汶川地震(Ms8．0)地表破裂及其同震右旋斜向逆冲作用

Co-seisimic Surface Rupture and Dextral-slip Oblique Thrusting of the M_S 8.0 Wenchuan Earthquake

2008年5月12日14时28分,青藏高原东缘龙门山地区发生了震惊世界的汶川地震(MS8.0),地震不仅造成巨大的人员伤亡和财产损失,并形成了迄今为止空间上分布最为复杂、长度最大的逆冲型同震地表破裂带。通过多次野外考查表明,汶川地震(MS8.0)在龙门山断裂带上至少使两条NE走向、倾向NW的映秀-北川断裂和灌县-安县断裂同时发生地表破裂,并沿映秀-北川断裂产生的地表破裂带长度约275km,以逆冲运动伴随右旋走滑为其破裂特征,最大垂直位移量约11m,最大右旋走滑位移量至少约12m;沿灌县-安县断裂产生的地表破裂带长度约80km,表现为纯逆冲运动的破裂特征,最大垂直位移量约4m;另外发育一条长约6km呈NW走向连接于映秀-北川破裂带和汉旺破裂带的小鱼洞破裂带,以左旋走滑兼有逆冲运动为特征。地表破裂基本沿袭早先活动断裂带上,并使早先抬高的地貌更加抬高,表明龙门山地区地震在同一断裂带上重复发生过,并且无数次地震活动(包括类似汶川MS8.0地震的强震)的累积,逐渐形成了现今的龙门山。根据同震断裂面以及断裂面上的擦痕分析表明,汶川地震是由两次破裂事件叠加而成,初期破裂以逆冲运动为主,后期破裂以右旋走滑为主,这种破裂过程与地震波数据反演结果(陈运泰等,2008;Ji,2008;王为民等,2008)一致。在地表破裂带南段(映秀—清平段)叠加了两次不同性质的破裂过程,北段(北川—南坝段)只反映了第二次破裂事件的过程。利用长期滑移速率与汶川地震同震位移对比,估算出在龙门山断裂带上类似汶川地震(MS8.0)的强震复发周期为3000～6000a。通过对比研究,西昆仑山、阿尔金山和东昆仑山与龙门山具有很相似的转换挤压构造特征,斜向逆冲作用是青藏高原周缘山脉快速崛起的主要机制。

At 2：28 pm, 12 May 2008, the Ms 8. 0 Wenchuan earthquake struck the Longmenshan area, the eastern margin of the Tibetan Plateau in Sichuan, China. Besides the huge casualties and property losses, a most complicated yet longest thrust-type co-seismic surface rupture zone was developed in the quake-hit area. Field surveys indicate that the Wenchuan earthquake produced at least two NE-trending NW-dipping surface rupture faults on the Longmenshan fault： Yingxiu-Beichuan fault and Guanxian-Anxian faults. The former about 275 km of surface rupture is characterized by dextral-slip thrusting, with a maximum vertical displacement of 11 m and a dextral-slip displacement of 12 m. The latter about 80 km of surface rupture is characterized by pure thrusting, with a maximum vertical displacement of 4 m. Furthermore, the earthquake also produced a 6 km-long NW-trending Xiaoyudong rupture zone, located at the Yingxiu- Beichuan and the Hanwang rupture zones, and characterized by sinistral-slip thrusting. Surface ruptures occur generally along the previous active faults and raise the elevated landforms again, indicating that earthquakes in the Longmenshan region had repeatedly occurred on the same fault belt and accumulation of numerous earthquakes including strong events like the Ms 8. 0 Wenchuan earthquake result in the formation of the present Longmenshan Mountains. The analysis of the co-seismic fault planes and striation reveals the Wenchuan earthquake is the compound result of two rupture processes. Thrusting and dextral strike-slip dominates the early and later stages, and this is in agreement with the results of seismic wave inversion （Chen et al. , 2008; Ji, 2008; Wang et al. , 2008）. The southern segment of the surface rupture zone （Yingxiu-Qingping segment） suggests two different processes, while the northern only reflects the second event. By comparing the long-term slip rate with the co-seismic displacement, it is estimated that the recurrence intervals of great earthquakes like the Ms 8. 0 Wenchuan earthquake can be 3000 ~ 6000 years. The comparative researches on the West Kunlun, the Altun, the East Kunlun and the Longmenshan Mountains reveal that the similar transpressional structural features the oblique thrusting ---- is the dominative mechanism for uplift of surrounding mountains in the Tibetan Plateau.