Time-series of scarp modification on the 2010 El Mayor-Cucapah earthquake rupture from repeat terrestial LiDAR surveys

We analyze the modification of fault scarps formed by the 4 April 2010 M 7.2 El Mayor-Cucapah earthquake through comparison of terrestrial LiDAR scans acquired within two weeks of the event to repeat scans one year later. From this comparison of annual scans we assess the mechanisms and rates of scarp modification in different substrates, and attempt to capture near-field post-seismic deformation. Constraining the manner and rate with which scarps degrade over time is necessary to understand the neotectonic record of past earthquakes, particularly in regions of diffuse faulting and infrequent ruptures. This earthquake provided an opportunity to scan freshly formed, nearly vertical scarps from which we can directly measure the initial rates and processes of scarp modification. We also seek evidence of afterslip and near-field subsidence that may have been caused by post-seismic relaxation of shallow stresses introduced by coseismic faulting. We conducted initial terrestrial LiDAR surveys of the fault rupture totaling ~2km along-strike beginning just 12 days after the earthquake, in April 2010. Our targets included four different sites chosen to document surface faulting through four different substrates: bedrock, abandoned alluvial gravels, active fluvial sands, and fine aeolian sand and silt. The initial surveys preserved fresh rupture free-faces, minute offset features, disrupted desert pavement, and fault surface topography including multiple sets of fault striae. The surveyed sites demonstrate systematic narrowing of the fault zone with degree of consolidation of the substrate. We repeated scans of our initial survey sites in March and April 2011, after one rainy season in the region. We also expanded our survey coverage of the fault by ~250% to establish a wide basis for comparison with surveys in the future. Stability of fault zone topography between the years again correlates strongly with the degree of consolidation of the substrate, with the scarps through sand dunes nearly buried and the scarps through bedrock barely modified. In addition to diffusion and aeolian erosion/deposition of unconsolidated sands, we observe mass wasting of oversteepened scarp faces and concentrated gullying where narrow channels intersect the scarp. Thus far we have yet to find evidence for afterslip or fault-zone subsidence in the epoch captured by our surveys.