Characteristics of Near‐Fault Vertical and Horizontal Ground Motion from the 2008 Wenchuan Earthquake

Abstract

AbstractCharacteristics of near‐fault vertical and horizontal ground motion from 2008 Wenchuan earthquake are investigated in this paper. Records from 40 strong motion stations along the Beichuan‐Yingxiu fault with fault distance smaller than 120km are used as database. Based on the analysis of peak ground acceleration, peak velocity, spectra acceleration and spectra ratio of vertical to horizontal ground motion, the following conclusions can be made: (1) Hanging wall effects are shown in PGA for near‐fault ground motions within 60 km to the surface rupture, the empirical model results in up to a 30%~40% increase in peak horizontal and vertical accelerations on the hanging wall over the distance range of 3 to 60 km relative to the median attenuation for the Wenchuan earthquake. The peak acceleration residuals on the hanging wall appear to be biased to positive values whereas the residuals of footwall sites are negative. The EW component of horizontal accelerations is larger than the NS component and attenuates more slowly. (2) There is a lack of long period component in near‐fault ground motion of Wenchuan earthquake, the value of spectrum acceleration decreases rapidly in the long period range. The maximum spectrum acceleration is only 0.5 g even at locations very close to the main causative fault at period 1.0 s. Large distinctions are shown in the distribution of acceleration spectrum ratio (Vertical/Horizontal) for near‐source ground motion at longer periods (T=0.2 s, 0.5 s, 1.0 s) with respect to short periods (T=0.05 s, 0.1 s). (3) High intensity of UD component is shown for near‐fault ground motion, the PGA ratio (Vertical/Horizontal) can be as large as 1.4. High intensity of UD component is more obvious on the hanging wall, and the PGA ratios on the hanging wall are much larger compared with the footwall. For some near‐fault records, the acceleration spectral ratio (Vertical/Horizontal) can even be larger than 1.5. It can also be concluded that the spectral ratio decreases with the increasing of fault distance at short periods (<0.1 s).