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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS014 Oral Presentation 2259 GPS Constrain on Active Faulting and Block Rotation in deforming Asia Dr. Wang Qi Institute of Seismology China Earthquake Administration IASPEI We explore a synthesized geodetic velocity field consisting ~2000 GPS velocities from the Crustal Movement Observation Network of China (CMONOC ) and various campaigns throughout a period of 1991-2006. Our analysis of densified velocity data reveals a lot of unspecified features of active deformation. A velocity profile along the direction of N15E across Tibet shows a pattern of localized deformation on narrow zones of 50-100 km-wide associated with major Quaternary faults and sutures. The maximum velocity gradient is located on the E-W trending Gangdes, Gaize-Amdo and Fenghuoshan Thrusts. The thrusting slip rates are in range of 6-8 mm/yr comparable with striking-slip rates on the sub-parallel Karakorum-Jiali, Manyi-Ganzi and Kunlun faults. The geodetic slip rates of major faults in Tibet are 4-9 mm/yr. The fast rate of 14-18 mm/yr occurs on frontal thrusts at Himalaya, Pamir and southwest Tienshan , and less than 4 mm/yr elsewhere outside Tibet. The overall geodetic velocity field is predicted well within the uncertainty of 1-2 mm/yr in terms of rigid rotation model of 30 crustal blocks intersected by the Quaternary faults. The GPS inferred rates are in general agreement with that of long term faulting or its lower bound on the most faults. For example , the current geodetic rates are l one factor of 2-3 smaller than the long tern rates of 20-30 mm/yr inferred from fault offset feature since Holocene on the Altyn Tagh fault and Karakorum fault. Internal deformation within ~30 intervening blocks hundreds of kilometers in dimension is relatively minor at level of several nano-strain /yr. The GPS-inferred slip rates on the major faults in Tibet indicate that the fault slip associated with a combination of crustal-scale block subducting, rifting and lateral extruding, has played a dominating role in accommodating continental convergence on India-Eurasia plate boundary. Our block model suggests that the tenet of plate tectonics may be applicable to characterize crustal deformation in Asia, though the large-scale extrusion out of Tibet through rapid slip-rate faulting is not necessarily invoked. Keywords: gps, active deformation, china

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS014 Oral Presentation 2260 Plate deformation and crustal structures at the southern end of the Ryukyu subduction zone off eastern Taiwan Prof. Char-Shine Liu Institute of Oceanography National Taiwan University Philippe Schnurle, Serge Lallemand, Yvonne Font Multichannel seismic reflection profiles and swath bathymetry data reveal detailed morphotectonic structures of the southernmost Ryukyu arc-trench system east of Taiwan. Based on the different structural styles, the southernmost section of the Ryukyu arc-trench system can be divided into three zones. The first zone lies to the east of 123oE. It presents structures of a typical oblique subduction zone with frontal accretion of trench sediments, imbricated thrusts and folds in the accretionary wedge (the Yaeyama Ridge), and slightly folded forearc basin strata. A prominent right-lateral strike-slip fault is developed at the rear of the accretionary wedge which accounts for, at least particially, the lateral component of the oblique plate convergence, and may indicate the location of thesouthern boundaryof the overriding Eurasia plate. The second zone lies between 123oE and 122o10E where the Ryukyu subduction system is terminated by the impinging Luzon volcanic arc. Series of NW-SE trending shear faults cut through the thrusted and folded ridges in the accretionary wedge. A NW-SE trending forearc basin (the Hoping Basin) lies between the accretionary wedge and the Ryukyu arc.Large earthquakes occur frequently along the eastern edge of the Hoping Basin. Seismic reflection data reveal that there is a basement high lies under the eastern half of the Hoping Basin, called the Hoping basement rise This basement rise could be an uplifted feature due to the subduction (or the underplating) of some local asperities. Further investigation using deep seismic profiling and OBS array observation are planned in 2008 to better understand the seismogenic processes at this plate boundary and to evaluate the potential risks of earthquake hazards. Structures in the studied forearc region are mainly controlled by the high obliquity of convergence between the Philippine Sea and Eurasia plates, and by the collision of the Luzon arc with the Eurasia continental margin that terminates the Ryukyu subduction system to the west. Subduction of asperities may also play important roles in shaping the structures of the accretionary wedge and forearc basins here. Keywords: ryukyusubduction zone, forearc structure, seismic reflection

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