基于V_(P)/V_(S)波速比模型约束的张渤地震带深部电性结构研究

Inversion MT data for the electrical structure beneath the Zhangbo seismic belt based on constraint of the V_(P)/V_(S) model

张渤地震带是中国东部地区一条重要的北西向地震活动带,获取深部精细的电性结构有助于探究该区域深部孕震环境及动力学机制等科学问题.为了提高大地电磁法深度分辨率,本文提出基于V_(P)/V_(S)波速比地震学模型约束的二维大地电磁反演法,通过理论模型合成数据检验了算法的可靠性.将算法应用于张渤地震带大地电磁测深资料,对比分析大地电磁无约束和有约束反演结果,检验算法在张渤地震带应用的有效性和实用性.最后结合已有的地质-地球物理资料,发现:反演获取的电阻率模型的电阻率梯度带与断层的空间分布吻合;唐山断裂带中上地壳表现为高阻特征,在下地壳底部有上涌的高导异常体,推测该区域深部高导区域与幔源物质侵入有关;三河—平谷断裂带浅部低阻异常与深部低阻异常不连续,地震主要分布于不连续区域;太行山山前断裂电阻率结构上表现为明显的电阻率变化梯度带,高阻异常体规模大、延伸到下地壳;张家口断裂带中下地壳高导异常区域比怀来盆地深部高导异常区域规模大,这可能暗示张家口断裂带的深部物质作用更为强烈.

The Zhangjiakou-Bohai(Zhangbo)seismic belt is one of the major NW trending feature in eastern China.Probing the electrical structure beneath this seismic belt can help to explore the seismogenic environment and the dynamic mechanism in this region.In this paper,we present a 2-D MT inversion algorithm based on constraint of the V_(P)/V_(S) model.The validity of this constrained inversion method is verified by the synthetic model test.The inversion results by the non-constrained and constrained methods are compared and analyzed to verify the practicability of the algorithm.Finally,combined with the existing geological and geophysical data,the MT data inversion suggests that the resistivity gradient zone in the inverted model is consistent with the fault distribution.The middle and upper crust of the Tangshan fault zone shows a high resistivity anomaly.There exists an upwelling low-resistivity anomaly at the bottom of the lower crust.It is speculated that the deep low-resistivity upwelling anomaly is related to the intrusion of mantle material.The low-resistivity anomalies in the shallow and lower crust are discontinuity beneath the Sanhe-Pinggu fault zone.Earthquakes mainly occurred in the discontinuous areas,which might be caused by the upwelling of mantle material.At the piedmont fault of the Taihang Mountains,there is an apparent resistivity gradient zone,where the high resistivity anomaly body is large in scale and extends to the lower crust.The scales of low-resistivity anomalies in the middle and lower crust beneath the Zhangjiakou fault zone are larger than that of the Huailai basin,which may indicate that the upwelling mantle activity beneath Zhangjiakou is stronger than that of the Huailai basin.