汶川Ms8.0地震后龙门山断裂带地壳应力场及其构造意义

CRUSTAL STRESS FIELD AND ITS TECTONIC SIGNIFICANCE NEAR THE LONGMENSHAN FAULT BELT,AFTER THE WENCHUAN Ms8. 0 EARTHQUAKE

2008年5月12日在青藏高原东缘龙门山断裂带中段发生汶川8.0级特大地震。大震发生时释放应力并对震源区及外围构造应力场产生影响,受汶川地震断层破裂方式和强度空间差异性的影响,震后龙门山断裂带地壳应力场也应表现差异特征,至今鲜有针对该科学问题深入的分析和讨论。经过系统收集、梳理汶川地震后沿龙门山断裂带水压致裂地应力测量数据与2008年汶川地震中强余震序列震源机制解资料,对汶川地震后龙门山断裂带中上地壳构造应力场进行厘定,通过与震前构造应力场对比,深入探讨了汶川8.0级地震对龙门山断裂带地壳应力场的影响,进而对汶川震后应力调整过程及青藏高原东缘龙门山地区深部构造变形模式进行研究,研究结果表明:受汶川8.0级地震的影响,震后龙门山断裂带地壳构造应力场空间分布具有差异性,近地表至上地壳15 km深度范围,映秀—青川段最大主应力方向为北西西向、地应力状态为逆走滑型,青川东北部最大主应力方向偏转至北东东向、应力状态转变为走滑型;15~25km深度范围,龙门山断裂带最大主应力方向仍为北西—北西西向、应力状态以逆冲型为主。汶川8.0级地震后,龙门山断裂带中地壳北西西向逆冲挤压的构造应力特征进一步支持了青藏高原东缘龙门山地区东西两侧刚性块体碰撞挤压、逆冲推覆的动力学模式。

The great Wenchuan M s8.0 earthquake of May 12, 2008 occurred on the central segment of the Longmenshan Fault Belt. Stress was released when the earthquake struck and stress field of seismic source and peripheral areas were affected. Due to the spatial difference in rapture mode and intensity of fault in the Wenchuan earthquake, the crustal stress field in the Longmenshan Fault Belt should also show various features after the earthquake. So far, there have been few in-depth analysis and discussion on this scientific problem. In this study, the hydraulic fracturing in situ stress measurement data along the Longmenshan Fault Belt as well as the focal mechanism solutions of the strong aftershocks sequences after the Wenchuan earthquake are collected and analyzed, and the mid-upper crustal tectonic stress field of the Longmenshan Fault Belt is determined. The influences of the Wenchuan earthquake on the crustal tectonic stress field are thoroughly discussed through comparing with the regional tectonic stress before the great earthquake, which provides a scientific reference for understanding the stress adjustment process after the Wenchuan earthquake and the deep structural deformation model of the Longmenshan area on the eastern margin of Qinghai-Tibet Plateau. The results reveal that some distributional diversity of the crustal tectonic stress field does exist. At the depth of 0-15 km, the maximum principal stress is orientated at NWW near the Yingxiu-Qingchuan segment of the Lonmenshan Fault Belt and the stress state is characterized by TS, whereas, it is NEE with the SS stress regime at the northeastern Qingchuan area. At the depth of 15-25 km, the principal compressive stress is still orientated at NW-NWW with the broad stress state of TT, which is consistent with the regional tectonic stress field before the Wenchuan earthquake. The NWW-trending compression tectonic stress field beneath the middle crust of the Longmenshan Fault Belt further supports the rigid collision tectonic deformation in the Longmenshan area with collision-compression and thrust-nappe structures.