Spatio-temporal variation of the stress field in the Wenchuan aftershock region

Focal mechanism solutions and centroid depths of 312 <i<M</i<≥4 aftershocks from the 2008 Wenchuan earthquake sequence have been derived by CAP (Cut and Paste) method from broadband waveform data with relatively high signal-to-noise ratio (SNR). Following this, we have analyzed the distribution of focal depths and the stress tensors, as well as the types of focal mechanisms. The major results are: (1) different cross-sections show that the depth ranges of the aftershocks at the southern and northern ends of the aftershock area along the Longmenshan fault zone are wider than those on the central segment, where rare <i<M</i<≥4 aftershocks occurred at depths shallower than 10 km. The main faults trend to the NW on the southern and central segments, and for the northern segment, no dominant trend direction has been determined; (2) stress tensor distribution demonstrates that the majority of the aftershock areas on the cross-section along the major axis are mainly under compressive stress perpendicular to the profile; however, for the areas near Lixian, Beichuan, Qingchuan and the shallow parts of its northern segment, large principal stress components are parallel to the major axis profile direction. On the cross-sections perpendicular to the major axis, the three areas above can be divided into two parts: one with dominantly compressional stress near the major faults of the Longmenshan fault zone on the SE side, and the other with NE-direction push along the fault zone on the NW side; (3) the stress tensor distribution in map view is very similar to those on the vertical cross-sections. In map view, the orientation of the principal compressional stress axis <i<S</i<<sub<1</sub< on the central segment of the aftershock area presents an SE-trending arc shape; (4) the stress tensor slices at different depths show that the orientation of <i<S</i<<sub<1</sub< axis mainly changes on the central segment and at the northern end, indicating that the two segments have different seismogenic structures at different depths; (5) with the exception of the northern end of the aftershock region, the orientation of the <i<S</i<<sub<1</sub< axis changes little during the early and late stages, illustrating the seismogenic structures are relatively stable; (6) preliminary analyses for the seismogenic structures at the northern end indicated that deeper strike-slip quakes occurred on the ENE-striking branch at first, and then the NNE-striking branch faults at the northern end were activated and generated a series of relatively shallow strike-slip earthquakes due to subsequent stress-triggering; (7) the aftershock triggering mechanism that occurred near Lixian is different between the shallow and deep depths, and between the early and late stages, indicating that the main faults and the branch faults responsible for aftershocks are at different depths. Consequently, the relaxation effect of the main shock particularly impacts the branch faults.