2013年芦山MS7.0地震前甘孜台地电阻率变化分析

甘孜台地电阻率N30°E测道观测资料自2011年7月开始出现趋势上升变化,N60°W测道地电阻率则从2012年出现趋势上升变化.2011年测区原317国道实施扩建工程,N30°E测道测量电极分别向供电极方向移动10m.采用甘孜台电测深曲线以水平层状模型反演了测区的电性结构,理论计算表明,测量电极的移动将会引起N30°E测道4Ω·m的上升变化,扣除这部分变化后,甘孜台两测道观测值于2012年同步上升.以水平层状模型计算了甘孜台两测道各层介质的影响系数,两测道浅层两层介质影响系数均为负,能合理地解释甘孜台地电阻率在雨季降水量增加时观测值上升、旱季降水量减少时观测值下降这一“夏高冬低”的年变现象.建立三维有限元模型计算了317国道拓宽部分对观测的影响,计算结果表明,拓宽部分仅能引起N60°W测道约0.15Ω·m的下降变化和N30°E测道约0.1Ω ·m的上升变化,其对观测的影响非常小.同时2013年1月甘孜台两测道年变低值显著高于2008年以来各年的年变低值,在芦山地震前呈同步的上升变化,但是与汶川地震前的下降变化相反,因此甘孜台自2012年的趋势上升变化是不是芦山地震的前兆异常还难以确定.

Analysis on Apparent Resistivity Variations of Ganzi Station before 2013 Lushan MS7.0 Earthquake

Apparent resistivity data of N30oE monitoring direction observed at Ganzi station began to increase since July 2011. And data of N60oW direction also increased since 2012. In 2011 the potential electrodes of N30oE direction moved 10m towards the current electrodes due to the expansion construction of national road No. 317. By using a horizontal layered model our analysis indicates that the potential electrodes shift can cause 4 raise to measurement of N30oE direction. Therefore, apparent resistivity of the two directions increased in the same time of 2012 after offset the effect from electrodes shift. Sensitive coefficients of two directions were also obtained using the model. Sensitive coefficients of shallow layers were minus for the two directions and the unexpected annual variations of Ganzi station were well explained. In order to quantitatively analyze effects on observation from the national road, we constructed an finite element model based on the electric structure. Analysis results also suggested that, the national road expansion construction can only cause 0.15 decrease on N60oW monitoring direction and 0.1 increase on N30oE direction. Additionally, the valley values of annual variation of 2013 were distinctively higher than other years from 2008, meaning that there existed abnormal rise of apparent resistivity of the two directions at Ganzi station before the Lushan earthquake. However, the rise was contrary to the decline variation before Wenchuan earthquake. Therefore, it was still unconfirmed that whether or not the rise variation was related to Lushan earthquake.