【摘要】 针对汶川8.0级地震的主破裂面是否以陡立倾角延伸至地壳深部的争议,我们的研究旨在明确发震断层形态随深度分布的二维结构特征,即断层的倾角及相应段落的深度,并在此基础上分析发震断层的孕震和发震机理.利用子空间置信域非线性反演方法,通过拟合近场的同震水准变形,获得了分别对应于清平北川和南坝一青川发震断层的二维弹性位错模型.结果显示,此次龙门山中央断裂带的发震断层系统存在明显的南北分区特征.以北川南坝为过渡带,北川以南至清平的发震构造为二元结构,包括两部分:一是浅部高倾角的逆断层,倾角在70°～80°以上,底部深度可达10～15 km,同震位错主要发生在10 km以上深度,平均位错超过6 m;二是深部缓倾角的逆断层,反演得其倾角约25°,底部可达30 km深度,位错主要分布于断层的顶部和底部,平均位错约4 m.南坝以北的发震断层为单一结构的逆断层,倾角约60°～70°,逆断层位错分布于10 km深度以上,平均位错小于2 m.余震分布和主震震源机制也支持清平以南发震断层为二元结构的推论.有限元模拟显示,在二元结构的逆断层系统中,通过提高断层面上正的库仑应力,深部缓倾角的逆断层活动对浅部高倾角逆断层有明显的促震作用.模拟还显示地壳缩短不是现今松潘地块地表垂直变形的主要原因,垂直变形更可能反映了青藏高原东缘相对四川盆地的差异抬升.对高倾角逆断层的库仑应力作用显示,差异抬升对龙门山逆断层活动有重要的促进作用.更多还原

【Abstract】 The investigations on the surface rupture zone of Wenchuan M_W8.0 earthquake revealed that the shallow portion of Yingxiu-Beichuan fault slipped co-seismically and reversely with a dip-angle of 70°, which is significantly steeper than both the dip of initial focus and the theoretical speculation on maximum dip of thrust faults.Although the view that the shape of high dip-angle may extend to deep crust is supported by trapped wave study, some others argue that the steep fault plane merely dwells within upper crust (<10 km) and the majority of co-seismic fault is a listric fault with lower dip-angle in deeper crust.In this study, we focus on the 2D character of fault shape versus depth, i.e., the dip of fault segments and corresponding depth. On the basis of geometrical features of co-seismic fault, we further discuss how stress accumulated before the earthquake and why such steep reverse fault is capable of moving.Using subspace trust region method for non-linear problems to fit the co-seismic near field leveling data, we obtain 2D elastic dislocation models respectively corresponding to two co-seismic fault segments of Qingping-Beichuan and Nanba-Qingchuan segment.The results display apparent geometrical diversity along middle Longmenshan thrust belt from south to north.Lying south to the Beichuan-Nanba transition zone, the sub-fault breaking through south Beichuan to Qingping is featured by a duplex faulting system, which is divided into two layers in depth.The upper portion steeply dips 70°～80° with bottom depth of 10～15 km and suffered on average over 6 m co-seismic reverse dislocation above 10 km depth.In contrast, the lower and gentler portion, standing with dip-angle of 25° and bottom at the depth of 30 km, suffered 4 m average co-seismic dislocation near its top and bottom edges.To the north of Nanba, the co-seismic sub-fault dips 60°～70° in a shape of simple plane, on which the average dislocation is lesser than 2 m and is mainly distributed above 10 km depth.Moreover, we speculate that the sub-fault lying south to Qingping is also a duplex faulting system, which is consistent to previous studies on the distribution of aftershocks and the focal mechanism of main earthquake.As uncovered by finite-element modeling, within the duplex reverse-faulting system, the thrust events on the lower and gentler portion obviously increase the positive Coulomb stress on the upper portion, through which a major earthquake with primary reverse component and simultaneously high dip-angle can be stressed to happen.As indicated by the modeling results, rather than crustal shortening the present vertical deformation of Songpan block may reflect the differential uplift of Eastern Tibet versus Sichuan basin.The process of differential uplift is calculated to be positive function of Coulomb stress on high dip-angle reverse fault, which exerts significant enhancement on Longmenshan earthquakes on steep reverse fault.On the other hand, the vertical extension caused by the differential uplift may balance a certain amount of lithostatic pressure, which is prone to cause deficient vertical principle compression against fluid pressure and consequently provides an essential condition for earthquakes on high dip-angle reverse fault.更多还原