Analysis on variation of electric field spectrum at cut-off frequency before strong earthquakes：Taking 2006 Tonga MW8.0 earthquake as an example
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摘要: 以汤加MW8.0地震为例，着重分析了DEMETER卫星记录中截止频率附近电磁场频谱在震前的异常演化特征. 根据地震前后该区域3个月的数据对比，发现电场频谱增强与地震孕育的关系很不明显. 而长时间、大范围的电场频谱减弱现象可能更值得关注，白天和夜间的电场频谱显示同一特征．2006年夏秋季节38次6级以上强震短期异常的统计平均结果显示，白天的电场频谱在震前24小时距离震中2000 km左右可能有一定增强，达到背景值的3倍均方差. 但其它时段多以下降为主，而夜间无论震前震后比较明显的超过3sigma;的异常均以下降为主．最后讨论了地震-电离层异常孕育耦合机理．Abstract: Taking a Tonga MW8.0 earthquake as an example, we focus on the analysis of evolutional features of electromagnetic field spectrum in a band around cut-off frequency recorded by DEMETER satellite. The result indicates that the increase in electric spectrum amplitude were not closely related to earthquake preparation process based on comparison of 3-month data above the study region. However, we should pay more attention to the long period and large scale decreasing variation of the electric field in this frequency band. The image in local nighttime shows decreasing characteristic similar to that in local daytime. A statistical analysis on the data associated with 38 Mge;6.0 earthquakes in summer and autumn of 2006 demonstrates that for daytime data at the time 24 hours before the earthquakes electromagnetic emissions strengthened near the distance of about 2000 km, reaching 3 times RMS deviation (sigma;) relative to the background value, but weakened mostly in other time periods. While for nighttime data most anomalies above 3sigma; showed decrease whatever before or after the earthquakes. Finally, the earthquake-ionosphere coupling mechanism was discussed.
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