基于断层虚位错模式讨论2008年汶川MS8.0地震前视电阻率变化

Apparent Resistivity Variations before 2008 Wenchuan MS8.0 Earthquake Based on Fault Virtual Dislocation Model

摘要:

2008年汶川M_S8.0地震前，距震中450km范围内有7个视电阻率台站运行，震前记录到了不同形态和幅值的变化。本文采用断层虚位错模式，在模型中将汶川地震同震滑动位移按大小相等但方向相反的方式进行加载，计算震前产生这些同震位错所需积累的应力应变分布。计算结果表明：在100m深度，震前正应力和剪切应力主要积累区域位于龙门山断裂带两侧约100km范围内，最大应力变化量为0.5MPa；体应变积累区域与应力积累区域基本一致，最大应变值为10^-5，显示这些区域在震前存在较高的挤压应力应变积累，而在这些区域外，应力应变积累程度较低。成都台和江油台位于震前主要的应力应变挤压积累集中区域，视电阻率观测数据均为震前下降-准同震阶跃-震后回升，与震前应力应变积累-震后释放的变化形式一致，这2个台站震前的异常变化与汶川地震关系密切；而甘孜台、武都台、冕宁台、小庙台和天水台位于应力应变积累较弱的区域，其视电阻率变化与汶川地震的关联性较弱。

Abstract:

There were 7 apparent resistivity stations operating within 450km from the epicenter of 2008 Wenchuan M_S8.0 earthquake. These stations recorded different types of changes before the main shock. In this paper,a fault virtual dislocation model is adopted by loading the coseismic sliding displacement of the earthquake in the opposite direction,in order to calculate the stress-strain distribution that can be accumulated to produce these coseismic dislocations before the Wenchuan earthquake. The calculated results showed that at a depth of 100m,the main accumulation areas of normal stress and shear stress before the earthquake were located within about 100km on both sides of Longmenshan Fault zone,and the maximum stress variation was up to 0.5MPa. The volume strain accumulation areas were approximately as same as the stress accumulation areas,and the maximum strain value was up to 10^-5. The degree of stress-strain accumulation was rather low outside these areas. Both stations in Chengdu and Jiangyou were located in the main stress-strain compression accumulation area before the earthquake. The apparent resistivity data recorded at Chengdu and Jiangyou stations declined before the main shock,stepped quasi-coseismicly,and rose after the earthquake,which was accompanied by the stress-strain accumulation before the earthquake and release after the earthquake. The abnormal changes of these two stations were closely related to the Wenchuan earthquake. However,for other 5 stations,i.e. Ganzi,Wudu,Mianning,Xiaomiao and Tianshui,which were located in areas with weak stress-strain accumulation,in consequence,the apparent resistivity changes might be correlated rather weakly with the Wenchuan earthquake.