Variation of Electromagnetic Tornogram Before and After Earthquakes  

作　　者：Sun Cichang,Wang Xusheng,Liu Qingfang,Tao Yulu,Yu Ping,and Feng RuiInstitute of Geophysics,SSB,Beijing 100081,China Department of Geophysics,Peking University,Beijing 100871,China Seismnlogical Bureau of Hunan Province,Changsha 410001,China Center for Seismological Data and Information,SSB,Beijing 100045,China 

出　　处：《Earthquake Research in China》1998年第2期70-76,共7页中国地震研究（英文版）

基　　金：Contribution No.97A0058 Institute of Geophysics, SSB, China

摘　　要：Cross-well electromagnetic tomography has been used to monitor the changes of the abso rption coefficient of electromagnetic waves before and after earthquakes for 5 years at Dahuichang fault since 1992.It has been found that 3 months before the occurrence of moderately strong earthquakes(ML≥4.0,△【210 km),several special changes in tomogram appeared as follows:The mean-value of the absorption coefficient dropped about 10% compared with the annually decreasing tendency; the mean difference of pixels suddenly decreased or increased in different directions; and the entropy of the paragenic matrix in a tomogram decreased and the number of low absorption pixels increased.It is inferred that such changes in electromagnetic tomograms are attributed to the variation of micro-cracks and porosity in rocks.Electromagnetic tomography can be studied as a new technique to detect earthquake precursors.Cross-well electromagnetic tomography has been used to monitor the changes of the abso rption coefficient of electromagnetic waves before and after earthquakes for 5 years at Dahuichang fault since 1992.It has been found that 3 months before the occurrence of moderately strong earthquakes(ML≥4.0,△<210 km),several special changes in tomogram appeared as follows:The mean-value of the absorption coefficient dropped about 10% compared with the annually decreasing tendency; the mean difference of pixels suddenly decreased or increased in different directions; and the entropy of the paragenic matrix in a tomogram decreased and the number of low absorption pixels increased.It is inferred that such changes in electromagnetic tomograms are attributed to the variation of micro-cracks and porosity in rocks.Electromagnetic tomography can be studied as a new technique to detect earthquake precursors.

关 键 词：ELECTROMAGNETIC TOMOGRAM Dahuichang fault Earthquake PRECURSOR Imageanalysis. 

分 类 号：P315[天文地球—地震学]