Seismic Moment Accumulation Response to Lateral Crustal Variations of the San Andreas Fault System

Lauren A. Ward, Bridget R. Smith-Konter, Xiaohua Xu, & David T. Sandwell

Published February 22, 2021, SCEC Contribution #10963

Rheologic variations in the Earth’s crust (like elastic plate thickness or crustal rigidity) modulate the rate at which seismic moment accumulates for potentially hazardous faults of the San Andreas Fault System (SAFS). To quantify rates of seismic moment accumulation, GNSS and InSAR data were used to constrain surface deformation rates of a 4D viscoelastic deformation model that incorporates rheological variations spanning a 900‐km section of the SAFS. Lateral variations in elastic plate thickness, estimated from surface heat flow and seismic depth to the lithosphere‐asthenosphere boundary, were converted to lateral variations in rigidity and then used to solve for seismic moment accumulation rates on 32 fault segments. We find a cluster of elevated seismic moment rates (11‐20x1015 Nm yr‐1 km‐1) along the main SAFS trace spanning the historical Mw 7.9 1857 Fort Tejon earthquake rupture length; present‐day seismic moment magnitude on these segments ranges from Mw 7.2‐7.6. We also find that the average plate thickness in the Salton Trough is reduced to only 60% of the regional average, which results in a ∼60% decrease in moment accumulation rate along the Imperial fault. Likewise, a 30% increase of average plate thickness results in at least a ∼30% increase in moment rate and even larger increases are identified in regions of complex plate heterogeneity. These results emphasize the importance of considering rheological variations when estimating seismic hazard, suggesting that meaningful changes in seismic moment accumulation are revealed when considering spatial variations in crustal rheology.

Ward, L. A., Smith-Konter, B. R., Xu, X., & Sandwell, D. T. (2021). Seismic Moment Accumulation Response to Lateral Crustal Variations of the San Andreas Fault System. Journal of Geophysical Research: Solid Earth,. doi: 10.1029/2020JB021208.