Fault Slip Rate Studies in Southern California

The hazard posed by earthquakes is strongly related to the geologic slip rate of active faults.  To determine slip rates, geologists use high-resolution digital elevation maps (DEMs) to reconstruct landforms offset by meters to hundreds of meters by active faults.  Scientists at the Earthquake Science Center in Pasadena, California, are working to improve methods for obtaining low-cost but high-resolution DEMs using structure from motion (SfM) photogrammetry.  SfM software utilizes a set of overlapping photographs to determine the relative camera position and orientation of each photo viewpoint, creates a DEM that includes the topography of the scene, and then “drapes” the imagery over the model.  Point clouds created with balloon and helicopter camera platforms produce point densities that are higher than expensive aerial lidar datasets, and when combined with a small number of ground control points, SfM-produced DEMs can approach lidar accuracies. SfM DEMs of alluvial fans offset 30 m by the San Andreas Fault were combined with cosmogenic radionuclide dating to establish the relative importance of different strands of the fault over the last 20,000 years. SfM DEMs showing  stream channels offset by 4 m during the 1999 magnitude 7.1 Hector Mine earthquake are being compared with aerial lidar to analyze scarp degradation.  In both cases, research indicates SfM provides a low-cost option for creating high-resolution DEMs in areas of sparse vegetation.