Estimating Fault Slip From Radar Interferograms

Estimating Fault Slip From Radar InterferogramsJay Parker, Andrea Donnellan, Margaret Glasscoe, and Tim StoughJet Propulsion Laboratory, California Institute of Technology Radar interferogerams can measure near-surface fault slip with sub-cm accuracy. Common analysis considers a repeat-pass interferogram from a single viewing angle, which maps fault slip projected into the line-of-sight of the radar. Nonetheless fault motion estimates are signed: whatever the mechanism of slip, one side of the fault moves relatively toward the radar, the other away. Line-of-slight slip estimates are compared (in some cases) with field observations of surface fracture projected into the same radar line-of-sight direction. Views from two sufficiently distinct directions allows separate estimates for vertical and horizontal motion but by necessity leaves one component of slip undetermined. View from more than two angles is rare, but resolves three components of fault slip. In contrast with field measurements of surface fractures, radar interferograms allow estimating the motion of cross-fault patches of tens of meters extent. Many such faults have discernable shear-zone width, allowing modest inversion for slip at depth down to tens of meters. This allow characterization of near-surface slip deficit. Also when there are multiple fractures across a fault zone, the interferogram will detect the overall mean fault motion, while field measurements may only discover one strand among many. Algorithmic estimates using a uniform set of control parameters are applied to California faults, including artifacts of the El Mayor Cucapah M7.2 2010 event and aftershocks, the La Habra M5.1 2014 Event, and the South Napa M6.0 2014 event.