A site-conditions map for California based on geology and shear-wave velocity

One simple way of accounting for site conditions in calculating seismic haz ards is to use the shear-wave velocity in the shallow subsurface to classif y materials. The average shear-wave velocity to 30 m (V-s(30)) has been use d to develop site categories that can be used for modifying a calculated gr ound motion to account for site conditions. We have prepared a site-categor y map of California by first classifying the geologic units shown on 1:250, 000 scale geologic maps. Our classification of geologic units is based on V -s(30) measured in 556 profiles and geological similarities between units f or which we have V-s data and the vast majority of units for which we have no data. We then digitized the geologic boundaries from those maps that sep arated units with different site classifications. V-s data for California shows that several widespread geologic units have r anges of V-s(30) values that cross the boundaries between NEHRP-UBC site ca tegories. The Franciscan Complex has V-s(30) values across UBC categories B and C with a mean value near the boundary between those two categories. Ol der alluvium and late Tertiary bedrock have V-s(30) values that range from about 300 to about 450 m/sec, across the boundary between categories C and D. To accommodate these units we have created intermediate categories, whic h we informally call BC and CD. Geologic units that have, or are interprete d to have, V-s(30) values near the boundary of the UBC categories are place d in these intermediate units. In testing our map against the available V-s(30) measurements, we have foun d that 74% of the measured V-s(30) values fall within the range assigned to the V-s category where they fall on the map. This ratio is quite good cons idering the inherent problems in plotting site-specific data on a regional map and the variability of physical properties in geologic units. We have a lso calculated the mean and distribution of V-s(30) for each of our map uni ts and prepared composite profiles, showing the Variation of V-s in the upp er 100 m from the available V-s data. These data show that the map categori es that we have defined based on geologic units have different V-s properti es that can be taken into account in calculating seismic hazards.