THE IMPORTANCE OF IN SITU SHEAR-WAVE VELOCITY MEASUREMENTS IN DEVELOPING URBAN AND REGIONAL EARTHQUAKE HAZARD MAPS

Since 1997, we have been developing scenario and probabilistic earthquake ground shaking hazard maps in the U.S. at a local and regional scale. These hazard maps depict the intensity and<br>variability of surficial ground shaking due to the variability in the type, thickness, and distribution of soil and rock units in a given map area. An essential input in site response analyses to estimate surficial ground motions is shear-wave velocity (VS) information. For each of the map areas, we have attempted to acquire, compile, and/or evaluate VS data collected by a variety of geophysical techniques. Based on these data we have developed a mean VS profile that is representative of each site response unit in a particular map area. Using a randomized suite of profiles derived from the mean profile, we perform site response analyses based on a random vibration theory-based equivalent-linear model to develop strain-dependent and if possible, depth-dependent amplification factors. The importance of developing urban and regional hazard maps that include site response effects cannot be overstated because as demonstrated by past earthquakes, the most damaging ground shaking occurs in areas underlain by soils and unconsolidated sediments. Thus the acquisition of VS data for soils, unconsolidated sediments, and rock in earthquake-prone areas has become an essential element in developing urban and regional hazard maps.