Abstract: Aftershock activity ofMay 12 M8.0 W enchuan Earthquake sequence shows an obvious segmen-ted feature,most of the large aftershocks are d istributed in the south and north parts of aftershock zone.Thrusting is dom inant in the south partwith a small strike slip component.The aftershock activ-ity decays gradually and presents the sequence feature ofmainshock-aftershock pattern.The north part is the end ing area of the mainshock fracture,where the strike slipping is dom inant and shows an obvi-ous swarm feature,it therefore became the major area for large aftershocks.Modulation of earth tide on aftershock activity is remarkable,most of large aftershocks occur during the period of flood and neap tide.The time period around 16:00 is the dom inant occurring time for large aftershocks.Thep-val-ue,a parameter ofmod ified Omori formula,increases gradually with time,and reaches about 1 in the final.Based on the previous study,the sequence patterns,magnitude ofmaximum aftershock,as well as the duration of aftershock activity have been d iscussed.The primary results also show that the mag-nitude d ifference between mainshock and the maximum aftershock is proportional to the rupture size of the mainshock for huge earthquakes of about M8.0.This means that,when the magnitudes of earth-quakes are nearly the same,large rupture size corresponds to the sufficient energy release.

Key words: the May 12^th M8.0 Wenchuan earthquake, segmentation features, sequence pattern, magnitude ofmaximum aftershock, duration of aftershock activity, rupture size

摘要： 汶川8.0级地震序列具有明显的分段特性,较强余震分布于茂县、绵竹以南及平武以北。主破裂过程在中南段以逆冲为主,序列逐渐衰减,呈主余型的序列衰减特征;北段是汶川地震破裂过程的终止区域,主破裂过程在该区域以走滑为主,形成多震型的序列特征,也成为汶川序列较大余震的主体活动区域。序列较强余震活动明显受引潮力调制,大多数较强余震发生在固体潮大、小潮时段,并且16时前后是较强余震的优势发震时段。序列衰减系数p值随时间增加而逐渐增大,最终基本稳定在1附近变化。结合以往的研究,对序列类型及最大强余震震级、强余震活动持续时间等进行了初步讨论。初步的统计结果还显示,8级左右强震序列中主震与最大余震之间的震级差正比于主震破裂尺度,这意味着当震级大体接近时,较大的破裂尺度与较为充分的能量释放相对应。

关键词: 汶川8.0级地震, 分段特征, 序列类型, 最大强余震震级, 强余震活动持续时间, 破裂尺度