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利用20301条全国及区域地震台网的Pn波射线的走时资料,采用地震层析成像方法反演了华北地区上地幔顶部Pn波速度结构。结果表明,华北地区Pn波平均速度为7.92km/s,横向速度变化量从-0.21～+0.29km/s,Pn波高、低速异常区沿NNE向相间排列,从西向东有鄂尔多斯地块中部高速区、山西地堑低速区、冀中坳陷高速区、鲁西隆起及渤海湾低速区、郯庐断裂带以东高速区,华北地块商界除秦岭一带里高速异常外其余为低速异常。大同附近新生代火山区呈现较大的波速各向异性,快波方向呈NNE-SSW向;渤海湾周围也显示出显著的各向异性,并呈旋转趋势,反映了该区地幔流动变形的迹象。华北地区Pn波速度与大地热流之间具有负相关关系,Pn波低速异常区,对应着高热流值,如:山西地堑、渤海湾;而在Pn波高速异常区,对应着低热流值,如:冀中坳陷。这表明本区Pn波速度变化主要是上地幔顶部存在温度差异的结果。强震主要发生在低速异常区以及高、低速异常区的交界带上面的地壳内,在低速异常区发生的地震,其震源深度较浅;在高、低速异常区的交界带发生的地震,其震源深度较深。
Based on the traveltime data of 20301 Pn wave radiation from national and regional seismograph networks, the velocity structure of Pn wave at the top of upper mantle in North China was retrieved by seismic tomography. The results show that the average Pn wave velocity in North China is 7.92km / s, lateral velocity variation is from -0.21 ~ + 0.29km / s, Pn wave height and low-velocity anomalous zone are arranged along NNE direction from west to east, Low speed area of Shanxi Graben, High Speed Area of Jizhong Depression, Luxi Uplift, Bohaiwan Low Speed Area, East High Speed Area of Tanlu Fault Zone, and North China Block Commercial Area except for the high-speed anomalies in the Qinling Mountains. The Cenozoic volcanic area near Datong shows a large anisotropy of wave velocity, with a fast-wave direction of NNE-SSW; remarkable anisotropy also appears around the Bohai Bay and shows a rotation tendency reflecting the signs of the flow deformation in this area . There is a negative correlation between the Pn wave velocity and the geothermal flow in North China. The Pn low-velocity anomalous zone corresponds to the high heat flow values, such as Shanxi Rift and Bohai Bay. In the Pn anomalies, Such as: Jizhong depression. This shows that the Pn wave velocity in this area is mainly the result of the temperature difference at the top of the upper mantle. The strong earthquakes mainly occur in the crust above the boundary zone between the low-velocity anomalous zone and the high-low-velocity anomalous zone. The earthquakes that occur in the low-velocity anomalous zone have a shallow depth of source. The earthquakes that occur at the boundary zone between high and low-velocity anomalous zones, Deep source depth.