Surface fractures nearby the Ridgecrest earthquakes: what can we learn on the regional stress and fault properties?

Abstract:

We analyzed repeat-pass Sentinel-1 and ALOS-2 synthetic aperture radar images to measure broad-area surface deformation and high-resolution surface rupturing from the combined July 4-5 2019 Ridgecrest earthquake sequence. Sentinel-1 phase gradient maps were analyzed to illuminate the fine details of the main rupture as well as triggered deformation on nearby faults. Compared to previous events in the Eastern California Shear Zone (e.g. 1992 Landers and 1999 Hector Mine) we found a much more complex set of surface fractures, including orthogonal, synthetic and antithetic faults. We constructed fault slip model and computed static Coulomb stress change for a suite of receiver fault orientations consistent with those exhibiting surface fractures. Comparing static stress change models to the phase gradient maps, we were able to determine that these surface fractures can largely be explained with static stress change from the earthquake sequence, with most antithetic motions are associated with compliant fault deformation, where most prograding ones being shallow frictional slip. One important implication from this is that much of the “off-fault” strain in the Mojave shear zone is due to permanent inelastic deformation on many small faults. We further explore a conceptual model that can explain these observations but a few important questions are yet to be resolved. 1) What can we learn about the regional stress from these fractures? 2) Are we able to derive a lower envelope of stress where regional shallow faults were able to hold. 3) Can the different amount of compliant motion from the sets of faults be used to derive parameters like damage zone depths and their moduli reduction? We’ll probe these possibilities and report at the meeting.