铲形逆冲断层地震破裂动力学模型及其在汶川地震研究中的启示

摘要

The 12 May 2008 Mw7.9 Wenchuan earthquake occurred on the Longmenshan fault,which is a classic listric-reverse fault. Using a 2-D elastic finite element model, we obtain some generic understanding on listric fault, and rupture process of the Wenchuan earthquake: (1) If fault strength does not vary along depth, the earthquake would initiate from a spot near the brittle-ductile transition zone of the fault, and propagate upward. The rupture could reach the surface when there is enough energy stored in the crust around the fault system. (2) Once the earthquake rupture breaks to the surface, the coseismic slip would be peaked at the surface, as the case of the Wenchuan earthquake; the shallower the dipping angle of the fault is, the greater the dislocation at the surface would be. (3) Unlike the maximum coseismic slip, the maximum strain energy release density would be located at downdip near the brittle-ductile transition zone of the fault at more than 10 km depth. (4) The shallower the dipping angle of the fault is, the more complete the release would be for the strain energy accumulated during the interseismic time period, and the greater destruction the quake would result at the surface for the near-field region at the hanging wall. (5) Thrust earthquakes occurred on a listric fault could not release all the energy accumulated during the interseismic time period, and furthermore higher strain energy density would be loaded at two regions: one is under the brittle-ductile transition zone of the fault, at a depth range of 15～22 km within the foot wall, and the other is in the shallow crust of the hanging wall, in a depth range of 0～5 km and at about 10+ km horizontal distance from the surface break point. Spatial concentration of the Wenchuan aftershocks reflected the continued strain energy release in these related regions in addition to the continued rupture around the main rupture plane.%2008年发生了汶川地震的龙门山断层带是典型的铲形逆冲断层带.利用二维线弹性有限元模型,得到关于铲形逆冲断层带一些具普适性的认识:(1)如果断层带强度不随深度变化,则地震从断层带转换层附近开始发动,破裂沿断层向上传播,当地震蓄积能量足够大时,破裂可以冲破到地表,如汶川地震.(2)一旦到达地表,其最大同震位错将位于断层带出露地表处;且在断层带介质弱化或损伤大致相同条件下,铲形断层倾角越缓,地表的同震位错量越大.(3)与最大同震位错位于地表不同,同震应变能释放最大处位于地壳内十几公里深处的断层带转换层附近.(4)在断层带介质软化或损伤度大致相同条件下,铲形断层倾角越缓,地震对震间期积累应变能的释放越多、范围越广,对地表近断层区域造成的破坏可能越大.(5)冲破到地表的逆冲型大地震会进一步增加铲形断层带下盘转换层以下(约在15～22 km)地壳深处和断层上盘距离断层地表出露点约十几公里处地壳浅部的应变能积累密度;汶川地震之后的余震分布,除了沿主破裂面附近的继续破裂外,也体现了以上地区应变能的继续释放.本文得到的这些认识有助于深入理解汶川地震的动力学机制.