中国中强地震前地电阻率中短期异常特征及其可能原因

Middle-short Term Abnormal Characteristics of Apparent Resistivity before Strong Earthquakes in China and Its Possible Mechanism

摘要:

地电阻率已经发展成为我国地震地球物理场观测的重要方法,通过标示一定区域范围内介质电阻率随时间的变化特征和空间分布,应用于探索地震孕育发生的过程。在50多年的观测过程中,记录到了近百次5～8级地震前的中短期异常和部分强震前的临震变化以及地震发生之后的恢复过程。电阻率是地下介质重要的物性参数,与岩土骨架和裂隙内流体电阻率、裂隙率、裂隙结构以及流体饱和度等因素有关。其中,应力作用导致的微裂隙结构变化是引起电阻率变化的重要因素。已有相关研究分别从岩石物理实验、介质电阻率模型、震例研究等方面讨论了地震前地电阻率变化的可能机理。本文对其中具有代表性的研究结果进行梳理,以“介质变形-电阻率变化”为纽带,将岩石物理实验和电阻率模型给出的细观尺度介质电阻率变化机制与震中周围地电阻率异常变化的宏观现象联系起来,介绍地震晚期孕育阶段地电阻率异常变化的可能原因。地震孕育引起发震断层周围介质变形,由发震断层向外围方向附加变形程度逐渐衰减; 地表浅层介质处于低围压状态,新生微裂隙将大致沿最大主压应力方向扩展,进而引起介质电阻率发生变化; 同一台站不同方向观测的地电阻率呈现出与P轴方位有关的各向异性异常变化,与P轴方向夹角越大的测道异常幅度越大,与P轴方向平行或近于平行的测道异常幅度最小或无异常变化; 地震前挤压变形增强区域地电阻率异常呈现下降变化,相对膨胀区域呈现上升变化或变化不明显。

Abstract:

Apparent resistivity has been one of the important branches of seismic geophysical field observation in China,which is applied to explore the process of earthquake preparation by marking the variation characteristics and spatial distribution of medium resistivity with time in a certain area. Through over 50 years of observation,we have recorded a series of the medium-term and short-term anomalies before about one hundred of M_S5.0～8.0 earthquakes and the impending earthquake changes before some strong earthquakes,as well as the recovery processes after the earthquakes. Resistivity is an important physical parameter of underground media,which is related to resistivity of both rock-soil and fluid,crack rate,crack structure and fluid saturation in geotechnical framework and fissures. Tectonic stress is an important factor leading to the change of resistivity. In previous related literatures the possible mechanism of resistivity variation before earthquakes have been discussed from rock physics experiments,electrical resistivity models and case studies. In this paper we introduce the possible mechanism of apparent resistivity anomalies associated with strong earthquakes along the line of “medium deformation-resistivity change”,combining with the results from rock physical experiment,resistivity model,and macroscopic phenomena of apparent resistivity anomalies around the epicenter. Seismic preparation process causes the deformation of the medium around the seismogenic fault,and the deformation degree decreases gradually along the seismogenic fault. In shallow surface medium under low confining pressure,the new micro-cracks will roughly expand along the direction of the maximum principal compressive stress,and then the resistivity of the medium will change. The apparent resistivity observed at the same station in different directions implies anisotropic changes related to the P axis orientation. The bigger the angle between the arrays and the P axis,the greater the amplitudes of the anomalies are. When the arrays are parallel to or sub-parallel to the P axis,the arrays have the smallest amplitudes of anomalies or the changes are even unnoticeable. Before a strong earthquake,apparent resistivity in the area of compressive deformation enhancement decreases,while that in the area of relative expansion such increases or the changes are not obviously.