Segmentation characteristics of the Longmenshan fault -Constrained from dense focal mechanism data

The Longmenshan fault has obvious structural zonation and segmentation characteristics. Generally, the Longmenshan fault is divided into five structural zones from west to east with four main faults as boundaries. However, there is still no consensus on the segmentation characteristics of Longmenshan fault. In this paper, seismic waveforms of the earthquakes occurred in nearly ten years following the Wenchuan earthquake were collected from Sichuan earthquake network and 1495 focal mechanism solutions with M≥3 were determined by the full waveform inversions. According to the inverted dense focal mechanism solutions, we employed the "sliding window" scanning method to calculate the variation curves of the counts of different earthquake types along the strike of Longmenshan fault. The focal areas were finally divided into 9 segments which are named S1-S9. In addition, according to the obtained focal mechanisms, we had further inverted the tectonic stress field of high resolution in the Longmenshan fault and discussed the segmentation characteristics from aspects of faulting types, fault plane structures and tectonic stress filed. Our results show: (1) The faulting types have obvious segmentation characteristics. Proportions of thrust, strike-slip and normal faulting are the highest in S1, S8 and S9, respectively. While the proportion of strike-slip faulting in the late Wenchuan aftershocks has generally increased, which is closely related to the stress supplements and adjustments along the strike of Longmenshan fault. After the occurrence of Wenchuan earthquake, the different sections of Longmenshan fault show differential adjustments. The Lushan earthquake may have caused stress disturbance in S2. In addition, the post-seismic adjustment processes near the Wenchuan mainshock and in the far end of aftershock zone are longer than that in the middle section of aftershock zone, and the S9 at the far end of the aftershock zone has more complicated and more intensive post-seismic adjustment processes. (2) The fault plane structures have obvious segmentation characteristics. The fault plane structures reveal that the buried faults near the Wenchuan mainshock and in the far end of aftershock zone as well as the south end of Huya fault participated in the post-seismic activities. The dip angles show good relationships with strike-slip components, which the dip angles range of 50°~70° in the areas with obvious thrust components and are generally larger than 60° in the areas with obvious strike-slip components. In addition, the increase of dip angles is coincided with narrowing of the width of Wenchuan aftershock zone. (3) The stress environment of Longmenshan fault is very complex. The segmentation differences of σ1 directions lead to the crustal tearing and mantle material upwelling in the seismic gap between Wenchuan and Lushan earthquakes, the seismicities in the buried faults near the Wenchuan mainshock and in the far end of aftershock zone as well as a large number of thrust earthquakes in the south end of Huya fault. Combined the stress filed determined in this paper with previous geodetic surveys, we deduce the uplift of Longmenshan may be mainly caused by the shortening and thickening of upper crust under the tectonic stress field. © 2021, Science Press. All right reserved.