Effects of Surface Geology on Seismic Motion

Methods of site response analysis require theoretical and empirical testing and validation before they can be used in seismic hazard assessment. Comparison of site amplification function (SAF) calculated using earthquake data recorded by seismic arrays with SAF obtained using analytical approaches represent the most important tests of reliability of those methods. The Treasure Island (TI) downhole array was installed in 1992 by the California Geological Survey (CGS) to study the response of soft-soil over rock geologic structure to earthquake motion. The TI array has sensors located in the Franciscan bedrock (104 and 122 m depths), bay sediments (16, 31 and 44 m depths), artificial fill (7 m depth) and at the surface. High-quality low-amplitude earthquake data not exceeding 0.03 g recorded by the array between 1993 and 2010 were used for comparisons with the two versions of the equivalent linear (EQL) site response methods implemented in the computer program STRATA (Kottke and Rathje, 2008, 2010): • Time Series (TS) approach • Random Vibration Theory (RVT) approach. Use of the TS approach in STRATA produces the same results as SHAKE, and matches well empirically determined SAF between bedrock and surface. The STRATA version of the RVT approach can produce significantly different site amplification function from the empirically determined and the TS approach when input motion is located in the downhole (within the media with upand downgoing waves). The differences in the calculations of site amplification using TS and RVT approach were also observed when both input and output motions are outcrops. Further testing of different realizations of RVT method is desirable to assess method’s reliability and limitations.