Surface slip variations and off-fault deformation patterns in complex
cross-fault systems revealed from 3D high-resolution satellite optical
image correlation: the 2019 Ridgecrest earthquakes (California, 2019)
Abstract
The Ridgecrest sequence (Mw6.4 and Mw7.1, July 2019, California) is a
cross-fault earthquake that has been observed using a wide range of
geophysical and geological methods. The sequence ruptured consecutively
two orthogonal cross-fault systems within 34 hours (northeast- and
northwest-trending). It raised the question of the relation between the
two systems of faults both at depth and at the surface, and its impact
on the surface displacement pattern. Here we use high-resolution (50 cm)
satellite optical image correlation to measure the 3D surface
displacement field at 0.5 meters ground resolution for the two
earthquakes. Because our images bracket the whole sequence, our
displacement and deformation maps include both earthquakes. Our data
allow for measuring series of slip profiles in the components parallel
and perpendicular to the rupture, and in the vertical direction, to look
at the correlation between slip distribution and rupture complexity at
the surface. We point out significant differences with previous geodetic
and geological-based measurements and show the essential role of
distributed faulting and diffuse deformation in the comprehension of
surface displacement patterns. We discuss the segmentation of the
rupture regarding the fault geometry and along-strike slip variations.
We image several surface deformation features with similar orientation
to the deeply embedded fabric identified in seismic studies. This
northeast-trending fabric influenced the surface deformation both during
the foreshock and the mainshock earthquakes. We also derive strain
fields from the horizontal displacement maps and show the predominant
role of rotational and shear strains in the rupture process. We finally
compare our results to kinematic inversions and show that the foreshock
did influence the mainshock by clamping the fault and encouraging
off-fault diffuse deformation rather than fault slip in some areas.