基于汶川MS8.0地震的岩石圈—大气层—电离层电磁耦合初探

At present, the lithosphere–atmosphere–ionosphere coupling (LAIC) among the three involved layers of the Earth system has gradually been a scientific frontier as the satellite Earth Observation develops. Obvious electromagnetic abnormal variations were recorded by ground-based ULF electrical field and geomagnetic systems, GPS and DEMETER satellites three days before the Wenchuan MS8.0 earthquake, which indicates the occurrence of an LAIC. In this investigation, firstly, combined with the observed electrical field data, a horizontal surface current source involved in the ‘Earth–ionosphere’ model is employed to simulate and calculate the magnitude of the ‘energy source’ (seismotelluric current) which gives rise to the LAI electromagnetic coupling prior to the Wenchuan main shock; Secondly, the additional electrical field at the ionospheric bottom can be attained by solving the current continuity equation when the seismotelluric current propagates through the atmosphere from the surface of the Earth. Finally, the basic equations of electrical field influence on ionosphere are solved to attain the variation magnitudes of ionospheric parameters. These results can be compared with the data recorded by GPS and DEMETER satellites at the same period. This subject attempts to explore the mechanism of the LAI electromagnetic coupling associated with seismic activities and tentatively investigate the transmission and interaction of electromagnetic energy between lithosphere and the above layers of atmosphere and ionosphere. This subject also attempts to search some ways to interpret real-time observed data and methods to pursue seismic precursors, which is of most importance for earthquake prediction and natural disaster defense.

随着地震天地一体化观测的发展，地震相关岩石圈—大气层—电离层耦合的机制和过程成为当前科学前缘问题。汶川MS8.0地震前3天，地面ULF电磁扰动和地磁、空间GPS、DEMETER卫星观测均记录到显著电磁异常，这些现象意味着电磁耦合发生。本课题首先利用“地—电离层”模式地下水平面电流源模型，结合地面实测数据，模拟和计算引起汶川地震电磁耦合的“能量源”(地电流)大小；然后，利用空间电流的连续性方程，计算地电流在电性非均匀大气层中向上传播，到达电离层底界面产生的附加电场；最后，利用附加电场引起电离层等离子体变化的基本方程，研究附加电场引起的电离层参数变化幅度，与GPS、DEMETER卫星实测数据对比分析。本课题探索性地研究汶川震前空间电磁耦合机制，尝试定量研究电磁能量在三层间的传输和交换过程，探寻地震天地一体化实测数据的解释途径及捕捉地震前兆的方法，对地震预测和灾害防御具有重要的现实意义。

当前，地震相关电磁异常信息产生和传播机理不甚明了，地震电磁天地一体化观测和资料研究仍处于初步阶段，开展典型内陆强震相关多参量电磁异常资料收集和整理，建立物理模型，探索性地研究地震电磁信息产生机制、作用过程和实测资料解释途径，有利于提高资料利用和地震前兆的探寻。这顺应了当前地震天地一体化观测技术的发展趋势和空间电磁现象耦合机制的前沿研究方向，对地震预测和灾害防御具有重要的现实意义。收集汶川MS8.0地震异常高潮期地面电磁和空间电离层观测资料以及相关发表文献，重点分析各参量异常时序演化特征，定性推断汶川地震引起的LAI电磁耦合基本过程。建立考虑电离层影响的 “地—电离层”模式等效源物理模型(I)和半空间模型(II)，研究地下有限长电偶源模型产生的电磁场，结果显示，在一定范围内，电离层影响可以忽略；当收发距大于这个范围时，电离层与入射电磁场发生相长干涉，使入射电磁场衰减变慢，影响可达1-2个数量级，这在实际的观测中相当于增强了仪器的探测能力，很好地解释了远距离观测到地震电磁信号的观测事实；随发射源频率的增大，电离层的影响会在更小的接收范围显示出来。结合震中距1440km的高碑店台记录到1.3mV/m的电场强度，反推两种情况下地下瞬间电流分别为5.3×104 kA 和3.7×105 kA，二者差一个数量级，完成汶川地震LAI电磁耦合“能量源”计算。汶川源产生的电磁场到达电离层底界面中心点处的附加电场可达V/m量级，1000km范围内大于0.1mV/m，与前人研究结果相比较，这一附加电场引起的电离层变化幅度与实际观测相符合，理论计算仍在建模中，结果还需进一步验证。鉴于课题的主要研究内容为国际科学前沿问题，具有较强的探索性，在项目实施过程中，增加三维“地—电离层”式球形空腔模型研究，该模型已经完成设计并获得一次场初步模拟结果。