Vertical Ground Motion Characteristics of Wenchuan Earthquake

The Wenchuan earthquake of May 12, 2008(M-w=8.0) was the most significant earthquake that had affected Chinese Mainland since the Tangshan earthquake of 1976. The dense network of seismographs deployed in this region recorded ground motion acceleration data with good quality. In this paper, vertical ground motion characteristics of Wenchuan Earthquake are investigated from the database including 282 records at 94 stations. Special attention is focused on the behavior of the recorded vertical ground motion and its relation to the horizontal ground motion counterparts. The results indicate that the average value of the ratios of vertical to horizontal peak ground acceleration (a(v)/a(h)) is 0.58, but the ratios of over 30% stations exceed the rule-of-thumb value of 2/3. This a(v)/a(h) is much discrete in the near-fault region and tends to decrease with the increase of epicentral distance. Furthermore, site condition also influences this ratio. The curve of vertical to horizontal acceleration response spectral ratio (S-av/S-ah) with period ranging from 0 to 6s appears to be saddle-shaped overall and S-av/S-ah value varies within different period range. It is also can be seen that the vertical acceleration response spectrum (S-av) looks thin compared with the horizontal acceleration response spectrum (S-ah). Except several stations, the ratio of vertical to horizontal ground motion predominant period (T-pv/T-ph) is less than 1.0, and it increases with the increase of epicentral distance and becomes smooth gradually.