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 byrnCAP 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 Wcnchuan 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 (展开▼
