Seismic velocities and geological conditions at twelve sites subjected to strong ground motion in the 1994 Northridge, California, earthquake; a revision of OFR 96-740

Abstract

cr < 0, respectively. Although cr < 0 is theoretically possible (e.g. Fung, 1968), we decided that our P-and S-velocities should yield a between 0.0 and 0.5. We made corrections to the velocities using one or more of the following procedures: repicking shallow arrivals (usually P arrivals because small changes in P travel-times have greater effect on cr), adding a shallow layer, and/or adjusting layer thickness to ensure that Poisson's ratio was in the range 0.0-0.5. In most cases the greatest changes (compared to determination of velocity without consideration of a) were in the P-wave velocities at shallow depths. Overall, the changes in velocity required to produce acceptable values of a were small and were only in a few layers. Calculations of Poisson's ratios for the preferred models are contained in Appendix B. Because of items 1 and 2 discussed above, the values in Appendix B may not represent the true Poisson's ratios of the materials at shallow depths.