Submeter Resolution Surface Rupture Topography From Legacy Aerial Photographs-A Test Case From the 1992 Landers Earthquake

The 1992 M-w 7.3 Landers earthquake in the Mojave Desert (California) provided exceptional observations of surface faulting in a large, continental earthquake. The U. S. Geological Survey obtained nadir angle, overlapping aerial photographs at 1:6,000 scale for the entire similar to 85 km rupture length. Recent advances in Structure from Motion photogrammetry allow for archival photographic data sets such as these to be reprocessed, generating digital topography that can be reanalyzed quantitatively in a way that was not previously possible. In this proof-of-concept study, we generated a georectified, similar to 10 points/m(2) topographic point cloud over nearly the entire Landers rupture length and a higher-resolution similar to 40 points/m(2) point cloud over a smaller (similar to 5 km) rupture segment along the Emerson fault. We estimated the accuracy and explore the utility of our point cloud in two tests. First, we observe close geometric agreement (average closest point distance 2.1 cm and standard deviation 14.0 cm) between our point cloud and a 2008 terrestrial lidar survey of the Galway Lake Road site on the Emerson fault. Second, we made 173 vertical offset measurements within a small, structurally complex pull-apart basin, also on the Emerson fault, and find visual and statistical similarity with 21 local field measurements. These two tests demonstrate that point clouds generated from legacy aerial surveys and georeferenced using free Google Earth and National Elevation Dataset imagery are geometrically accurate and can be used to densify geomorphic offset measurements even along well-studied surface ruptures. Applied to other historical events, such measurements could provide new insights into earthquake rupture processes.