CONSTRUCTION OF LIQUEFATION SUSCEPTIBILITY AND NEHRP SITE CLASS MAPS FOR CLARK COUNTY, WASHINGTON USING GIS DATA OVERLAYS

We have developed 1:24,000-scale liquefaction susceptibility and NEHRP site class maps for Clark County, Washington as part of a statewide earthquake hazards mapping program. Both maps show a more complex distribution of hazards than preliminary maps based strictly on 1:100,000-scale geologic mapping, but with an overall lower countywide hazard level. Regional earthquake hazard maps such as these support hazard mitigation, emergency planning and response, planning of local zoning ordinances, and building code enforcement. These maps were developed using ArcGIS 8.3, and are distributed via ftp as PDF plot files and ESRI shapefiles.

We first compiled geologic, water resources, and soil maps to produce a new simplified surficial geologic map for the county. Field observations confirmed our interpretations. Geotechnical boring and water well logs were interpreted, and thicknesses of stratigraphic units were contoured, digitized, and then gridded using a natural neighbor interpolation method. The resulting 3-D stratigraphic models were arranged to yield a subsurface geologic column for each grid cell of the map.

Liquefaction susceptibilities for specific geologic units were determined using a standard engineering analysis of geotechnical boring data. The liquefaction analysis included evaluating the sensitivity of computed factors of safety over a range of groundwater depths and earthquake scenarios. From these results we determined threshold groundwater depths to be used in defining areas of differing liquefaction susceptibilities. We then used static groundwater depth models developed for the study area to map liquefaction susceptibility based on our threshold criteria.

To develop the NEHRP site class map, we assigned characteristic shear wave velocities, based on the mean and the mean minus one standard deviation (lower bound) values of available shear wave velocity data, to each stratigraphic unit in our 3-D models. We then determined depth-weighted average shear wave velocities for the upper 30 m of the soil column, using both the mean and lower bound velocity values. Site class maps were developed for both the characteristic and lower bound velocity assignments, and ultimately combined to produce a map reflecting the statistical variation in shear wave velocities.