Elastic and Large-Strain Nonlinear Seismic Site Response from Analysis of Vertical Array Recordings

摘要

Strong ground motions from the M_w = 6.6 2007 Niigata-ken Chuetsu-oki earthquake were recorded by a free-field downhole array at a nuclear power plant. Site conditions consist of about 70 m of medium-dense sands overlying clayey bedrock, with groundwater located at 45 m. Ground shaking at the bedrock level had a geometric mean peak acceleration of 0.55g, which reduced to 0.4g at the ground surface, indicating nonlinear site response. One-dimensional ground response analysis of relatively weak motion aftershock data provides good matches of the observed resonant site frequencies and amplification levels, provided small-strain damping levels somewhat larger than those from laboratory tests are applied. Nonlinear ground response analyses of strong-motion data using laboratory-based modulus reduction and damping relations valid up to moderate strain levels (＜～0.5%) produce unrealistic strain localization at a velocity contrast. A procedure is presented to more realistically represent the large-strain portion of backbone curves by asymptotically approaching the shear strength at large strains, which removes strain localization for this application and provides reasonable matches of observed and computed ground motions.