WESTERN RAINIER SEISMIC ZONE AIRBORNE LASER SWATH MAPPING

Airborne laser swath mapping (ALSM) of the Puget Lowland, conducted by TerraPoint, LLC for the Puget Sound Lidar Consortium (PSLC), has been successful in revealing Holocene fault scarps and landslides hidden beneath the dense, temperate rain forest cover and in quantifying shoreline terrace uplift (Haugerud et al, 2003, GSA Today, v 13(6)). Expanding the PSLC efforts, NASA-USGS collaboration is now focusing on topographic mapping of seismogenic zones adjacent to volcanoes in the western Cascades range in order to assess the presence of active faulting and tectonic deformation, better define the extent of lahars and understand their flow processes, and characterize landslide occurrence. Mapping of the western Rainier zone (WRZ) was conducted by TerraPoint in late 2002, after leaf fall and before snow accumulation. The WRZ is a NNW-trending, ~ 30-km-long zone of seismicity west of Mount Rainier National Park. The Puget Lowland ALSM methods were modified to accommodate challenges posed by the steep, high relief terrain. The laser data, acquired with a density of ~ 2 pulses per sq m, was filtered to identify returns from the ground from which a bare Earth digital elevation model (DEM) was produced with a grid size of 1.8 m. The RMS elevation accuracy of the DEM in flat, unvegetated areas is ~ 10 cm based on consistency between overlapping flight swaths and comparisons to ground control points. The resulting DEM substantially improves upon Shuttle Radar Topography Mission and USGS photogrammetric mapping. For example, the DEM defines the size and spatial distribution of flood erratics left by the Electron lahar and of megaclasts within the Round Pass lahar, important for characterizing the lahar hydraulics. A previously unknown lateral levee on the Round Pass lahar is also revealed. In addition to illustrating geomorphic features within the WRZ, future plans for laser mapping of the Saint Helens and Darrington seismic zones will be described.