USING SUAS PHOTOGRAMMETRY AND OPTICAL IMAGE CORRELATION TO QUANTIFY FAULT SLIP IN THE 2019 RIDGECREST EARTHQUAKE SEQUENCE

Optical image correlation is a valuable technique for quantifying fault displacements in the aftermath of surface-rupturing earthquakes. The application of optical image correlation relies on precisely orthorectified and coregistered pre- and post-event images of the same scene, ideally collected using the same platform. We demonstrate that viable optical image correlation results can be produced using disparate source imagery combinations such as orthomosaics derived from low-altitude aerial photographs collected from both sUAS (small Unmanned Aircraft System) and helicopter platforms. Combining imagery across different platforms allows for flexibility in deriving optical image correlation maps. We use helicopter photographs collected on 7/5/2019, between the Mw 6.4 foreshock and the Mw 7.1 mainshock in the Ridgecrest earthquake sequence, and taken on 7/12/2019 after the Mw 7.1 to quantify displacements on fault strands activated in both events near the intersection zone of the NW- and NE-striking surface ruptures. Several sUAS surveys were collected on portions of the surface rupture in the days after the earthquake before high quality satellite images were available, demonstrating the value of this platform in obtaining short temporal baselines in key locations. Using National Agriculture Imagery Program (NAIP) and WorldView imagery as pre-event imagery and sUAS as post-event imagery resulted in correlation maps and slip distributions that agree with those derived from high-resolution satellite images. Along the central four kilometers of the Mw 6.4 surface rupture we detected 0.1 – 1.4 m of left-lateral slip. Our work demonstrates the utility of analyzing imagery datasets collected across different platforms using optical image correlation in event response and further motivates the rapid collection and processing of low-altitude aerial photographs.