川滇菱形块体东边界断层闭锁程度与滑动亏损动态特征研究

利用1999—2007期和2009—2013期中国大陆GPS速度场数据,采用DEFNODE负位错反演程序估算了川滇菱形块体东边界——鲜水河—安宁河—则木河—小江断裂带在汶川地震前后的断层闭锁程度和滑动亏损空间分布动态变化特征,讨论了汶川地震对该断裂系统的影响范围和程度,并结合b值空间分布和地震破裂时-空结果分析了断裂系统的强震危险段.结果表明,汶川地震前鲜水河断裂最南端为完全闭锁(闭锁深度25 km),中南段地表以下10~15 km深度为强闭锁状态,中北段基本处于蠕滑状态;安宁河断裂最南端闭锁很弱,其余位置闭锁深度为10~15 km;则木河断裂除最南端闭锁较弱以外,其余位置基本为完全闭锁;小江断裂在巧家以南、东川以南、宜良附近、华宁以北等四处位置闭锁较弱,其余位置为强闭锁.10年尺度的GPS速度场反演所得断层闭锁程度所指示的强震危险段,主要为鲜水河断裂道孚—八美段、安宁河断裂中段、则木河断裂中北段、小江断裂北段东川附近、小江断裂南段华宁—建水段,该结果与地质尺度的断层地震空区和30年尺度的b值空间分布所指示的危险段落具有一致性.汶川地震后断裂带远、近场速度分布和块体运动状态发生变化,这种区域地壳运动调整使得负位错模型反演得到的断裂带闭锁情况发生一定变化.汶川地震前后川滇菱形块体东边界平行断层滑动亏损速率均为左旋走滑亏损,且在安宁河断裂北端、则木河断裂中北段滑动亏损速率最大;除鲜水河断裂中南段与最南端和小江断裂东川附近以外,其余断裂震后滑动亏损速率均有所增加.垂直断层滑动亏损速率既有拉张亏损也有挤压亏损,且鲜水河断裂最南端由震前挤压转变为震后拉张,其余断裂除了安宁河断裂和小江断裂中段与最北端存在挤压滑动亏损速率外均为拉张速率.

Study on dynamic characteristics of fault locking and fault slip deficit in the eastern boundary of the Sichuan-Yunnan rhombic block

Some research results about Coulomb stress change and co-seismic and post-seismic strain accumulation show that the Wenchuan earthquake does make certain impact on the eastern boundary of the Sichuan-Yunnan rhombic block. The Wenchuan earthquake has happened for more than five years by the end of 2013. For better understanding its influence on the eastern boundary seismic potential, we inverted the spatial fault locking and fault slip deficit in the boundary which contains four major fault zones named Xianshuihe, Anninghe, Zemuhe and Xiaojiang before and after the Wenchuan earthquake, and made some discussions. The GPS velocity field during 1999-2007 and 2009-2013 in China mainland were used in the inversion process and the DEFNODE software package was used as the negative dislocation inversion model. We discussed the influence scope and extent on the fault zones caused by the Wenchuan earthquake, and analyzed the seismic potential sections combining with the spatial distribution of b-values and the spatial-temporal pattern of earthquake rupture.The results show that before the Wenchuan earthquake the southernmost segment of Xianshuihe fault is completely locked to 25 km, the middle-southern segment is tightly locked from the surface to 10~15 km depth, and the middle-northern segment is in creeping state. The southernmost segment of Anninghe fault is weakly locked, while the locking depth of other segments are between 10 km to 15 km. Zemuhe fault is completely locked except the locking fraction of the southernmost segment being about 0.8. Xiaojiang fault is weakly locked in four regions including the south of Qiaojia, the south of Dongchuan, the neighborhood of Yiliang and the north of Huaning, while other segments are tightly locked. The velocity distribution far and near fault zone and the block movement changed after the Wenchuan earthquake, and such adjustment of regional crustal movement made the fault locking state inverted by the negative dislocation model change. The parallel fault slip deficit of the eastern boundary of the Sichuan-Yunnan rhombic block are all positive which means sinistral strike-slip before and after the Wenchuan earthquake, and the slip deficit are maximum in the northern segment of Anninghe fault and the middle-northern segment of Zemuhe fault. The slip deficit of the fault zones all increase after the earthquake except the middle-southern and the southernmost segments of Xianshuihe fault and the segment of Xiaojiang fault near Dongchuan. The vertical fault slip deficit are tensional and compressional deficit, and the movement of the southern segment of Xianshuihe fault changes from compression before the earthquake to tension after the earthquake. Other faults are tensional except Anninghe fault and the middle and northernmost segments of Xiaojiang fault. The GPS sites distribution and fault structure setting didn't have fundamental influence on the inversion results. Although the observation sites of 2009-2013 increased more than that of 1999-2007, but it didn't change the overall trend of the fault locking distribution. The fault locking inverted by GPS velocity between 10 years indicates that the Daofu-Bamei segment of Xianshuihe fault, the middle segment of Anninghe fault, the middle-northern segment of Zemuhe fault, the northern segment of Xiaojiang fault around Dongchuan, and the Huaning-Jianshui segment of Xiaojiang fault are seismic potential segments, which is consistent with the potential segments indicated by seismic gaps along the fault zones of geological scale and the spatial distribution of b-values during 30 years.