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

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.

Earthquakes associated subionospheric VLF anomalies recorded at two low latitude stations in the South Pacific region

“…These anomalies were observed for EQs with magnitude more than 5.5, shallow depth up to 32 km, and epicenter position less than 185 km off the TRGCP. Our results are consistent with the earlier results of Hayakawa et al (2011), Kasahara et al (2010) and Khadka et al (2017) who found a simultaneous decrease in the T and increase in the NF and D. The T showed a decrease in the average of nighttime dA(t) values on EQ day because of the reduction in the ionospheric VLF reference height (Molchanov et al, 1998;Zhao et al, 2020).…”

Reduction of the VLF Signal Phase Noise Before Earthquakes

“…Studies of the ionospheric changes as precursors of earthquakes report changes usually a few days before events [9,13,35]. These lower ionosphere disturbances are detected as the solar terminator shift [13,36,37] and deviations of the time evolutions of signal characteristics from values recorded during days with unperturbed conditions [5,38] (in time domain), and as variations in the wavelet power spectrum [5,39] (in frequency domain), and all of these changes are shown for both signal amplitude and phase. The recent analysis presented in [33] shows reduction of the amplitude noise of the VLF signal less than one hour before the earthquake occurred near Kraljevo, Serbia, on 3 November 2010 (the seismotectonic model of this event is presented in [40]), as well as excitation and attenuation of the acoustic waves.…”

Potential Earthquake Proxies from Remote Sensing Data

“…In Ref. [73], seismic anomalies before two strong earthquakes with M w > 7 in western China were detected using ground-based observation of VLF signals from monitoring stations. A full wave model was used to research possible factors inducing seismic anomalies on the received VLF electric field.…”