汶川M_S8.0地震前成都台NE测线地电阻率异常的进一步研究
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摘要
汶川MS8.0地震前,位于震源区内的成都台NE测线地电阻率显示了数年大幅度趋势下降异常,其年变化甚至严重畸变到消失的程度,它和多年来我国7级以上大地震记录到的异常形态有极大的相似性,被认为是最清晰的异常之一.然而,长期的观测表明,成都台NE测线地电阻率与测区潜水水位年变化呈正相关.因此,关注异常发展过程中测区水位变化,成为汶川MS8.0地震震例地电阻率异常科学总结的重要课题之一.本文通过对成都台建台以来NE测线地电阻率的全程分析,确认与汶川MS8.0地震有关的地电阻率异常变化属于全程变化的第三阶段(1999—2008年).分析了第三阶段同步观测的地下水位数据,并进行了地电阻率和地下水位趋势变化的综合分析.最终确认与汶川MS8.0地震有关的地电阻率异常经历了前、后两个时段:前期2004年2月—2006年2月,小幅趋势上升,幅度约为1.9%;后期2006年2月—2008年2月,趋势下降,幅度达-6.8%,如此大幅度快速下降现象,是成都台建台30多年来从未见到的.基于前人开展的试验工作的启示,推测与此下降相应的测区底层(h>96.6m)介质电阻率减小量可达22.3%左右.其量级之大,须用承载标本试验中岩石破裂前微裂隙广泛发育和排列趋同化导致的地电阻率变化来解释,由于成都台位于汶川地震震源区内的特殊位置,可以作为汶川地震孕震物理过程研究中的重要约束条件之一.
Chengdu geoelectric station is located merely 35 km from the epicenter of the 12 May 2008 Wenchuan MS8.0 earthquake. This paper deals with the remarkable descending tendency of apparent resistivity with strong distortion of seasonal variations at the station associated with the Wenchuan earthquake on a new point of view. Firstly, the long-term observation data since 1978 were applied to the investigation in order to determine the stage features in the whole course of the apparent resistivity observation; secondly, more attention was paid to the close relationship between apparent resistivity and water-table changes to recognize the possible influence of the water-table variations on the determination of tendency anomalies in apparent resistivity at Chengdu station; and thirdly, the previous results of in-situ experiments at the station were utilized to give a nearly quantitative analysis of the tendency anomaly in the third stage of the whole courses of observation. The analysis has identified two segments of anomalous behaviors in apparent resistivity which could be associated with the Wenchuan MS8.0 earthquake after checking the influence of water-table variations. The first segment showed a slight ascending of 1.9% from Feb. 2004 to Feb. 2006; the second one had a very huge descending of-6.8% from Feb. 2006 to Feb. 2008. The deduced variations of-22.3% in true resistivity in the deep media, which had never been seen in the previous time of the whole course of observation at the station, could be taken as one of the constraints for researches on the preparation processes of the earthquake. The results have shown important complements to the case studies of apparent resistivity anomalies in China, especially to those with strong distortion of seasonal variations in the anomaly development without synchronous observation of water-table variations.
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