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Chen C.-Y.,Institute of Geology | Chen C.-Y.,First Crustal Monitoring and Application Center | Chen C.-Y.,Institute of Earthquake Science | Ren J.-W.,Institute of Earthquake Science | And 6 more authors.
Earthquake | Year: 2012

We have collected GPS data in the period of 2001-2009 from the Crustal Motion Observation Network of China in northwest of Sichuan, and the newest observation data from observed campaign stations which were built under the support of the National Key Basic Research Development Program, to study the characteristics of present-day crustal horizontal motion velocities in the research zone. Strain rate components are computed in spheric coordinate system. Analyses of the spatial distribution strain rate fields derived from GPS measurements are carried out in contrast with seismological and geological results. Results show that the horizontal motion of the GPS stations are consistent with the clockwise rotation of the whole Tibetan Pleatau around the Eastern Himalayan syntaxis. From west to east and northeast, the velocity decreases obviously. The principal compression strain rates are apparently greater than principal extension strain rates, especially in Tazang fault, the eastern part of Dongkunlun fault and the middle part of the Longmenshan fault. The principal compress strain is obviously in the eastern segment of the Dongkunlun fault. Combining analysises with the data of earthquake geology and active tectonics, we suggest that the motion characteristics may be changed from west to east, in the west, the fault shows an obviously left lateral strike-slip, but in the eastern segment, it not only show a left lateral motion but also an compress movement. The motion characteristics of the boundary fault plays an important part in adjusting the relative motion with the blocks. Based on the velocity filed and strain filed from GPS data, we think that the motion in the research zone can be thought as a block motion, most obvious deformation mainly on the boundary faults of the active blocks. Source

Li J.,China Earthquake Administration | Tang T.-M.,China Earthquake Administration | Jing Q.,China Earthquake Administration | Lou G.-S.,First Crustal Monitoring and Application Center | Liu W.-Y.,Second Crustal Monitoring and Application Center
Earthquake | Year: 2010

Based on the statistics of cross-fault deformation anomalies before strong earthquakes during the last forty years, this article analyses the characteristics of the time duration, spatial distribution, amplitude of the anomalies and the relationship of all the three factors, it disgusses the anomaly characteristics of the measurement of cross-fault deformation as well as the maximum spacing of the cross-fault sites and observation cycle of the monitoring of different magnitude of earthquakes. The results suggest that: The anomalies are mainly of medium term as well as short and medium terms, occur more frequently one year before the earthquakes. The time duration of the anomalies is directly proportional to the earthquake magnitude. The spatial distribution of the anomalies also has a phenomenon that it extends as the augment of the epicenter distance. The amplitude of the anomalies enlarges with the increase of time duration and reaches to higher level when the epicenter distance is within 160 to 350 km. The anomaly shape is devided into two main types:trend and mutant. The spacing of monitoring cross-fault sites of different earthquake magnitude can be different; The spacings for monitoring magnitude 7 earthquakes should be less than 450 km, and 300 km, 200 km, 100 km for magnitude 6,5,4 earthquakes respectively. The measurement cycle is supposed to be within three months. Source

Chen C.-Y.,China Earthquake Administration | Chen C.-Y.,First Crustal Monitoring and Application Center | Ren J.-W.,China Earthquake Administration | Meng G.-J.,China Earthquake Administration | And 5 more authors.
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2013

Based on the concept of active blocks, we analyze the major active faults and secondary faults, historical earthquakes, field geological investigation, and other data, then divide the eastern segment of Bayan Har block and its adjacent regions into four first grade blocks: Bayan Har block(I), South China block(II), Sichuan-Yunnan block(III), and West Qinling block(IV). According to GPS velocities, data of geophysical fields and geomorphological features, we further divide the Bayan Har block into three subblocks, Aba (I1), Maerkang (I2), and Longmenshan (I3), and divide the Xiqinling block into two subblocks, Minxian (IV1) and Lixian (IV2), and then check the relative independence among those blocks by F-test. The modern translation and rotation rates of those blocks were calculated using the GPS data from the Crustal Motion Observation Network of China between 1999 and 2007. The results show that besides the eastward or southeastward translation the subblocks also rotate around the Euler pole; the slip rates for the eastern Kunlun fault, Ganzi-Yushu fault, and the Xianshuihe fault are much higher than that for the Longmenshan fault; the faults trending northwest or northwest-west show apparently left-lateral strike-slip and with a tensile characteristic, and those faults trending northeast show obviously right-lateral strike-slip and compression characteristics, except for the Chengxian-Taibai fault which shows left-lateral and compression characteristics. The eastward motion of Bayan Har block is accommodated mainly by the Longriba fault and Minjiang fault, and the right-lateral horizontal component of Longriba fault is obviously ~4.8±1.6 mm/a, and this may indicate that the eastward motion of Bayan Har block is discontinuous. The Longmenshan fault was divided into three segments by Maerkang subblock, Longmenshan subblock and Minxian subblock. The strain rate on the middle part of the Longmenshan fault is smaller than that on the southern part, and the deformation northwest of the fault shows that near the fault the strain rate becomes smaller, it maybe imply that high strain energy had accumulated before the earthquake, which promoted the surface rupture. The change from thrust motion to thrust and right strike-slip motion revealed by the surface ruptures and focal mechanism of aftershocks of Wenchuan earthquake may be related to the change of the stress on the middle part of the Longmenshan fault from southwest to northeast. The low velocity of Maerkang, Longmenshan, and Minxian subblocks relative to the South China block, the low strain rate and the tightly locked character of Longmenshan fault may determine the low slip rate of the Longmenshan fault. Source

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