摘要
本文对一条布设在滇西盈江—龙陵地区的大地电磁剖面(苏典—中山剖面)数据进行了精细处理和二维反演解释,得到了测区较高置信度的二维电性结构.该电性模型纵向上表现为高阻-低阻-高阻的"三明治"式岩石圈电性结构,上地壳为平均厚度约为10km的高阻地层,在约6～16km地壳深度范围发育有电阻率为几欧姆米的显著高导层,下地壳底部和上地幔顶部表现为电性较为均匀的相对高阻层.横向上自西向东划分出以大盈江断裂带、龙陵—瑞丽断裂带为限的3个主要构造区域.壳内分布的高导层沿剖面表现出一定的横向不均匀性,其在龙陵—瑞丽断裂带下方消失,在该处形成了腾冲地块和保山地块的电性构造边界.电性结构表明,大盈江断裂附近高导层顶界面浅,两侧高阻体厚度小,因此难以形成较大规模的相互作用,致其附近浅震源、小震级的地震活跃;龙陵—瑞丽断裂两侧的高阻体较厚,易积累较大的应力,具有大震的深部孕震环境,故其附近发生过多次7级以上强震.
A 2 D Magnetotelluric(MT)profile(Sudian-Zhongshan MT profile)located at YingjiangLongling area,western Yunnan,has been interpreted in refined processing techniques,and results in a robust two-dimensional resistivity model.Characterized by an electrical"sandwich"structure,this model consists of a resistive layer with mean thickness of 10 km in the upper crust,and an obvious high conductive layer in the depth of 6~16 km,under which is the relative resistive and homogeneous lower crust and uppermost mantle.Deduced from the resistivity model,this region was divided into three micro-tectonic elements bounded by the Dayingjiang fault and the LonglingRuili fault respectively from west to east along the MT profile.The distribution of mid-crustal conductive layer shows somehow lateral heterogeneity,which disappears beneath the Longling-Ruili fault.Therefore,the Longling-Ruili fault acts as the electrical tectonic boundary between the Tengchong block and Baoshan block.Our model shows that the top surface of the high conductive layer under the Dayingjiang fault is very shallow,where the bilateral resistive rock of the fault is too thin to accumulate tectonic stress of huge magnitude earthquake hence resulting in active seismicity of shallow and small magnitude earthquakes.However,the thick resistive rock on both sides of the Longling-Ruili fault,which could act as effective mechanical carrier and amass huge tectonic stress,indicates the seismogenic electrical structure of several large earthquakes above magnitude 7 occurred nearby.