This paper presents a case study of very low frequency (VLF) radio signal anomaly during a moderate geomagnetic storm (Kp = 6, Ap = 80 and Dst = − 60 nT) on September 10, 2018 followed by a moderate earthquake (M = 5.3) on September 12, 2018 which occurred near to (~ 80 km) a subtropical low latitude station Cooch Behar (CHB), India (26.345^°N, 89.448^°E). Signal amplitudes from the two transmitters, VTX (18.2 kHz) and NWC (19.8 kHz), are analyzed for this study. Seven VLF parameters, such as nighttime amplitude fluctuation (dA_n), amplitude difference between pre-sunrise signal and sunrise minimum signal (dA_m), amplitude difference between sunset minimum signal and post-sunset signal (dA_e), time (T_m) and amplitude (A_m) during sunrise minimum, time (T_e) and amplitude (A_e) during sunset minimum have been examined. Anomalous behaviors of all the parameters have been observed during September 10–12, 2018. D-region electron density is estimated using the long wave propagation capability code which revealed a maximum increment of ~ 20 electron/cm³ at altitude 85 km over the VTX-CHB path ~ 2 days before the earthquake event. The anomalous behaviors of the VLF signals ~ 1–2 days before the earthquake are identified as possible precursor of the earthquake which could be linked with various pre-earthquake seismo-electromagnetic processes.

本文介绍了一个案例研究，该案例研究是在2018年9月10日的中等地磁风暴（Kp = 6，Ap = 80和Dst = − 60 nT）以及随后的中等地震（M  = 5.3 ）期间的极低频（VLF）无线电信号异常）发生在2018年9月12日，发生在印度Cooch Behar（CHB）的一个亚热带低纬度站附近（〜80 km）（北纬26.345 ^°，东经89.448 ^°）。本研究分析了来自两个发射机VTX（18.2 kHz）和NWC（19.8 kHz）的信号幅度。七个VLF参数，例如夜间振幅波动（dA _n），日出前信号与日出最小信号之间的振幅差（dA _m），日落最低信号与日落后信号之间的振幅差（dA _e），日出最小时间的时间（T _m）和振幅（A _m），日落最小时间的时间（T _e）和振幅（A _e）已进行了检查。在2018年9月10日至12日期间观察到了所有参数的异常行为。使用长波传播能力代码估算了D区电子密度，该代码揭示了VTX上85 km的高度最大〜20电子/ cm ^3的增量-CHB路径〜地震发生前2天。地震发生前1-2天的VLF信号异常行为被认为是可能的地震前兆，这可能与地震前的各种地震电磁过程有关。