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本文应用最大熵谱估计方法(MEM)等处理了2008年四川汶川M_S8.0地震震中周围电磁台的观测数据,研究了地电、地磁场变化,得到:(1)在汶川大震前,位于青藏高原东北缘的电磁台记录了显著的自然电场变化、地电场功率谱值增大以及地磁场日变化波形畸变等电磁现象,特别是地电场周期约小于3h的低频成分的谱值增大了3个数量级以上.(2)在距离主震震中约500km范围内,记录了地电、地磁场低频谱值增大为主的电磁现象,特别是距离主震震中最近的成都台地电场谱值增大了4个数量级.(3)震前电磁现象的时空分布与青藏高原的大震活动(包括2008年新疆于田M_S7.3地震)有大空间尺度的构造关联性.本文讨论了电磁现象的机理,认为在震前地下介质内部的微裂隙发展、定向排列过程中,微破裂“机-电转换”作用和地下水运移等多因素引起了地电、地磁场变化.在本次大震前电磁现象的时空强图像很复杂,可能原因是台站所在处的地下局部介质作用和震源孕育激发的电磁辐射引起了复杂的电磁现象.
In this paper, the maximum entropy spectral estimation method (MEM) and other processing of the 2008 Wenchuan M_S8.0 Sichuan earthquake epicenter observations, the study of the geoelectric and geomagnetic field changes, get: (1) before the Wenchuan earthquake, located in Electromagnetic stations on the northeastern margin of the Qinghai-Tibet Plateau recorded notable electromagnetic phenomena such as significant changes in the natural electric field, increase in the power spectrum of the geoelectric field, and waveform distortions in the diurnal variation of the geomagnetic field. Especially, the spectral values of low frequency components with geoelectric field periods of less than about 3 hours increased 3 orders of magnitude or more. (2) Electromagnetic phenomena whose geoelectric and geomagnetic field are dominated by low frequency spectrum are recorded in the range of 500km from the epicenter of the main shock, especially the increase of the terrestrial electric field in Chengdu from the epicentral distance (3) The temporal and spatial distribution of the electromagnetism before the earthquake has a large spatial scale of tectonic correlation with the large earthquakes in the Qinghai-Tibet Plateau (including the Yuda M_S7.3 earthquake in Xinjiang Uygur Autonomous Region in 2008) Mechanism, it is believed that the micro-cracks in the underground medium before the earthquake, the development of micro-cracks in the process of orientation, micro-cracks “machine-electricity conversion ” effect and groundwater movement caused by many factors such as geoelectric and geomagnetic field changes in this large Pre-earthquake electromagnetic now Strong spatiotemporal image is complex, probably due to electromagnetic radiation and the role of underground local media source at the station where the breeding excitation caused by the complexity of electromagnetic phenomena.