2008年汶川MS8.0地震前成都台NE向地电阻率趋势异常的数值模拟

本文针对汶川大地震前位于源区特殊部位的成都台NE测向地电阻率的大幅度趋势下降过程, 以测区地表地电阻率变化与测区内部介质(真)电阻率变化的物理联系的理论为依据, 模拟了测区底层因受孕震过程影响出现一个电性变化区域, 计算了该区域电阻率变化的程度以及其上界面逐步(向上)扩展对成都台地表装置系统地电阻率变化的影响。 结果表明, 当底层电性变化区域介质电阻率发生某种减小时, 在成都台的装置系统上测到的地电阻率确实会下降, 下降的幅度随着变化区域上界面向上的扩展而单调增大; 变化区域电阻率减小的程度越大, 这种影响也会更显著。 对于成都台7%的趋势下降异常而言, 从数值模拟的角度来看, 变化区域的电阻率变化可以取多种可能的量级, 不过不同量级的变化与其相应的上界面位置有关, 例如: 底层变化区域真电阻率下降幅度达20%左右时, 变化区域上界面到距地表430 m时, 地表装置视电阻率下降可达1%, 其影响才开始为地表装置系统所识别; 而要解释地表视电阻率有7%左右的视电阻率下降变化, 则要求这个变化区域的上界面上升到距地表122 m左右。

Simulation of the Geo-electricity Anomaly in the NE Direction at Chengdu Station Associated with the 2008 Wenchuan MS8.0 Earthquake

This paper dealt with the anomalous changes in geo-electricity (apparent resistivity) at Chengdu station (Δ=35 km), a two-year descending tendency of 7.2% in NE direction associated with the Wenchuan M_S8.0 earthquake. We used an 1D model of negative resistivity changes occurring in the lower part of the bottom layer, so that a new interface appearing in certain level of the bottom layer. In the model, the negative resistivity changes are supposed to be due to the influences from the preparation processes of the quake, and also the consequence of the interface upward with the different magnitudes of the resistivity changes has been considered for comparison. The results show the appearances of the new interface with the negative resistivity changes would lead the geo-resistivity descending in a monotone behavior in the ground observation configuration system, with the increase of the magnitude of the resistivity descending and the interface level upward in the part of the bottom layer. For example, for the negative resistivity change being 20%, the interface level has to be up to the 122 m below the surface. As for other changes, for example, 15%, the level would be up to the 86 m shallower than 122 m, while for 30% the level could be 175 m, deeper than 122 m.