Shallow Seismic and Geotechnical Site Surveys at the Turkish National Grid for Strong-Motion Seismograph Stations

Abstract eng:
We conducted shallow seismic site survey at 161 strong-motion stations of the national grid in order to estimate P- and S-wave velocities down to a depth of 30 m. At each station site, we also drilled a geotechnical borehole down to 30-m depth. The P- and S-wave velocities were then correlated with geotechnical borehole data. The ultimate objective is to use shallow seismic velocities and geotechnical soil profiles to correct recorded seismograms for the local site effects, and determine geotechnical earthquake engineering parameters, including wave attenuation, site response spectrum, TA-TB periods, change of maximum acceleration with depth, soil amplification, and susceptibility to liquefaction. An integrated data acquisition system that includes an accelerated impact source with a 50-kg weight, a 48-channel receiver cable with 2-m geophone interval and 4.5Hz vertical geophones, and two 24-channel, 24-bit Geode recording units was used to acquire the shallow seismic data. We acquired three 48-channel seismic records at each of the station sites using the commonspread recording geometry. By applying nonlinear traveltime tomography to the first-arrival times picked from the shot records, we estimated a near-surface P-wave velocity-depth model for the site along the receiver spread. By applying Rayleigh-wave inversion to the surface waves isolated from the shot records, we estimated an Swave velocity-depth profile for the site. In the borehole at each station site, we performed standart penetration tests (SPT) at 1.5-m depth interval, determined core recovery and rock quality, measured the ground-water level. We then conducted geotechnical laboratory measurements and determined the soil profile for each station site. All of the field data and the results of our investigations from the shallow seismic and geotechnical site surveys will eventually be made available via a centralized data base on the internet to scientists and engineers for future investigations in earthquake engineering. Specifically, the reported results from the seismic and geotechnical investigations will be used to determine geotechnical earthquake engineering parameters for each station site, including site-dependent response spectrum, short- and long-corner periods of design spectrum, change of maximum ground acceleration with depth, soil amplification, and investigation of susceptibility to liquefaction. The reported results will also be of great use in the derivation of ground-motion prediction equations that consider the shear-wave velocity dependent site influence.

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