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Abstract

Due to a general lack of exposed faults it is difficult to predict the location and timing of future earthquakes in the Pacific Northwest region. Other geologic and geo-technical site conditions that control soil behavior and consequent surface damage can, however, be readily identified. Earthquake-hazard maps that reflect these conditions thus can be used as a predictive tool for seismic policy development and emergency planning. Many seismically prone jurisdictions have not yet implemented earthquake hazard mapping programs, largely reflecting a lack of awareness of the utility of these maps. Demonstrated applications of the maps include identification of geologically vulnerable areas with critical facilities and selection of suitable areas for new facilities, prioritization of seismic upgrading programs, recognition of high hazard areas requiring special study or restricted development, assessment of property insurance, estimation of risk, and establishment of more stringent regulatory requirements where needed.

A pilot program in southwest British Columbia illustrates the methodology used to develop an earthquake-hazard map. Existing and new geotechnical data are integrated with surficial geology mapping in a geographic information system (GIS) format. Liquefaction and ground motion amplification hazards within each geological map unit are assessed both qualitatively and quantitatively.

Probabilistic assessments of liquefaction potential, using ground acceleration data from the National Building Code of Canada, include a measure of the severity of potential surface disruption that in turn is a function of the geotechnical characteristics, thickness, and depth of each liquefiable unit. Potential for ground-motion amplification was estimated by comparison of geological map units with soil classes adopted by the U.S. National Earthquake Hazards Reduction Program. The hazard for each map unit was expressed as a range that reflects observed geological variation. The final hazard map, designed for land use and emergency planning purposes, shows a conservative default to the highest rating of either the liquefaction or amplification hazard.

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