<p><strong>Abstract.</strong> Earthquakes may disturb the lower ionosphere through various coupling mechanisms during their seismogenic and coseismic periods. The VLF signal radiated from ground-based transmitters will get affected when it penetrates the disturbed region in the ionosphere above the epicenter area, and this anomaly can be recorded by low earth orbit satellite under certain conditions. In this paper, the temporal and spatial variation of the Signal to Noise

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... (SNR) of the VLF transmitter signal in the ionosphere over the epicenter of 2010 Yushu Ms 7.1 earthquake in China is analyzed. The results show that the SNR over the epicenter of Yushu earthquake especially in the southwestern region decreased (or dropped)revealed by one satellite revisit period before the main shock, which is consistent with the observed TEC anomaly at same time, implying that the decrease of SNR might be caused by the enhancement of TEC. A full-wave method was used to study the mechanism of the change of SNR before the earthquake. When the electron density in the lower ionosphere increases by four times, the electric field will decrease about 1 dB, indicating that the disturbed electric field decrease 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 important factor influence the variation of SNR.</p>