Site classification and Vs30 estimation of free-field TSMIP stations using the logging data of EGDT

Abstract

The Engineering Geological Database for TSMIP (EGDT), the Taiwan Strong Motion Instrumentation Program, has been under construction by the National Center for Research on Earthquake Engineering and the Central Weather Bureau in Taiwan since 2000. Site characterization, comprising surface investigations and logging measurements, was carried out throughout the project. We provide a set of specifications and a description to help users understand the subject matter of the database. EGDT contains 469 surveyed stations, 439 of which were drilled and the logging measurements completed. Of these, 385 had logging data reaching at least 30m, and we used these to examine and determine the most accurate extrapolation of Vs30 (the average S-wave velocity of the top 30m of strata) for the other 54 stations with velocity profiles less than 30m. The chosen method assumed that the bottom velocity is identical from the actual depth of the hole to a distance of 30m, that is, the Bottom Constant Velocity (BCV) method. In order to utilize other existing boreholes which have only N values but no velocities in the future, the empirical S-wave velocity equations for seven different regions and the whole of Taiwan were evaluated by a multivariable analysis. Henceforth, for those existing boreholes which have an N profile less than 30m, the S-wave velocity profile can first be calculated by empirical S-wave velocity equations, and then Vs30 can be estimated by reliable extrapolation. Some other studies of site classifications of TSMIP stations were compared with our results to demonstrate the necessity of reclassification. Ultimately, the Vs30 values of the 439 drilled free-field TSMIP stations were derived and the new site classification was achieved according to the Vs30-based provisions of the National Earthquake Hazards Reduction Program.