汶川8级地震前空间电离层VLF电场异常现象

Ionosphere VLF electric field anomalies before Wenchuan M 8 earthquake

基于法国DEMETER卫星记录的空间电场频谱资料,提取分析了汶川地震前可能存在的电场异常信息。首先利用轨道的重访特性分析了四川汶川8级地震前震中上空固定纬度点(31°N)的VLF电场信号2年多的正常频谱变化背景,结果显示1～6月各频点变化比较稳定,但在7～10月份各频点谱值扰动比较剧烈,存在整体幅度上升或者下降的现象,因此电场可能存在年周期变化。在背景分析的基础上,收集了2008年4月1日～5月12日震中区上空2000km范围内所有夜间的降轨轨道在北纬31°记录的电场所有频点资料,计算结果显示震前1周内即5月6～10日相对4月份呈现明显的电场能量增强现象,增强幅度达到3σ,频率集中分布在2～6kHz范围内,异常区域的空间统计分布反映该电场增强现象位于震中以南较大范围内。这次异常发生在空间电磁活动平静时期,可能与汶川地震的孕育过程有关。最后讨论了空间地震电场异常的可能形成机理,对地震电离层耦合机理的深入认识还需要地基和空间的联合立体观测来支持。

Based on the information of spatial electric field spectrums recorded at DEMETER satellite, the possible electric field anomalies before Wenchuan earthquake were extracted and analyzed. Firstly using the revisited feature of satellite orbis, background spectrum of VLF electric field during more than 2 years at certain latitude（31°N）above the Wenchuan M8 earthquake in Sichuan were analyzed. It is found that ,the spectrum curves are steady during January to June, but during July to October, the spectrums are disturbed violently, uprising wholly or decreasing quickly,so electric field in this area may exist annual variation. On the basis of background information analysis, using electric filed spectrums at all frequency points recorded at 31°N on all up-orbits in local nighttime in a distance of 2000 kmto Wenchuan epicentre during 1 April-12 May 2008, it was shown that during 1 week before the earthquake,that is 6-10 May,the amplitude of electric field spectrums increased apparently,reaching 30 at the band of 2-6 kHz,while with the anomalous area distributing in a large area at the south of the epicenter. This anomalous phenomenon occurred in the quiet period of spatial electromagnetic activity, so it may be related to the preparation process of Wenchuan earthquake. Finally the forming mechanism of spatial seismo electric field anomalies was discussed,and it is thought that further understanding on the coupling mechanism between earthquakes and ionosphere needs the support of united and stereoscopic observation at surface and space.