Shallow fault geometry from differential lidar: examples from the 2010 Mw 7.2 El Mayor-Cucapah and 2016 Mw 7.8 Kaikoura earthquakes

Three-dimensional surface displacements from differential lidar provide a new means of characterizing shallow faulting in large earthquakes, bridging an observational gap between far-field deformation imaged by traditional satellite geodesy and surface slip measured in the field. Here, we show how lidar displacement fields can resolve shallow fault geometry, using the multi-segmented 2010 Mw 7.2 El Mayor-Cucapah (Mexico) and 2016 Mw 7.8 Kaikoura (New Zealand) earthquakes to illustrate. By profiling E-W, N-S and vertical displacement fields at densely-sampled intervals along the mapped surface ruptures, and computing fault offsets in each component, we map the along-strike slip vector distribution. Since the computed slip vectors must lie in the plane of the fault, whose local strike is known, we determine the fault dip at each measurement point. In the El Mayor-Cucapah earthquake, we seek to resolve the discrepancy between field-based inferences of widespread low angle (<30°) oblique-normal slip beneath the Sierra Cucapah, and geodetic models which support steeper (>50°) faulting. Our results confirm that low-angle slip occurred along a 5 km stretch of the Paso Superior fault and along shorter sections of the Paso Inferior and Borrego faults. This implies that deep-seated rupture of steep faults (resolved by coarse geodetic models) transfers at shallower depths onto low-angle structures. However, we also find that slip is pronounced along steep sections of fault and inhibited along low-angle sections and that at several fault segment boundaries, the propagating rupture jumped from low-angle faults onto steeper structures nearby. This highlights the important role of local structural fabric in controlling the surface expression of large earthquakes. In the Kaikoura example we focus on the 19 km-long Papatea fault, which may have played a key kinematic role in the evolution of the earthquake. We measure peak slip of 12 m and average slip of 8-9 m along the central, Clarence valley fault section, confirming earlier inferences of an extremely high slip-to-length ratio on the Papatea fault. We determine that the Papatea fault dips 55° - 70° W in the Clarence valley, but 2 m of back-thrusting along a secondary fault strand, 1.5 km to the west, hints at a reduction in the main fault dip angle at depths of 1 - 2 km.