【摘要】 龙门山断裂带在先后5年中发生了2008年汶川M S8强震和2013芦山M S7强震。研究龙门山断裂的地震活动特点,分析地震发展形势成为人们关注的焦点。文中从龙门山断裂带40多年的地震时空演化出发探讨了汶川地震孕育过程中的几个现象:1)孕震范围远小于发震范围,说明在有限断层段上孕育的地震在快速失稳过程中可以连接和扩展成规模很大的失稳断层;2)汶川地震前孕震区小震密集,持续了8年以上,而无明显错动和变形,意味着汶川断层段可能经历了强烈的碎裂方式的挤压应变,在碎裂达到一定程度后失稳;3)为进一步了解震前主震区附近的变化,研究了紫坪铺水库专用台网记录的2004年以来主震附近区域的小震活动的时空过程。结果显示,震前主震附近区域的地震活动曾在2005年10月和2006年10月发生过2次沿断层走向的扩展,扩展时间恰巧与紫坪铺水库2次高水位相呼应。其中,2006年10月地震活动的扩展范围大,震级高。这对说明汶川地震最后的失稳过程十分重要。此外,文中还讨论了汶川地震的发生过程与断层几何相关以及芦山地震没有直接发生在汶川地震后,而是发生在多年后的可能原因。研究结果有助于认识逆断层型强震孕育过程。更多还原

【Abstract】 Within almost five years,the 2008 Wenchuan MS 8.0 and 2013 Lushan MS 7.0 earthquakes ruptured the Longmenshan Fault zone successively.The characteristics of earthquakes and their development tendency on this fault zone have been a focus of subject of research.This article attempts to explore some features of seismic preparation process of the 2008 Wenchuan event from temporalspatial evolution of earthquakes along the Longmenshan Fault zone during more than 40 years.(1) The spatial range of the earthquake preparation,or seismic nucleation,is much smaller than that of coseismic rupturing.It indicates that the seismic source,probably consisting of some small asperities or barriers,prepared on a finite fault segment can be connected and expand into a large-scale rupture section along the fault when the fast instability occurs at the source.(2) Prior to the 2008 Wenchuan giant shock,its preparation area had experienced a dense distribution of small earthquakes for eight years or more,while no conspicuous slip and deformation were observed on the surface.This implies that the seismogenic fault segment of the Wenchuan event on the Longmenshan Fault was undergoing probably compressive deformation,accompanied with cataclastic process.When the cataclastic deformation of the great-shock source reached a critical state,fault instability occurred along the fault with rapid rupturing.(3) To further study the variations of the main-shock area prior to the event,this article analyzes the temporal-spatial processes of small earthquakes around the main shock since 2004 recorded by a special seismic network in the Zipingpu reservoir.The results indicate that the scope of the seismicity expanding along the fault took place along the fault in October 2005 and October 2006,respectively,in accordance with the time when the reservoir reached its high water level.Among them,the second expanding from October 2006 covered a relatively large area and with relatively big magnitudes,implying great importance for the study of the final instability process of the 2008 Wenchuan huge earthquake.Besides,this paper discusses the correlation of the rupturing process of the 2008 Wenchuan giant event with the geometry of the fault and the reason why the 2013 Lushan earthquake occurred many years after the Wenchuan event rather than immediately following this giant shock like usual big aftershocks.The research results are helpful for understanding of seismogenic processes of major earthquakes of the thrust type.更多还原