expected based on the linear correlation between seismicity rates and strain rate in the slower deforming parts of the plate boundary. We also observe this along the Indo-Mediterranean plate boundary. Understanding this discrepancy is important for advancing the understanding of earthquakes and improving seismicity forecasts based on strain rates.

The source of the discrepancy may involve different factors including: faster straining areas might have relatively more aseismic deformation, significantly lower seismogenic thickness and shear modulus), or seismicity that follows a fundamentally different frequency-magnitude distribution (FMD). A combination of these factors is also possible. It is further possible that instead of a fundamental difference between fast and slow straining areas, the source of the discrepancy may be driven in part by significant variations of factors within the fast-straining areas (e.g., along-strike the SAFS). These variations may (partly) control significant seismicity variations along the SAFS (e.g., locked and creeping fault sections) without discernible variations in expected loading rates.

Here, we present preliminary analysis on two of the possible explanations for the discrepancy. (1) We incorporate the seismogenic thickness map of Zeng et al. (SRL, 2024). We find that the thickness is only considerably less than the plate boundary’s average for the slowest deforming areas. More importantly, however, the SAFS does not have a significant deviation in thickness (i.e., smaller than average) and a relatively small seismogenic thickness there can thus not explain the observed discrepancy. (2) We consider a new seismicity catalog for the western US (Trugman & Ben-Zion, TSR, 2024), which has homogeneous magnitudes assigned based on the estimated seismic potency. This catalog overcomes potential pitfalls of using catalogs with mixed magnitudes in performing FMD analysis (e.g., ComCat). We use this catalog to investigate systematic spatial variations in FMD.

Key Words
earthquake rate, crustal properties, geodetic strain rate, frequency-magnitude distribution

Citation
Kreemer, C., Trugman, D. T., & Ben-Zion, Y. (2024, 09). What Does the Discrepancy Between Seismicity Rate and Strain Rate Tell Us About the Seismogenic Properties of the San Andreas Fault System?. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)