安徽霍山地震区深部电性结构和发震构造特征

Characteristics of deep electrical structure and seismogenic structure beneath Anhui Huoshan earthquake area

霍山地震区位于大别造山带北缘华北板块与扬子板块接触带上,是大别造山带及周边地震活动最频繁、最集中的地区.83个大地电磁测点组成的大地电磁三维阵列覆盖了整个霍山地震区.用多重网格法、印模迭代重构法和非线性共轭梯度法对阵列数据进行三维带地形反演,获得了地震区深部三维电性结构.电性结构显示,北大别、北淮阳区的中上地壳为电阻率1000Ωm以上的高阻区,中下地壳为电阻率数十欧姆米的相对低阻区;六安盆地电阻率整体较低,中地壳存在显著的电阻率为几欧姆米的壳内高导层.北西向的晓天—磨子潭断裂分隔了北大别高阻层和北淮阳高阻层,在浅部向NE倾,深部向SW倾;北东向的落儿岭—土地岭断裂切穿北大别上地壳高阻层.小震双差定位结果表明,地震主要发生在NE向延伸的落儿岭—土地岭断裂附近的北大别、北淮阳中上地壳的高阻区,并集中于NW向的晓天—磨子潭断裂运动所造成的构造薄弱带中;2014年M S4.3霍山地震震源深度较深,位于北大别高阻区内部的电性梯度较大的区域.综合上述结果我们认为,霍山地震区的主要发震断裂为落儿岭—土地岭断裂,断裂的运动变形充分利用了晓天—磨子潭断裂早先活动所形成的构造薄弱带,断裂下方壳源高导体中的流体沿断层传播使断层强度弱化,使得这些薄弱带区易于发生小地震.由于北大别、北淮阳构造区显著高阻层的存在,我们认为霍山地震区存在发生6级以上中强震的深部孕震环境.

Huoshan earthquake area is located in the northern margin of Dabie orogen,i.e.,the contact zone between North China plate and Yangtze plate,where earthquakes are the most frequent and most concentrated in whole Dabie orogen and its surrounding area.A three-dimensional magnetotelluric array composed of 83 magnetotelluric stations covered the Huoshan earthquake area.Applying multi-grid method,impression and iterative reconstruction inversion technique and standard minimum structure nonlinear conjugate gradients method(NLCG),we inverted the array data and obtained the 3D electric structure beneath the area.The electrical structure reveals that mid-upper crust beneath North Dabie and North Huaiyang region is a high resistance area with the resistivity above 1000Ωm and the mid-lower crust is a relatively low resistance area with tens ofΩm resistivity.The northwest-strike Xiaotian-Mozitan fault separate the North Huaiyang high resistivity layer from the North Dabie high resistivity layer.Fault dip is NE in the shallow and changes to SW in the deep.The northeast-strike Luoerling-Tudiling fault cuts through the high resistivity layer of North Dabie upper crust.Background seismicity relocated using the double difference earthquake location algorithm indicates:earthquakes mainly occurred in North Dabie and North Huaiyang high resistivity layer near the Luoerling-Tudiling fault,and were concentrated in the tectonic weak belt formed by early activities of Xiaotian-Mozitan fault.M S4.3 Huoshan earthquake has a deeper focal depth,and is located at a big electrical gradient zone in North Dabie high resistivity layer.Based on the above results,we think the primary seismogenic fault in Huoshan earthquake area is Luoerling-Tudiling fault.Fluid from mid-crustal conductive body below the fault propagates along the fault.It weakens the fault strength.The kinematic and deformation of the fault makes full use of the tectonic weak belt along Xiaotian-Mozitan fault.Since the existence of significant high resistivity layers in North Dabie belt and North Huaiyang belt,we consider there is a deep seismogenic environment beneath Huoshan earthquake area where earthquakes of magnitude above six are possible.