PRELIMINARY INVESTIGATION ON CHARACTERISTICS OF VERTICAL GROUND MOTION DURING WENCHUAN EARTHQUAKE
94 suites of acceleration records from the stations in Sichuan,Gansu and Shaanxi provinces during the Wenchuan earthquake of May 12,2008,which were released by China Strong Motion Networks Center,were selected to the process for extracting some essential ground motion parameters.The ratios of vertical to horizontal peak ground accelerations,the acceleration response spectra and predominant period were mainly investigated with the method of statistical analysis.The results indicated that the average ratio of vertical to horizontal peak ground motion is 0.58,and the ratios of more than 30% stations are greater than 2/3.This ratio decreases with the increasing epicentral distance and it is much discrete in the near-fault region.Furthermore,site condition also influences this ratio.The curve of vertical to horizontal acceleration response spectrum ratios with periods from 0 to 6 second appears to be saddle-shaped generally.The shapes of vertical and horizontal acceleration response spectrum vary within different period ranges.And vertical acceleration response spectrum looks thin comparing to the horizontal counterpart.Except several stations,the ratio between vertical to horizontal predominant periods is less than 1.0.This ratio increases with the increasing epicentral distance and becomes smooth gradually.The study results can provide a reference for choosing the vertical ground motion input during a time history analysis.