关键词: 强震, 活动地块, GPS基准站, 地壳形变, 位移时间序列曲线

Abstract: The most remarkable feature of Cenozoic and present-day tectonic deformation of the continental lithosphere of China is that the crust has been cut by huge late Quaternary active faults, forming active crustal blocks of different orders. Various active crustal blocks exhibit different horizontal movement and different deformation styles. The inner part of the active crustal block is relatively stable. Deformation commonly takes places along their boundary structures, and most of the great earthquakes (M≥7) occur along these boundaries. In order to monitor crustal movement in China mainland, China Crustal Movement Observation Network has disposed 25 continuous GPS base stations in the main tectonic units all over the country. These stations had been run for 3 years from March 1999 to December 2001. On 14 Nov. 2001, an earthquake of M_S 8.1 occurred to the west of the Kunlun Mountain Pass. This event has produced a surface rupture zone of more than 350km in length with a general strike of 70°～90°. The rupture zone is dominated by left-lateral strike-slipping, and the largest horizontal displacement is about 6m. The observation data of continuous GPS measurement stations show that various GPS stations in different active blocks around this earthquake site had different responses to the earthquake. The GPS station within the active block where the earthquake occurred, such as the Delingha station, exhibited very obvious displacement. However, no obvious displacement was observed at the GPS stations located in the active blocks that are secluded by one active block from the earthquake site,such as the Lhasa GPS station. If the GPS stations are located on the boundary structures of the active blocks adjacent to the earthquake site, such as the Xiaguan GPS station, then they would record obvious displacements several days after the occurrence of the earthquake. If the stations are located within the active blocks, such as the Xining and Kunming GPS stations, no obvious displacement would be observed. However, no obvious displacements was observed at the Xiaguan GPS station after Burma earthquake (M=7.2) occurred in the north of Burma active block, although the epicentral distance of this earthquake (about 370km) is significantly less than that of the west of Kunlun Mountain Pass earthquake. This can be attributed to the relative small magnitude of the Burma earthquake, which did not cause the compression of the Sichuan-Yunnan active block. This fact may indicate that the deformation on the boundary zone of the active block is obviously stronger than that occurs within the block, and it is independent to the epicentral distance. The difference of the effects of great earthquakes on its adjacent active blocks depends mainly on the mode of action on the adjacent block by the movement of active block where the great earthquake occurs. If the movement of the block results in compression of the adjacent block, then the effect of the earthquake will be obvious, while the movement does not result in compression of the adjacent block, no obvious effect can be recorded by the GPS station in this block, because the effect may rapidly decrease when it passes through the boundary zone of the block. The observation data of the GPS stations in response to great earthquake-demonstrate that more effective monitoring of earthquake related crustal movement can be fulfilled, provided that the GPS stations are reasonably disposed within the active blocks and on their boundary zones.

Key words: great-earthquake, active crustal blocks, crustal deformation, continuous GPS measurement