Attenuation characteristics of peak ground acceleration and spectrum of near-field ground motions

The peak ground acceleration and three kinds of frequency content parameters are calculated and analyzed based on 333 groups of near-field strong motion records collected from earthquakes in Europe and its adjacent areas. The epicentral distance of these strong motion records are less than 40 km. Using the regression analysis method, the influences of site, magnitude and epicentral distance on characteristics of the near-field ground motion are discussed. And the attenuation relationship of the peak ground acceleration and spectrum period of near-field ground motions at different sites and magnitudes is given. The results show that the epicentral distance is an important factor affecting the near field earthquake peak ground acceleration and spectral characteristic. With the increase of the epicentral distance, the peak ground acceleration decreases gradually but the characteristic period increases gradually. The site and magnitude factors have some influence on the peak ground acceleration and characteristic period, but with uncertainty. In the near-field, the average ratio of vertical to horizontal peak ground acceleration is 0.60, but in all records the peak acceleration ratio of about 1/3 vertical ground motion's is larger than 2/3. The characteristic period of vertical ground motion is about 0.8 times that of horizontal ground motion. The softer the site is, the smaller the value is.