Attenuation characteristic of the response spectrum platform value for 2008 Wenchuan M_S8.0 earthquake

After the 2008 Wenchuan M_S8.0 earthquake, a large volume of strong motion records were acquired, which provides important basic data for the study on attenuation characteristics of seismic response spectrum parameters. This paper calculates the acceleration response spectra of strong motion records from 174 seismic stations with known site conditions. The spectra are calibrated via piecewise fitting with the least-squares method in order to fit the attenuation relationships of a response spectrum platform value. This paper also comparatively analyzes the attenuation characteristics of both vertical and horizontal response spectrum platform values so as to demonstrate how the quantitative study of site condition affects response spectrum platform values by means of the site attenuation influence coefficient of platform value. It also quantitatively analyzes the attenuation rates of different attenuation curves by means of their descending rates. The calculation results show that the site attenuation influence coefficients of horizontal platform value has an average value that is different for the three site types. The value for site type Ⅰ is 0.5358, type Ⅱ is 1;and type Ⅲ is 1.579, which is the minimum attenuation rate of the platform in all site types.