APPLICATIONS OF AIRBORNE AND TERRESTRIAL LIDAR TO PALEOSEISMOLOGY

Documenting earthquake characteristics such as location, magnitude, frequency, and ground shaking intensity is critical to understanding earthquake processes and validating seismic hazard analyses. Airborne and terrestrial laser scanning (ALS and TLS) document these characteristics effectively because they permit 3-dimensional (3D) representation of earthquake-related features at submeter scales. Two paleoseismic case studies are presented: (1) characterization of the 3D form and geomorphic setting of precariously balanced rocks (PBRs) and (2) high-resolution 3D imaging of a paleoseismic trench. PBRs are negative indicators for large earthquakes and are used to evaluate the spatial extent and infer the intensity history of strong ground motions. A 1.5-m tall by 1-m wide PBR was scanned using TLS. The aligned scans are used to generate a 3D surface model of the PBR from which geometric parameters can be computed and compared to their 2D estimates from photographs. Landscape morphometry computed using a 0.25 m ALS-generated digital elevation model (DEM) reveals the geomorphic situation of PBRs at the drainage basin scale. Hillslope angles between 10° and 45°, and contributing areas (per unit contour length) between 1 m²/m and 30 m²/m appear to be conducive to preserving PBRs. This can help refine interpretations of PBR exhumation rates and thus their effectiveness as paleoseismometers. In paleoseismic trenching, tens to hundreds of photographs are manually merged and used to create base maps on which faults, stratigraphy, and samples are logged. Photograph edge mismatch, lens distortion, and irregular trench walls limit the accuracy of these logs. Short-range TLS was used to scan the walls of a 1-m wide paleoseismic trench. The merged point clouds were projected onto vertical planes and colored using pixel values from the scanner’s camera. The rapidly produced TLS-derived high-resolution orthophotos have superior geometric accuracies compared to their photomosaic counterparts. This permits a more accurate and rapid documentation of sedimentary and earthquake-related structures revealed by paleoseismic trenches. TLS also facilitates the permanent digital archival, retrieval, and 3D visualization of trench walls for review and further quantitative analysis.