A Study On Crustal Seismic Characteristics Across Zhangjiakou-Bohai Sea Seismic Zone

Zhangjiakou-Bohai Sea seismic zone, one of the most important northwest strike tectonic zone in eastern China where regional principal compressive stress is approximately EW, consists of a series of echelon arrangement cracks which begin from Taihang Mountains at the west end and across Yanshan Mountain and North China plain to Bohai at the east end.When seismic waves spread through anisotropic medium, the phase velocity and group velocity vary with propagation direction. Shear wave will split into two wave trains, among which the polarization of fast shear wave is in the same direction with the trend of crack while the polarization of slow shear wave is almost vertical to that of fast shear wave. The time delay of fast and slow shear wave could reflect the extent of underground anisotropic media.This paper chooses seismic waveform data in capital area (2005.9-2010.9) as our research object and selects SAM method as the tool to analyze spatial distributions characteristics of regional crustal anisotropy. The paper divides Zhangjiakou-Bohai Sea seismic zone into several areas to analyze the result. The results show that the total predominate polarization direction of stations in the middle and eastern areas of Zhangjiakou-Bohai Sea seismic zone is near EW obviously. Crossing the both sides of Zhangjiakou-Bohai Sea seismic zone, the spatial distributions of predominate polarization directions for fast shear wave display that: In the areas of Yanshan uplift in the northern part of the seismic zone, the seismic zone and North China basin, the predominate polarization directions for fast shear wave are NEE to near EW, the average polarization directions differ 13°among for these three regions. There is a visible different predominate polarization directions, which is NNE, it may be related to region structures. Because the data in northern mountain area is not enough, it needs more data to analysis the anisotropic characteristic of northern part in Zhangjiakou-Bohai Sea seismic zone. The average polarization direction of the fast shear wave in the western section of Zhangjiakou-Bohai Sea seismic zone is 62.3°±42.3°, consistent with the result of Zhao et al (2011), it also show two predominate polarization directions: EW to near NWW and NE. The stations in the interchange among Yanshan Uplift, Taihang Uplift and North China Plain, their average polarization directions for fast shear wave is 40.8°±38.1°, inconsistent with regional principal compressive stress field. The predominate polarization direction of stations in the transition zone between Taihang Uplift and North China Plain is 95.7°±31.2°, the result may be affected by WNW fault and regional principal compressive stress field.In the middle and eastern areas of Zhangjiakou-Bohai Sea seismic zone, the average time delay between the fast and slow shear wave is 2.8±1.9ms/km, in the north part of this area, the Yanshan Uplift, the average time delay between the fast and slow shear wave is 2.1±1.0ms/km, in the seismic zone, the average time delay is 2.6±2.5ms/km, North China Plain which is in the south side of the study area, he average time delay is 2.0±1.1ms/km, the average time delay of the station DOH is 3.06±1.8ms/km. The average time delay is similar between the two sides of Zhangjiakou-Bohai Sea seismic zone, and lower than the value of the average time delay in Zhangjiakou-Bohai Sea seismic zone, it illustrates that the anisotropic extent may be stronger in Zhangjiakou-Bohai Sea seismic zone than both of its sides.This paper shows the data-processing of long time-series seismic ambient noise using cross-correlation to obtained the Green's functions, and get the Rayleigh wave signals between station-pairs, then estimate the Rayleigh wave velocity of media under the Zhangjiakou-Bohai See seismic zone. This paper use the waveform data from the broadband seismometers to calculate cross-correlation between station pairs, preliminary estimate the Rayleigh wave velocity distribution in Zhangjiakou-Bohai Sea seismic zone and its both sides. The paper divide the study area into three parts by the ray paths: on the south of Zhangjiakou-Bohai Sea seismic zone, Zhangjiakou-Bohai Sea seismic zone, on the north of Zhangjiakou-Bohai Sea seismic zone. The average value of Rayleigh wave in the study area is 3.97km/s, compared to the average value of the entire study area, there is low Rayleigh wave speed in the south, normal in the north, there is an abnormal high speed in the Zhangjiakou-Bohai Sea seismic zone. These results come from the data that signal to noise is not high, and the results is rough, it needs more cross-correlation data to stack to improve the SNR, and get more refined Rayleigh wave velocity distribution.This paper preliminary shows that, in the middle and eastern areas of Zhangjiakou-Bohai Sea seismic zone, it's a little different between the Zhangjiakou-Bohai Sea seismic zone and both of its sides. It provide a seismic evidence to prove whether the Zhangjiakou-Bohai Sea seismic zone is a structural belt, however, this study still need more evidence to support.