Abstract. Earthquakes may disturb the lower ionosphere through various coupling mechanisms during the seismogenic and coseismic periods. The VLF (very low-frequency) signal radiated from ground-based transmitters will be affected when it penetrates the disturbed ionosphere above the epicenter area, and this anomaly can be recorded by low-Earth orbit satellites under certain conditions. In this paper, the temporal and spatial variation of the signal-to-noise ratio (SNR) of the VLF transmitter signal in the ionosphere over the epicenter of 2010 Yushu Ms 7.1 earthquake in China is analyzed using DEMETER (Detection of Electro-Magnetic Emission Transmitted from Earthquake Regions) satellite observation. The results show that SNR over the epicenter of the Yushu earthquake especially in the southwestern region decreased (or dropped) before the main shock, and a GPS–TEC (Global Positioning System; total electron content) anomaly accompanied, which implies that the decrease in SNR might be caused by the enhancement of TEC. A full-wave method is used to study the mechanism of the change in SNR before the earthquake. The simulated results show SNR does not always decrease before an earthquake. When the electron density in the lower ionosphere increases by 3 times, the electric field will decrease about 2 dB, indicating that the disturbed-electric-field decrease of 20 % compared with the original electric field and vice versa. It can be concluded that the variation of electron density before earthquakes may be one of the important factors influencing the variation of SNR.

中文翻译：

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DEMETER卫星观测到的与2010年玉树7.1级地震相关的极低频发射机电波异常及其可能机制

摘要。在发震和同震期间，地震可能通过各种耦合机制扰乱低电离层。地基发射机辐射的甚低频（VLF）信号穿过震中区上方受干扰的电离层时会受到影响，在一定条件下，近地轨道卫星可以记录到这种异常。本文利用DEMETER（Detection of Electro-Magnetic Emission，2010年玉树7.1级地震震中）电离层VLF发射机信号的信噪比（SNR）时空变化进行分析。从地震区传输）卫星观测。结果表明，玉树地震震中特别是西南地区的信噪比在主震前下降（或下降），并且伴随着 GPS-TEC（全球定位系统；总电子含量）异常，这意味着 SNR 的降低可能是由 TEC 的增强引起的。使用全波方法研究震前信噪比变化的机制。模拟结果显示地震前信噪比并不总是降低。当低层电离层的电子密度增加3倍时，电场将减少约2 dB，表明干扰电场与原始电场相比减少了20%，反之亦然。由此可见，震前电子密度的变化可能是影响信噪比变化的重要因素之一。这意味着 SNR 的降低可能是由 TEC 的增强引起的。使用全波方法研究震前信噪比变化的机制。模拟结果显示地震前信噪比并不总是降低。当低层电离层的电子密度增加3倍时，电场将减少约2 dB，表明干扰电场与原始电场相比减少了20%，反之亦然。由此可见，震前电子密度的变化可能是影响信噪比变化的重要因素之一。这意味着 SNR 的降低可能是由 TEC 的增强引起的。使用全波方法研究震前信噪比变化的机制。模拟结果显示地震前信噪比并不总是降低。当低层电离层的电子密度增加3倍时，电场将减少约2 dB，表明干扰电场与原始电场相比减少了20%，反之亦然。由此可见，震前电子密度的变化可能是影响信噪比变化的重要因素之一。当低层电离层的电子密度增加3倍时，电场将减少约2 dB，表明干扰电场与原始电场相比减少了20%，反之亦然。由此可见，震前电子密度的变化可能是影响信噪比变化的重要因素之一。当低层电离层的电子密度增加3倍时，电场将减少约2 dB，表明干扰电场与原始电场相比减少了20%，反之亦然。由此可见，震前电子密度的变化可能是影响信噪比变化的重要因素之一。