Co-seismic slip and slip partitioning of multi-faults during the Ms8. 0 2008 Wenchuan earthquake

In order to study the kinematical mechanism of the co-seismic rupture of the 2008 Wenchuan earthquake, we implemented a detailed surveying and mapping in some typical sites with geological and geomorphologic marks, and calculated the three dimension slip vectors. The northwestern hanging wall of Baishuihe-Gaochuan rupture associated with the Beichuan-Yingxiu fault slipped in an azimuth of 88 degrees, with 2.58 m of horizontal slip and 3.70 m of vertical slip; The northwestern hanging wall of Bailu-Hanwang rupture associated with the Anxian-Guanxian fault slipped in an azimuth of 134 degrees, with 1.63 m of horizontal slip and 2.00 m of vertical slip; The southwestern hanging wall of Xiaoyudong rupture slipped with an azimuth of 76 degrees similar to 79 degrees, 2.15 similar to 2.71 m of horizontal silp and 1.36 similar to 1.51 m of vertical slip. There are totally about 1.72 m of strike slip and 3.49m of horizontal shortening perpendicular to the fault zone from the paralleling Baishuihe-Gaochuan and Bailu-hanwang ruptures. The integrated slip vector (trending 106 degrees) has an angle of 64 degrees with the fault zone (trending 42 degrees), and the Longmenshan fault zone was under a transpression. Combining the result from source inversions, we propose that the oblique-slipping Baishulhe-Gaochuan rupture and the dip-slipping Bailu-Hanwang rupture were produced in the same time during the largest sub-event with a slip partitioning, and the mechanism of slip partitioning made the two faults ruptured with an oblique slip and a dip slip, but not all the same oblique slip. The Xiaoyudong rupture slipped in a relatively small dip angle, which acted as a trigger structure for the simultaneous rupture of Beichuan-Yingxiu fault and Anxian-Guanxian fault in a manner of slip partitioning. The Xiaoyudong fault was the production of the Longmenshan fault zone under the transpressional tectonics in a long term, not only a tear fault among the thrust system.