Shallow Aseismic Slip and Stress/Strain Budgets on the Creeping Faults in the Imperial Valley, California

Shallow slow slip events have long been observed along the strike slip faults of the San Andreas fault system and are now increasingly observed on many other faults on Earth. Creep events are thought to episodically release a portion of the fault’s interseismic stress budget that has accumulated over the earthquake cycle. However, it is not known what portion of strain these events release, and what residual strain remains available to drive earthquake occurrence. Near-field surface creep measurements, like alignment arrays and creepmeters, are unable to constrain the depth of creep, meaning that is difficult to constrain the release of strain or stress in each creep event without additional assumptions. In this study, we use radar data from InSAR platforms to resolve the depth of creep during creep events along the Superstition Hills fault in Southern California. We mitigate atmospheric noise by stacking co-event interferograms and by using empirically derived covariance matrices in the modeling. We apply a new nonlinear dislocation modeling method that constrains the slip distribution to be elliptical at each point along the fault and uses field and creepmeter data as lower bounds on surface slip. Using this model, we compute the strain drop throughout the rupture. We apply this technique to the 2006, 2010, 2017, and 2023 aseismic ruptures in Envisat, UAVSAR, and Sentinel-1 data. Lastly, we compare the resulting strain drops to strain accumulation rates calculated from backslip, testing the hypothesis that shallowly released strain is equal to the strain applied from deep dislocations in the crust. Using only the creep events in the instrumental record, we find that interseismic slip rates on the SHF must be above 10 mm/yr to explain the observations, a result consistent with regional-scale block modeling. Our results have implications for the strength of faults, the expected modes of seismic moment release in the shallow crust, and for seismic hazard analyses near creeping faults.