HIGH-RESOLUTION TOPOGRAPHY FOR GEOLOGIC HAZARDS STUDIES, WESTERN WASHINGTON

Good topography is perhaps the most significant data resource for investigation of geologic hazards. With support from NASA, USGS, and several local agencies, the Puget Sound Lidar Consortium is working with a commercial contractor to acquire lidar topographic data and create high-resolution digital elevation models (DEMs) of the forested terrain of western Washington. To date the Consortium has acquired 10,000 sq km of 6-ft DEM with vertical accuracy of circa 30 cm RMSE. For details on this effort, see http://pugetsoundlidar.org.

Several probable Holocene fault scarps have been identified from the new lidar DEMs. Subsequent trenching studies indicate several large (M 6.5 - 7.5) crustal earthquakes in the last few thousand years. Locally we have used the DEMs to measure marine shoreline terrace elevations and quantify uplift and tilting produced by these earthquakes. Geomorphic mapping from the DEMS is refining our understanding of surficial geology (e.g. the distribution of bedrock, alluvium, artificial fill) that controls earthquake site response and will lead to improved earthquake hazard maps. This mapping may delimit the shores of short-lived late-glacial lakes with sufficient detail that we can describe aggregate Holocene vertical deformation throughout the Puget Lowland at the 1-m level.

From the lidar DEMs we are developing a regional inventory of large, deep-seated landslides that is more complete and more consistent than the present inventory based on aerial photography and field studies. Several large, spectacular, previously unknown landslides have been recognized. Areas with significant potential for small landslides can be delineated. Lidar DEMs are serving to define areas prone to flood and tsunami inundation and to study mitigation measures. We are now acquiring lidar topography on the west flank of Mount Rainier and we expect these data to provide important information on lahar processes, deposits, and hazards. Finally, we have learned that high-resolution DEMs provide an extremely effective map base on which to communicate the results of hazards studies.