Analysis Of Block Strain And Fault Zone Deformation Characteristics In Sichuan-yunnan Region Using Block Deformation Model And Negative Dislocation Model

According to elastic rebound theory, earthquake occurrence is due to the crust rockfracture dislocation. The rock itself is elastic, and when rupture occurs, the rock havingelastic deformation overall rebounds in the opposite direction after the force disappears andis back to the undeformed state. This rebound can produce amazing speed and strength,release suddenly long-term savings of energy, and cause earthquakes. Therefore, the study ofcrustal deformation can be used as an important means of earthquakes study. When we studythe relationship between the crustal deformation and the earthquakes, we should not onlystudy the deformation characteristics of the crustal surface, but also study the strainaccumulation, fault locking and fault slip deficit of the fault inside. We focus on theSichuan-Yunnan region in this paper where there are plentiful GPS observations and seismicrisk is big. Studying on the model of rotation in the entire block and homogeneous strain(REHSM) and analyzing the GPS velocity profiles, we discussed the rhombic blocksubdivision, the block strain status, fault slip rate and strain accumulation before and afterthe earthquake. By using the negative dislocation model of Defnode, we inverted for thefault locking and fault slip deficit of the Longmenshan fault before the Wenchuan earthquake,and analyzed crustal deformation before earthquake combining with GPS horizontal strainrate around the Longmenshan fault. Rudimental results are showed as below.1. Studying on the model of rotation in the entire block and homogeneous strain(REHSM) and using the GPS horizontal velocity field data of the Sichuan-Yunnan regionduring the period of1999~2007, first, we discussed the rhombic block subdivision by erroranalysis and residuals distribution, and the results show it is better to divide theSichuan-Yunnan block into two subblocks by the Lijiang-Xiaojinhe fault. Then, we analyzedthe strain status of the west-northern segment of Sichuan block-the middle segment ofYunnan block-the west-southern segment of Yunnan block, the results display the directionof the principal compressive strain rate of the blocks is characterized by clockwise rotation.The principal compressive strain rate of the west-northern segment of Sichuan block in2007~2009is more doubling than that in1999~2007, which makes its surface strain changefrom tension to compression and shows that the Wenchuan earthquake affects it obviously. Meanwhile, the strain status of the middle segment of Yunnan block remains basicallyunchanged before and after the earthquake, which shows that the Wenchuan earthquakeaffects it little.2. Using the velocity field data of the Sichuan-Yunnan region during the period of1999~2007and2007~2009, we studied the movement and deformation characteristics ofprincipal faults by calculating the velocity of them with REHSM model and analyzing theGPS velocity profiles. The results show that the Anninghe fault and the Zemuhe fault havesome shear strain accumulation, and the southern segment of the Xiaojiang fault and thenorthern segment of it perform different deformation features, the former mainly beingstrike-slip, and the latter mainly being accumulating strain; the earthquake had relativelyobvious influence on the middle-southern segment of the Lijiang-Xiaojinhe fault, theAnninghe fault and the Jinsha River fault, meanwhile it affected the Zemuhe fault, theXiaojiang fault and the Red River fault less.3. By using the GPS horizontal velocity filed of1999~2007and the negative dislocationmodel of Defnode, we inverted for the fault locking and fault slip deficit of theLongmenshan fault before the Wenchuan earthquake. The inversion results show that themiddle-northern segment of the Longmenshan fault is very high locked before earthquakeand the locking depth is about21km (locking fraction is0.99), in which normalcompressional slip deficit rate is about2.2mm/a and parallel dextral slip deficit rate is about4.6mm/a. Meanwhile the southern segment of the fault is high locked from the surface to12km depth (locking fraction is0.99), in which normal slip deficit rate is about1.4mm/a andparallel slip deficit rate is about4.6mm/a. As the depth increases, the locking fractiondecreases to about0.83at12~16km depth, in which normal slip deficit rate is about1.2mm/aand parallel slip deficit rate is about3.8mm/a. The locking fraction is about0.75at16~21kmdepth, in which normal slip deficit rate is about1.1mm/a and parallel slip deficit rate is about3.5mm/a. The Longmenshan fault turns to creeping gradually between21km and24km.4. We analyzed crustal deformation before earthquake using negative dislocationinversion results of Defnode combining with GPS horizontal strain rate around theLongmenshan fault zone. The inversion results of negative dislocation are consistent with the regional strain results that strain accumulation rate of the middle-northern segment of theLongmenshan fault is low while the southern segment of it is high. They both show that themiddle-northern segment of the Longmenshan fault had accumulated much strain and is veryhigh locked while the southern segment of it is a little low locked and this characteristic canexplain well the rupturing process which propagated from the epicenter to the northeast inone direction.