Ding Zhi-feng

Surface wave tomography of the crust and upper mantle of Chinese mainland and its neighboring region

The three dimensional S wave velocity structure of the crust and upper mantle of Chinese mainland and its neighboring region is obtained by genetic algorithm of surface wave tomography, with smoothness constraint, based on 25 wave group velocities for the periods from 10 s to 92 s, measured from long period Rayleigh waves recorded by 11 stations of CDSN and 12 digital seismometers surrounding China. The S wave velocity image is shown on two latitudinal sections along 30°N and 38°N, two longitudinal sections along 90°E and 120°E, and four horizontal slices at the different depths.

Variations of shear wave splitting in the 2013 Lushan Ms7.0 earthquake region

In this paper, variations of shear wave splitting in the 2013 Lushan Ms7.0 earthquake sequence were studied. By analyzing shear wave particle motion of local events in the shear wave window, the fast polarization directions and the delay time between fast and slow shear waves were derived from seismic recordings at eight stations on the southern segment of the Longmenshan fault zone. In the study region, the fast polarization directions show partition characteristics from south to north. And the systematic changes of the time delays between two split shear waves were also observed. As for spatial distribution, the NE fast polarization directions are consistent with the Longmenshan fault strike in the south of focal region, whereas the NW fast direction is parallel to the direction of regional principal compressive stress in the north of focal region. Stations BAX and TQU are respectively located on the Central and Front-range faults, and because of the direct influence of these faults, the fast directions at both stations show particularity. In time domain, after the main shock, the delay times at stations increased rapidly, and decreased after a period of time. Shear-wave splitting was caused mostly by stress-aligned microcracks in rock below the stations. The results demonstrate changes of local stress field during the main shock and the aftershocks. The stress on the Lushan Ms7.0 earthquake region increased after the main shock, with the stress release caused by the aftershocks and the stress reduced in the late stage.