Effects of site conditions on earthquake ground motion and their applications in seismic design in loess region

The Loess Plateau is an earthquake prone region of China, where the effects of loess deposit on ground motion were discovered during the 2008 Wenchuan earthquake(Ms8.0) and the 2013 Minxian-Zhangxian earthquake(Ms6.6). The field investigations, observations, and analyses indicated that large number of casualties and tremendous economic losses were caused not only by collapse and damage of houses with poor seismic performance, landslides, but also amplification effects of site conditions, topography and thickness of loess deposit, on ground motion. In this paper, we chose Dazhai Village and Majiagou Village as the typical loess site affected by the two earthquakes for intensity evaluation, borehole exploration, temporary strong motion array, micro tremor survey, and numerical analysis. The aim is to explore the relations between amplification factors and site conditions in terms of topography and thickness of loess deposit. We also developed site amplification factors of ground motion for engineering design consideration at loess sites. The results showed that the amplification effects are more predominant with increase in thickness of loess deposit and slope height. The amplification mayincrease seismic intensity by 1 degree, PGA and predominant period by 2 times, respectively.

Probabilistic assessment of surface level seismic hazard in India using topographic gradient as a proxy for site condition

This paper presents spatial variation of seismic hazard at the surface level for India,covering 6-38°N and 68-98°E.The most recent knowledge on seismic activity in the region has been used to evaluate the hazard incorporating uncertainties associated with the seismicity parameters using different modeling methodologies.Three types of seismic source models,viz.linear sources,gridded seismicity model and areal sources,were considered to model the seismic sources and different sets of ground motion prediction equations were used for different tectonic provinces to characterize the attenuation properties.The hazard estimation at bedrock level has been carried out using probabilistic approach and the results obtained from various methodologies were combined in a logic tree framework.The seismic site characterization of India was done using topographic slope map derived from Digital Elevation Model data.This paper presents estimation of the hazard at surface level,using appropriate site amplification factors corresponding to various site classes based on V_（S30） values derived from the topographic gradient.Spatial variation of surface level peak horizontal acceleration（PHA） for return periods of 475 years and 2475 years are presented as contour maps.