Buried Aseismic Slip and Off-Fault Deformation on the Southernmost San
Andreas Fault triggered by the 2010 El Mayor Cucapah Earthquake revealed
by UAVSAR
Abstract
We use UAVSAR interferograms to characterize fault slip, triggered by
the Mw 7.2 El Mayor-Cucapah earthquake on the southernmost San Andreas
Fault in the Coachella Valley providing comprehensive maps of landscape
change that complement in situ measurements. Creepmeters and
geological mapping of fault offsets on Durmid Hill recorded 4 mm and 8
mm of average triggered slip respectively on the fault, in contrast to
radar views that reveal significant off-fault dextral deformation
averaging 20 mm. Unlike slip in previous triggered slip events on the
southernmost San Andreas fault, dextral shear in 2010 is not confined to
transpressional hills in the Coachella valley. Edge detection and
gradient estimation applied to the 50-m-sampled interferogram data
identify the location (to 20 m) and local strike (to < 4°) of
secondary surface ruptures. Transverse curve fitting applied to these
local detections provides local estimates of the radar-projected dextral
slip and a parameter indicating the transverse width of the slip, which
we equate with the depth of subsurface shear. These estimates are
partially validated by fault-transverse interferogram profiles generated
using the web-based UAVSAR tool of GeoGateway, and appear consistent for
radar-projected slip greater than about 5 mm. An unexpected finding is
that creep and triggered slip on the San Andreas fault terminate in the
shallow subsurface below a surface shear zone that resists the simple
expression of aseismic fault slip. We introduce the notion of a surface
locking depth above which fault slip is manifest as distributed shear,
and evaluate its depth as 6-27 m.