SCARP EROSION AND BURIAL IN REPEAT TERRESTRIAL LIDAR SURVEYS OF THE 4 APRIL 2010 EL MAYOR-CUCAPAH EARTHQUAKE SURFACE RUPTURE

Height-difference maps derived from repeat post-earthquake terrestrial LiDAR (T-lidar) surveys reveal the spatial and temporal distribution of scarp erosion and burial over a 3.5-yr span following the 4 April, 2010 M_w7.2 El Mayor-Cucapah earthquake. This analysis reveals emplacement mechanisms of common paleoseismic markers and informs the interpretation of discontinuous or eroded offset features. T-lidar surveys were collected 12 days, 1 year, and 3.5 years after the 4 April, 2010 M_w7.2 El Mayor-Cucapah earthquake at two ~250 m-long reaches of the surface rupture. Capturing localized erosion of a fault scarp requires extremely accurate collocation of sequential scans. Initially poor regional GPS coverage (during 2010 and 2011 scans) and coseismic backslip corrections by NOAA’s Horizontal Time-Dependent Positioning (HTDP) system resulted in relatively high errors (25-150 cm) among our GPS-measured network of tiepoints at each site. To accurately collocate t-lidar point clouds from different years, we aligned later surveys with earlier ones using an iterative closest point (ICP) matching algorithm, excluding portions of the sites with known, mapped topographic changes. The precision of this approach was aided by the extreme stability of this hyperarid landscape away from the fault and by the high amplitude of surface roughness relative to scan precision. Height differencing of aligned scans reveals rapid (>1 m yr ^-1) headward scarp retreat where active channels cross the surface rupture, and the pervasive formation of rills and minor (10-50 cm) headward scarp retreat along hillslope reaches. No erosion is detected along portions of the scarp that cut channel margins, where hillslope aspect is parallel to fault strike. Where uphill-facing rupture scarps block an active stream channel, height differencing reveals 20-70 cm of upstream aggradation by ponding and delta progradation, and 80 cm of downstream erosion due to overtopping of the scarp by floodwaters. These observations guide predictions of fault scarp morphology and preservation potential, and may aid in identifying paleoseismic markers in the hunt for earthquakes elsewhere.