Seismic Site Conditions of China Based on Geology

Abstract:

Site conditions are essential parameters for assessing seismic hazards related to strong ground motion. Due to generally limited availability of measured Vs30 data, regional scale site condition classification is usually based on surface geology, terrain, or topographic slope. As a step to develop a seismic hazard map of China, we make a 1:1,000,000 scale site condition map, which is also the scale of the active fault database of China.

Based on the geological database compiled by Li et al. (2006), we classify each geological unit by age and petrological description. Following Wills and others (2000, 2006), the geological units are assigned to NEHRP classes. A 1:250,000 scale Quaternary geological map, compiled by Zhang (1990) is also used to augment the classification of Quaternary sediments.

About half of the currently available Vs30 data, which were surveyed at the Strong Motion Network stations mostly in Gansu, Sichuan and Yunnan Provinces, do not match our results. We attribute the mismatches to poor location control and the downward extrapolation of shallow boreholes.

The preliminary results show a pattern characterized by regional tectonics and climate settings. The subsiding basins of the Jianghan Plain in central China, Hetao Basin in north China, Ruoergai Basin and the Yangtze River Delta have large patches of class E soil. South China is mostly class B and C due to stable tectonics and removal of sediments by heavy rainfall. Tectonically active southwest China is also mostly class B and C because of strong erosion and little local sedimentation. North and northwest China are dominated by class D and B, with some class C due to Early Pleistocene rocks. Inside the Tibetan Plateau, low relief and active compression of young rocks produce stripes of class B, C and D, with small patches of class E for lacustrine sediments.