MAKING GEOLOGICAL HAZARD MAPS OF MERCER ISLAND, WA USING HIGH-RESOLUTION MAPS AND A SUBSURFACE DATABASE

The City of Mercer Island, in the Puget Lowland of Washington, recently updated its geological hazard maps in accordance with State regulations and the Growth Management Act. Guidance for mapping geological hazards is provided in Washington Administrative Code 365-190-080. “Geologically hazardous areas”, by State definition, “include areas susceptible to erosion, sliding, earthquake, or other geological events. They pose a threat to the health and safety of citizens when incompatible commercial, residential, or industrial development is sited in areas of significant hazard.” The City's new suite of maps includes maps showing Seismic Hazards, Slope Hazards, and Erosion Hazards. This new set of maps represents an update of the 2002 Geologic Hazard Map Series and is based on a review of Best Available Science for the Seattle Fault and related events, a new high-resolution Geological Map of Mercer Island, a database of subsurface explorations, and depth to water data. City planners and building officials will use these maps to enforce their critical areas codes, building codes, and building permit requirements.

Comparison of the 2009 to the 2002 geological hazard maps reveals many differences. Significant differences relate to scale and base map. The 2002 maps are based on a 1962, 1:63,500-scale geological map. The 2009 maps are based on a 2006 1:12,000-scale geological map with 4 times the level of detail of the 1962 geological map. The new geological map was constructed using a database of 3054 subsurface explorations and 164 exposure data points. LIDAR data were used to provide a high-resolution base for geomorphic analyses, mapping of scarps, identification of landslide deposits, and slope classifications allowing the new maps to be both more precise and more accurate. Updated 2009 maps include inventories of landslides, mapping of spring locations, depth to water data, mapping of subaqueous landslides, mapping of critical geological contacts, mapping of infiltration potential, mapping of weak deposits, and mapping of fill materials. A major change to the seismic hazard map is the addition of the Seattle fault zone, an active crustal thrust fault. More than twice the land area is shown as geologically hazardous, compared to the 2002 maps.