Extent of Low-Angle Normal Slip in the 2010 El Mayor-Cucapah (Mexico) Earthquake From Differential Lidar

We investigate the 4 April 2010 M _w  7.2 El Mayor-Cucapah (Mexico) earthquake using three-dimensional surface deformation computed from preevent and postevent airborne lidar topography. By profiling the E-W, N-S, and vertical displacement fields at densely sampled (∼300 m) intervals along the multisegment rupture and computing fault offsets in each component, we map the slip vector along strike. Because the computed slip vectors must lie on the plane of the fault, whose local strike is known, we calculate how fault dip changes along the rupture. A principal goal is to resolve the discrepancy between field-based inferences of widespread low-angle (<30°) oblique-normal slip beneath the Sierra Cucapah, and geodetic and/or seismological models which support steeper (50°–75°) faulting in this area. Our results confirm that low-angle slip occurred along a short (∼2 km) stretch of the Paso Superior fault—where the three-dimensional rupture trace is also best fit by gently inclined planes—as well as along shorter (∼1 km) section of the Paso Inferior fault. We also characterize an ∼8-km fault crossing the Puerta accommodation zone as dipping ∼60°NE with slip of ∼2 m. These results indicate that within the northern Sierra Cucapah, deep-seated rupture of steep faults (resolved by coarse geodetic models) transfers at shallower depths onto low-angle structures. We also observe a statistically significant positive correlation between fault dip and slip, with slip pronounced along steep sections of fault and inhibited along low-angle sections. This highlights the important role of local structural fabric in controlling the surface expression of large earthquakes.