Investigation of Precursors in VLF Subionospheric Signals Related to Strong Earthquakes (M &gt; 7) in Western China and Possible Explanations

Shufan Zhao,Li Liao,Chen Zhou,Zhuangkai Wang,Hengxin Lu,Zeren Zhima,Xuhui Shen,Jing Cui

doi:10.3390/rs12213563

Abstract

Earthquakes may disturb the lower ionosphere through various coupling mechanisms during their seismogenic and coseismic periods. The VLF signal radiated from ground-based transmitters is affected when it passes near the disturbed region above the seismogenic area, and this anomaly can be recorded by ground-based VLF receivers. In this paper, the seismic anomalies before two strong earthquakes (M > 7) that occurred in western China were detected using the ground-based observation of VLF signal; the possible reasons for the anomalies were discussed using full wave simulation. The amplitude of the VLF signals observed by the link between NOV, KHA transmitter, and VLF receivers at Ya’an and Tonghai show obvious anomaly by nighttime fluctuation analysis. The simulated results demonstrate that the anomalies could have been induced by ascending/descending of the bottom height of the ionosphere, caused by depletion/increase in D region electron density. The simulated result also illustrates that terminator time shift could have been induced by descending of the bottom boundary of the ionosphere, which is due to modal interference between different wave modes.

Highlights

VLF (Very Low Frequency) radio waves radiated from powerful ground based transmitters could be reflected alternately by the earth and the subionospheric layer, forming a ‘zigzag’ path between the lithosphere-ionosphere waveguide [1]
The TT shift corresponds to shifts in sunrise and sunset time on the continuous record of the amplitude and phase of the VLF signal before the earthquake, which reveal that variation in the D region of the ionosphere could exist before the earthquake
Some researchers declare that the nighttime fluctuation of VLF/LF transmitter signals could predict seismic anomalies [28–31]

Summary

Introduction

VLF (Very Low Frequency) radio waves radiated from powerful ground based transmitters could be reflected alternately by the earth and the subionospheric layer, forming a ‘zigzag’ path between the lithosphere-ionosphere waveguide [1]. The emission of gases (mainly Radon) and particles, accompanied by the increasing stress of rock, produce an electric field/current that can disturb the ionosphere [15–17] in the period of earthquake preparation, which could explain the anomalies observed in the ionosphere [18–22] These seismic ionospheric disturbances could induce anomalies in signals from ground-based VLF/LF transmitters [23–25] by analyzing the received signal from existing known transmitters, we can detect seismic anomalies in the path between the VLF receiver and the transmitter. Lehtinen and Inan [34] proposed a full-wave finite element method to solve the electromagnetic field in the waveguide and ionosphere simultaneously [35,36], which is inherently stable against such “swamping” problem based on the recursive calculation of reflection coefficients and mode amplitudes This method can be used to study the TT shift and NF of the VLF signal in the waveguide between the transmitter and the receiver.

Instruments and Data

Full Wave Method

Findings

VLF Signal Analysis from the CAJ-1 Monitoring Station