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

The Coachella Section of the San Andreas Fault (SAF) has accumulated considerable stress since its last major earthquake, c1700 CE (discussed below), and for the past few decades has exhibited surface creep amounting to 2-4 mm/yr. The southern SAF and major plate boundary features are displayed in Figure 1. This southernmost ∼70 km segment of the SAF is the least-well understood part of the plate-boundary-defining ∼1,100 km long SAF, with respect to hazard and the timing of the next large event (Philibosian Abstract We use UAVSAR interferograms to characterize fault slip, triggered by the Mw 7.2 El Mayor-Cucapah earthquake on the 1 San Andreas Fault in the Coachella Valley providing comprehensive maps of short-term geodetic surface deformation that complement in situ measurements. Creepmeters and geological mapping of fault offsets on Durmid Hill recorded 4 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 GeoGateway UAVSAR tool, 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.Plain Language Summary An aircraft-mounted imaging radar relies on a highly sensitive reflected interference pattern to form precise maps of surface changes. Images obtained from flights before and after the April 4, 2010 magnitude 7.2 El Mayor-Cucapah earthquake view the San Andreas Fault in California's Coachella Valley. Although the earthquake occurred 75 miles to the south of this fault, computer vision brings out complicated reshaping near and on the fault. The quiet deformation is concentrated in patches along the fault between the Mecca Hills and the Salton Sea, and matches the sense of slip expected from long-known continental plate motions surrounding this region. Slip at the fault surface are radar-measured at less than 3/4" but when compared to measurements in the broader fault zone we find that slip triggered by the distant earthquake is usually confined below a level 30 feet beneath the su...