Group Velocity Distribution in Hebei and Its Adjacent Area Based on Seismic Ambient Noise

The group velocity distribution characteristic in Hebei and its adjacent area is studied using surface wave tomography method based on continuous vertical component ambient noise data of 24 months recorded by 190 broad band seismographs since January 2010 to December 2011. We firstly calculate the frequency dispersion using multiple filter method, then divide the study area into 0.2°×0.2° grid and inverse the Rayleigh wave group velocity within 7-34 s cycle of hebei province and adjacent regions using the method of O'ccam. The results show that the group velocity distribution characteristics show good correlation with surface topography and geological structure in the study area, can clearly reveal the horizontal velocity changes within the crust. Within short period (7-25 s), the basin with thicker sedimentary is characterized by obvious low speed characteristics, while Taihang mountains and Yanshan uplift region is characterized by high group velocity distribution. Within long period(>25 s), the group velocity distribution show the opposite characteristics:the plain areas with thinner crust have higher group velocity, while Taihang mountains and Yanshan uplift region with thicker crust have lower group velocity. There are obvious low-velocity layers in the middle and low crust of northern Shanxi seismic belt and Hebei plain seismic belt. On the group velocity distribution image, Taihang mountain is not only the boundary between north China basin and Taihang mountain terrain and tectonic belt,but also corresponds to the transition zone of low group velocity and group velocity. Most strong earthquakes in the study area distribute in the transition zone and near the high speed area. In the middle and lower crust in where strong earthquakes distribute, there are obvious abnormal low speed layer.