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

Rheologic variations in the Earth's crust (like elastic plate thickness [EPT] 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, Global Navigation Satellite Systems, and Interferometric Synthetic Aperture Radar data were used to constrain surface deformation rates of a four-dimensional viscoelastic deformation model that incorporates rheological variations spanning a 900 km section of the SAFS. Lateral variations in EPT, 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 (11-20 x 10(15) Nm year(-1) km(-1)) along the main SAFS trace spanning the historical M-w 7.9 1857 Fort Tejon earthquake rupture length; present-day seismic moment magnitude on these segments ranges from M-w 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 similar to 60% decrease in moment accumulation rate along the Imperial fault. Likewise, a 30% increase of average plate thickness results in at least a similar to 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.

Automated summary

This study quantifies seismic moment accumulation rates in the San Andreas Fault System by incorporating rheologic variations, revealing clusters of elevated seismic moment rates along certain fault segments. The results emphasize the importance of considering crustal rheological variations when estimating seismic hazard.