汶川M_S8.0地震余震震源机制时空分布特征

Spatial and temporal variation of focal mechanisms for aftershocks of the 2008 M_S8.0 Wenchuan earthquake

本文利用CAP波形反演方法,获取了汶川MS8.0地震序列中312个具有较高信噪比波形资料的4级以上余震的震源机制解和震源深度.基于震源深度空间分布与震源机制时空分布,分析了主震后余震区断层行为特征与应力场时空变化,并对龙门山断裂带中北段的发震断层面几何形态进行了初步探讨.获得的主要认识如下:(1)余震震源深度分布存在显著的空间分段差异.绵竹以西的余震区南段与平武以东的北段余震深度范围大于中段(绵竹—平武段),但深度小于5km的5级以上超浅源地震主要分布在明显偏离龙门山断裂带走向的理县NW向分支与余震区北端NNE向分支,而中段余震主要分布在7～19km深度.(2)余震机制类型存在明显的时空差异.余震区中段逆冲型地震占绝对优势,理县NW向分支余震则以走滑型为主,机制类型随时间变化不显著.沿龙门山断裂带走向的余震区南段,早期(2008年8月底前)逆冲型地震比例高于走滑型、晚期走滑型地震比例显著升高并超过逆冲型;而余震区北段早期走滑型地震占绝对优势、晚期逆冲型地震比例大幅上升且超过走滑型.南、北两段余震机制类型比例的显著变化,可能是余震区两端断层调整性运动的表现.(3)节面走向及P轴方位优势方向均存在显著的空间差异.南段NWW向P轴方位与区域应力场一致,中段及理县NW向分支P轴优势方向NEE,而北段具NWW和NEE两个优势方向,这种差异反映了余震活动除了受区域应力场控制外,还受到主震引发的局部应力场的控制.节面走向的多方位分布则反映不同走向的构造参与了主震后的余震活动.(4)沿龙门山断裂带走向,余震区南段具深部缓倾角、浅部高倾角的铲形断面特征;中段深部倾角均值较稳定、浅部倾角均值随深度减小而增大;北段倾角均值相对稳定,显示其断面几何形态相对简单.上述不同区段倾角均值随深度的变化揭示龙门山断裂带中北段断层面几何形态复杂.

Focal mechanism solutions and depths of 312 M≥4 aftershocks with relatively high signal-to-noise ratio waveforms of the 2008 Wenchuan MS8.0 earthquake have been derived by CAP method.Following this,we analyzed the aftershock faulting behavior and spatial-temporal variation of stress field,and further discussed preliminarily the geometry of the central-northern segment of the Longmen Shan fault zone.The major results are as follows.（1） The range of focal depths is significantly different between various segments of aftershock area.The depth ranges of aftershocks on the southern segment west of Mianzhu and northern segment east of Pingwu are greater than that on the central segment from Mianzhu to Pingwu.Relatively shallow（5 km） aftershocks mainly occurred along NW-striking Lixian branch on the southern segment and NNE-striking branch at the northern end of the aftershock area.On the central segment,aftershocks are concentrated at the depths of 7～19 km.（2） The proportion of different-type aftershocks varies with time on different segments,especially on the southern and northern segments.Thrust slip and strike-slip motions are dominant faulting behavior for the central segment and NW-striking Lixian branch,respectively.However,on the southern segment along the Longmen Shan fault zone,thrust-slip aftershocks are more than those of strike-slip in the early stage before Aug.31,2008,and slightly less than those of strike-slip in the later stage.Meanwhile,for the northern segment,the proportion of aftershocks of right-lateral strike-slip type is higher than that of reverse type with minor strike-slip component in the early stage,but it is less than that of reverse type in the later stage.The significant variation of the proportion of different-type aftershocks on the southern and northern segments may be resulted from readjustment of faulting behavior on both ends of the seismogenic fault zone.（3） Variation of dominant strike of fault planes and orientation of P-axes is significant on different segments as rose diagrams indicate.The dominant orientations of P-axes are NWW with sub-horizontal plunges on the southern segment,NEE on the central segment and NW-striking Lixian branch,and NWW and NEE on the northern segment.The above finding demonstrates that the aftershock activity was influenced by both regional stress field and local stress field from the Wenchuan mainshock.（4） Along the central-northern Longmen Shan fault zone,the mean dip of the fault plane on the southern segment of the aftershock area gradually increases as the depth decreases from 18 to 10 km,and becomes relatively stable for depths shallower than 10 km,which means that the fault plane changes from gentle to steep upwards as a listric fault plane.While on the central segment,the mean dips at depths 10 km are relatively steady,and obviously increase as depths decrease upwards（10 km）.However,the relatively stable mean dips within the range of 50°～65° on the northern segment imply that the fault plane there is relatively steep and therefore more favorable for strike-slip faulting.The above results indicate the very complicated geometry of the fault plane of the central-northern Longmen Shan fault zone.