1. Why North China is seismically active while South China remains largely aseismic? Youqing Yang & Mian Liu, Dept. of geol. University of Missouri-Columbia (Email: yangyo@missouri.edu) Conclusions The contrasting seismic activity between the NCB and SCB can be explained by their different tectonic setting and lithospheric structures. The NCB is seismically active because it is situated between the stable Mongolian shield and the Tibetan plateau, which transmits compressive stress from the indenting Indian plate. The seismicity is further enhanced by the thinner and weaker lithosphere in NCB, and the numerous active faults. The SCB is relatively stable because it lies in the pressure shadow of the Indo-Asian collision: the major strike- slip faults bounding the eastern side of the Tibetan plateau and between NCB and SCB, in addition to a relatively free SE margin of the Asian plate, allows a uniform southeastward motion of the whole SCB with little internal deformation. Numeric Modeling and Results Model B: Assuming a thinner and weaker lithosphere beneath NCP and SULU block, and including simulation of slip on the boundary faults, leads to concentrated shear stress and energy release within NCP and the margins of Ordos plateau. (a) Differential stress. (b) Rate of energy release. A 3-D viscoelastic (Maxwell) model is used to explore the factors contributing to the contrasting seismicity between the NCB and SCB. The model includes eastern Tibetan plateau (TP), most part of SCB, and the Alashan-Mongolia shield (AM). NCB is subdivided into Ordos plateau, North China plain (NCP) and Sulu from west to east, according to their distinctive tectonic history. Different units may have different rheology. Displacement boundary conditions, based on the GPS velocity, are applied to the four edges. Both surface and bottom are supported by Winkler’s spring mattress. Topographic load is explicitly calculated using Etopo5 data. Introduction: The North China block (NCB) is part of the Archean Sino-Korean craton that was reactivated since Late Mesozoic. Today, NCB is one of the most seismically active regions in the world at intraplate settings, with some of the most devastating earthquakes in human history, including the 1975 Tangshan earthquake (M=7.5) that killed >250,000 people. In contrast, the South China block (SCB), a Protozoic craton that collided with NCB in the Mesozoic, has remained largely aseismic, although it has stronger custal deformation than the NCB through the Mesozoic and Early Cenozoic and it is closer to the plate boundaries. We explored the causes of such contrasting seismicity between NCB and SCB using a 3-D finite element model constrained by geophysical and geodetic data from these regions. GPS velocities collected by Wang et al. (2001; black), Shen et al. (1999; yellow), and He et al. (2002, blue and red) with respect to the stable Eurasia. NCB has been marked by widespread volcanism and rifting in the Cenozoic, and the lithopshere has been thinned to ~75 km in places. The P-wave travel time tomography beneath China at 110 km depth (upper panel) shows a clear velocity low under NCB. The lower panel is a vertical transect of the upper mantle along 39 o N. (From Liu et al., Tectonophysics, in review) Left: schematic tectonic map of China and surrounding regions. Right: seismicity in NCB and SCB (M>5). The blue circles are the epicenters of historical earthquakes before 1900 AD, including the most deadly Huaxian M8 event in 1556, which killed about 800,000 people. The red circles are those from 1900 to 1985. The green focal mechanism is from 1976 to 2002 with magnitude > 5. Solid lines delineate the major faults separating the different tectonic units. These boundaries are included in our geodynamic models. Model A: homogeneous crustal rheology for all units. (a) NCB has larger differential stress than SCB because it is situated between the stable Alashan- Mongolian shield and the Tibetan plateau, which transmits compressive stress from the Indo-Asian collision. The short bars indicate the direction of maximum horizontal compression. (b) Rate of mechanical energy (=stress x strain rate) in the Tibetan plateau and NCB is higher than other units, consistent with the observed seismicity pattern. The “beach balls” are lower-hemisphere projection of the 3-D stress state in the upper crust (10 km beneath the surface). a b a b