Indications of Ground-based Electromagnetic Observations to A Possible Lithosphere–Atmosphere–Ionosphere Electromagnetic Coupling before the 12 May 2008 Wenchuan MS 8.0 Earthquake

Mei Li,Xuhui Shen,Xuemin Zhang,Jun Lu

doi:10.3390/atmos10070355

Abstract

A large number of various precursors have been reported since the Wenchuan MS 8.0 earthquake (EQ) took place on 12 May 2008 in China. In this work, previous investigations of both ground-based electromagnetic (EM) parameters and spatial ionospheric parameters were first examined. The statistical results showed that various anomalies presented different time-scale variations but tended to be characterized by a common feature – reaching their climax on 9 May, three days before the Wenchuan event, which indicates a lithosphere–atmosphere–ionosphere (LAI) electromagnetic coupling. Second, the fluctuations on 9 May based on the observational ground-based ultra low frequency (ULF) electrical field at the Gaobeidian (GBD) station and the direct current/ultra low frequency (DC–ULF) geomagnetic vertical Z field at the Chengdu (CD) station were comparably analyzed with those of ionospheric disturbances reported previously. The results showed that distinct electromagnetic changes, geomagnetic “double low-point” phenomena, and ionospheric disturbances above both sides of the Earth started in turn, respectively, but reached their climax simultaneously within dozens of hours on 9 May. This evolutionary process increases the probability that electromagnetic energy propagates from the epicentral area, via the atmosphere and ionosphere, to the equatorial plane, and through this plane finally to its magnetically conjugated area in the opposite hemisphere, causing electromagnetic disturbances on the Earth’s surface, in the atmosphere, and in the ionosphere and its conjugate point, in that order.

Highlights

Ground-based electromagnetic (EM) observation is one of the geophysical methods employed early to search precursors related to seismic activities
An important characteristic of probable EM precursors is their appearance in a wide frequency band, covering from the direct current/ultra low frequency (DC–ULF), very low frequency (VLF), and low frequency (LF) to very high frequency (VHF) ranges
We are convinced that the variations both on ground-based electrical fields and the geomagnetic vertical Z element reached their peak magnitudes in the same period of ≈1200 local time (LT), which is close to the registered time (≈1330 LT)

Summary

Introduction

Ground-based electromagnetic (EM) observation is one of the geophysical methods employed early to search precursors related to seismic activities. The ULF band has been gaining more attention because only EM signals originating from the epicenter depths of EQs can be recorded at the Earth’s surface without significant attenuation [1]. Unusual ULF (f < 10 Hz) EM emissions were recorded 12 days prior to the Loma Prieta Ms 7.1 EQ, on. 17 October 1989 and 3–5 days prior to the great crustal MS 6.9 EQ at Spitak, Armenia, on 7 December. The abnormities occurred a maximum of 3 h before the Spitak event, 4 h before the Loma Prieta event, and 5 h before the 8 September 2017 offshore Chiaps MW 8.1 EQ [2,3,4,5,6].

Results

Discussion

Conclusion