Evaluation of site shear stiffness and damping properties from downhole earthquake records

Shear stiffness and damping properties at the Lotung site (Taiwan) are identified from free-field downhole acceleration records. The Lotung site was instrumented in 1985 by the Electric Power Research Institute (EPRI) and the Taiwan Power Company (TPC), with an array of downhole and surface accelerometers. A total of 18 earthquakes were recorded during the period 1985-1986. Cross-correlation between downhole accelerations is used to evaluate the low-strain vertical shear wave velocity profile. Actual seismic shear stress-strain histories are directly evaluated from the recorded downhole accelerations. These histories are used to estimate variation of soil shear moduli and damping ratios with shear strain amplitude.