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