A Seismological and Geodynamic Study of the Seismic Zones Surrounding the Ordos Plateau, China

The fault zones surrounding the Ordos Plateau in China are marked by intense seismicity; the lithospheric structure of these active fault zones are also critical for understanding diffuse plate boundary deformation. On the west side of the plateau is the Liupan Shan thrust belt, which is structurally connected to the Altyn Tagh-Qilian-Haiyuan fault system bounding the northern margin of the Tibetan Plateau and marks the change from shortening and uplift in the Tibetan Plateau to widespread extension in the North China block. The Weihe graben on the south side of the Ordos Plateau is connected to the Qinling-Dabie strike-slip fault that plays an important role in accommodating the eastward escape of the South China block driven by the Indo-Asian collision. During summer 2004, a team from Nanjing University and the University of Missouri deployed 15 stations across the Liupan Shan fold and thrust belt. We have calculated receiver functions that are consistent with a rapid increase in the crustal thickness 20 km to the east of the Lupen Shan mountains. This suggests that the Ordos crust is substantially thicker than the surrounding crust, implying that the topography in the plateau may be overcompensated unless the lower crust consists primarily of dense mafic material. These results have important implications for the interaction of the stable Ordos block and the surrounding lithosphere. We have also observed preliminary evidence of changes in the seismic anisotropy across the Lupen Shan mountains. With additional instruments from Beijing University, we are currently in the process of installing another 30 seismometers across the Weihe graben that should further constrain the seismic structure in this important region.