汶川MS8.0大震前的电磁现象

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3 h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500 km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.

The electromagnetic phenomena before the MS8.0 Wenchuan earthquake

We studied the changes of the geo-electric and geo-magnetic fields for the M_S8.0 Wenchuan earthquake of 2008 in Sichuan Province, China, and got the following understandings: (1) Significant anomalies of the geo-electric and geo-magnetic fields, which included the self-potential changes, the increased Power Spectrum Density (PSD) of the geo-electric field and the waveform distortion of the geo-magnetic field diurnal variation, were recorded along the Northeastern edge of the Qinghai-Tibet Plateau before the M_S8.0 Wenchuan earthquake in 2008, and especially, the PSD anomalies of low frequency components with periods less than about 3 hours (T<3h) of the geo-electric field increased by 3～4 orders of magnitude. It obviously shows the tectonic relevance between the great earthquake activity in the Qinghai-Tibet Plateau, the M_S7.3 Yutian earthquake in Sinkiang and the Wenchuan earthquake, and the geo-electromagnetic anomalies in a large spatial range. (2) In the range of about 500 km from the M_S8.0 Wenchuan earthquake, the PSD anomalies of mainly increased T<3 h low frequency components of the geo-electric and geo-magnetic fields were recorded. In particular, at Chengdu station which was 35km from the earthquake the PSD anomaly of the geo-electric field was increased by 4 orders of magnitude, and yet, the anomalies appeared spatially non-uniformly and non-synchronously around the focal region and had some great differences in amplitude. The mechanism for the anomalies is studied, as a result, it can be understood that in the processes of the non-linear development of numerous micro cracks and their directional arrangement in the media of the focal region before the earthquake, such influencing factors as the underground water migration and the electro-mechanical transformation function and so on induced the geo-electromagnetic anomalies, and the focal process of the earthquake and the local media action under these geo-electromagnetic stations led to the spatially, temporally complicated evolution of the geo-electromagnetic anomalies.