A MULTIFACETED APPROACH TO HIGH-RESOLUTION GEOLOGIC MAPPING OF MERCER ISLAND, NEAR SEATTLE, WASHINGTON

High resolution geologic mapping in a vegetated/urbanized setting, such as 8-kilometer-long Mercer Island, Washington, requires a multitude of geologic mapping techniques for geologic hazards and resource evaluations. In the Pacific Northwest, geologic exposures are limited, so to bridge the data gaps we built a subsurface database and conducted geomorphic surface analyses.

The initial geologic map was constructed by: field mapping in the winter when vegetation is at a minimum, and building a subsurface database with 2,730 boreholes, test pits, and water well logs that allows data viewing, layer searching, and cross section generation through ArcGIS©. These data were used to create a draft geologic map at 1:12,000-scale.

To enhance the geologic interpretations and to address questions in the interpretations, we analyzed (2-m cell) LIDAR data and reviewed aerial photographs. Using 3D analysis software, the LIDAR data were processed to highlight slope curvatures, slope angles, slope roughness and other abrupt elevation changes. These analyses helped to locate denuded drumlins, recessional lake deposits, paleo-shorelines, recessional outwash channels, landslide deposits and scarps, and provided geomorphic hypotheses for further field investigation. Larger drumlins and flow channels were identified that were not visible using standard topographic maps and standard LIDAR digital elevation models. Two- and three-dimensional shaded relief images with multiple sun aspects and azimuths enabled identification of mass wastage features and abundant lineaments. Although the Seattle Fault Zone crosses Mercer Island, recent fault scarps were not identified.

Interpretations from the LIDAR and aerial photographic analyses were incorporated into the draft geologic map to prepare the final geologic map. Secondary field investigations verified that the integrated interpretations are good representations of actual conditions. Without the use of LIDAR geomorphic analyses, some of the surficial deposits, of the type that would be more susceptible to ground failure during an earthquake, would have been missed during the geologic mapping. In the Pacific Northwest, earthquake risk is 2nd only to California, so geologic mapping must use all available means when feasible to best delineate susceptible deposits.