Seismic moment variations of the San Andreas Fault System: Exploring sensitivity of crustal rheology in southern California

When considering seismic moment accumulation of potentially hazardous faults like the San Andreas Fault System, it is important to recognize the probable variations in rheology that can influence rates that control the stored moment. Geologically unique provinces, such as the Salton Trough in southern California, host faults like the Imperial segment which may store seismic moment at a different rate than a companion fault residing in a region comprised of contrasting crustal structure. Using a new 4D viscoelastic crustal deformation model that incorporates rheological constraints of the southern California lithosphere, we use Sentinel-1 InSAR and GAGE GPS data to constrain present-day surface deformation rates. We adopt a simplified representation of crustal rheology derived from provisional heat flow estimates and seismically imaged lithosphere-asthenosphere boundary (LAB) depths. These results are then used to calculate seismic moment accumulation rate for 43 fault segments of the San Andreas Fault System. In the Salton Trough, for example, a lower than average crustal rigidity (17.5 GPa) reduces the seismic moment accumulation rate by 60% along the southern Imperial fault, to 2.4x10¹⁵ Nm/km/yr, when compared to rates previously determined by a homogenous crustal rigidity model of 30 GPa. When accounting for the time transpired since the last major earthquake on the southern Imperial fault, we also compute a decreased accumulated moment magnitude from Mw = 6.9 to 6.6. Alternatively, a higher crustal rigidity along the northern San Bernardino segment is responsible for an increased seismic moment accumulation rate, to 12x10¹⁵ Nm/km/yr and a moment magnitude increase from Mw = 7.5 to 7.7. These results emphasize the critical need for a unified representation of lithosphere rheology for southern California, suggesting that considerable variations in earthquake magnitude forecasts are possible when considering these effects of variable crustal rheology.