关键词: 背景噪声, 相速度, 群速度, 层析成像, 山西断陷带

Abstract: In order to acquire a better velocity structure of the crustal and uppermost mantle beneath Shanxi area, we obtain the group and phase velocities of Rayleigh wave of the periods 8s to 50s in Shanxi and adjacent area using ambient seismic noise recorded at 216 broad-band stations. All available vertical-component time series for 2014 have been cross-correlated to yield estimates of empirical Rayleigh wave Green's function. Group and phase velocity dispersion curves for Rayleigh wave are measured for each interstation path by applying frequency-time analysis. It describes finer velocity structure of the crust and upper mantle in Shanxi, which reflects the geological structure characteristics at different depths. The resolution is within 50km and the resolution of part periods can reach 40km.The Rayleigh wave group and phase speed maps at short periods(8~18s and 10~22s)show clear correlations with shallow geological structures. Mountain areas on both sides of Shanxi depression zone show apparent high-velocity anomaly, except for low-velocity anomaly in the Taiyuan Basin, Linfen-Yuncheng Basin and Weihe Basin. Especially, the areas of Youyu County-Pianguan County-Kelan County-Shuozhou City and Jingle County-Lishi District of Lüliang City in Lüliang Mountains, and Yu County-Fuping County-Yi County and Yangcheng County-Licheng County in Taihang Mountains, present higher velocity anomaly. In addition, the velocity is lowest in the Weihe Basin, and the amplitude of low velocity decreases gradually from the south to the north of the basins in Shanxi, which probably is related to the process of gradual stretching and development of the Shanxi rift zone from the southwest to the northeast. The obvious velocity difference across the latitude of 38°N exists at 18~30s period of phase and 24~35s period of group velocity maps, which is probably related to the deep and shallow Moho depth variation in the south and north of Shanxi and the suture zone of ancient blocks including "hard" southern block and "soft" northern block. At the same time, the research result of receiver function reveals that partial melting of the lower crust occurs in the northern Taihang Mountains, while the southern section remains stable(Poisson's ratio is above 0.3 in the northern Taihang Mountains and 0.25~0.26 in the southern section). The phase velocity map at 30~50s period clearly shows NW velocity gradient belt, and the low velocity anomaly in the northeast side may be related to Cenozoic volcanism. Meanwhile, the eastern border of Ordos block is the western faults of central basins in Shanxi depression zone. However, some research results indicate that the above border is Lishi Fault in the surface, inferring that the Ordos block shows a shape of wide in the upper and narrow in the lower part from the surface to deep. The Datong volcanic area at 18~45s period of phase and 24~35s period of group velocity maps shows low velocity of trumpet shape from shallow to deep, related to the upwelling of hot material from lower mantle in the Cenozoic causing a large area of intense magmatic activity. It indicates the more specific upwelling channel of Datong volcanoes simultaneously.

Key words: ambient seismic noise, phase velocity, group velocity, tomography, Shanxi fault depression zone