An analytical expression for early electromagnetic signals generated by impulsive line-currents in conductive Earth crust, with numerical examples
An analytical expression for early electromagnetic signals generated by impulsive line-currents in conductive Earth crust, with numerical examples
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DOI：
10.4401/ag-6867
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发表时间：
2016-05
影响因子：
1
通讯作者：
K. Yamazaki
中科院分区：
文献类型：
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作者：
K. Yamazaki
地震破裂后但地震波到达之前的电磁场变化（“早期电磁信号”）有时也有报道。定量评价是澄清观测到的现象是否可以用已知的理论解释，并评估该现象是否可以应用于地震预警的必要条件。因此，导出了导电半空间模型和两层模型中脉冲线电流产生磁场的解析表达式；还考虑了导电全空间的稍微简单的情况。通过对产生的电磁场表达式的分析，讨论了早期电磁信号的一些预期特征。首先，我验证了早期的电磁信号在地震波之前到达，除非电导率相对较高。其次，我表明，当震源靠近地面时，早期的电磁信号可以很好地近似于全空间模型，但当震源在深处时则不然。第三，我表明，早期电磁信号的预期幅度在常用电磁传感器的检测范围内，前提是地电导率不是很高，源电流足够强。然而，这并不意味着电磁信号很容易区分，因为探测器的灵敏度不考虑附加噪声或假阳性检测。
Changes in the electromagnetic (EM) field after an earthquake rupture but before the arrival of seismic waves (“early EM signals”) have sometimes been reported. Quantitative evaluations are necessary to clarify whether the observed phenomena are accounted for by known theories and to assess whether the phenomenon can be applied to earthquake early warning. Therefore, analytical expressions for the magnetic field generated by an impulsive line-current are derived for a conductive half-space model, and for a two-layer model; the somewhat simpler situation of a conductive whole-space is also considered. By analyzing the expressions obtained for the generated EM field, some expected features of the early EM signals are discussed. First, I verify that an early EM signal arrives before the seismic waves unless conductivity is relatively high. Second, I show that early EM signals are well approximated by the whole-space model when the source is near the ground surface, but not when it is at depth. Third, I show that the expected amplitudes of early EM signals are within the detection limits of commonly used EM sensors, provided that ground conductivity is not very high and that the source current is sufficiently intense. However, this does not mean that the EM signals are easily distinguishable, because detector sensitivity does not account for additive noise or false positive detections.