Spatial and temporal variations of stress field in the Longmenshan Fault Zone after the 2008 Wenchuan, China earthquake

We investigate the temporal and spatial patterns of the regional stress field along the Longmenshan Fault Zone (LFZ) following the 2008 Wenchuan Mw7.9 earthquake. Regional broadband seismic records are used to determine the focal mechanisms and centroid source depths of >400 aftershocks of magnitudes 3.3 ≤ Mw ≤ 5.9. The focal mechanisms are then used to invert for the temporal and spatial variations of the regional stress field. Results show clear spatial changes in the directions of principle stress axes as well as the faulting types along the LFZ. The regional stress field is in an overall thrust regime with nearly horizontal maximum compressional axis. In the southern section of the LFZ, in addition to the primarily thrust background with maximum compressional axis perpendicular to the main fault zone, strike-slip component is also present. In the middle section, the maximum compressional axis varies spatially, changing abruptly from perpendicular to parallel to the LFZ. In the northernmost section, the stress environment becomes primarily strike-slip. Our result suggests that the collision of Indian and Eurasian plates led to the first-order feature of the stress field in the study area. The geometry of blocks and faults, the lower crustal flow as well as the shallow crustal property difference are the major factors to controlling the local stress field. Temporal variations of stress field suggest that the mainshock caused a large stress disturbance at early stage in certain areas of LFZ, and the stress field variations with depth imply a different deformation pattern and different stress background in the shallow and lower crust.