Abstract
The precursory atmospheric gravity wave (AGW) activity in the stratosphere has been investigated in our previous paper by studying an inland Kumamoto earthquake (EQ). We are interested in whether the same phenomenon occurs or not before another major EQ, especially an oceanic EQ. In this study, we have examined the stratospheric AGW activity before the oceanic 2011 Tohoku EQ (Mw 9.0), while using the temperature profiles that were retrieved from ERA5. The potential energy (EP) of AGW has enhanced from 3 to 7 March, 4–8 days before the EQ. The active region of the precursory AGW first appeared around the EQ epicenter, and then expanded omnidirectionally, but mainly toward the east, covering a wide area of 2500 km (in longitude) by 1500 km (in latitude). We also found the influence of the present AGW activity on some stratospheric parameters. The stratopause was heated and descended; the ozone concentration was also reduced and the zonal wind was reversed at the stratopause altitude before the EQ. These abnormalities of the stratospheric AGW and physical/chemical parameters are most significant on 5–6 March, which are found to be consistent in time and spatial distribution with the lower ionospheric perturbation, as detected by our VLF network observations. We have excluded the other probabilities by the processes of elimination and finally concluded that the abnormal phenomena observed in the present study are EQ precursors, although several potential sources can generate AGW activities and chemical variations in the stratosphere. The present paper shows that the abnormal stratospheric AGW activity has also been detected even before an oceanic EQ, and the AGW activity has obliquely propagated upward and further disturbed the lower ionosphere. This case study has provided further support to the AGW hypothesis of the lithosphere-atmosphere-ionosphere coupling process.
Highlights
It is recently agreed that precursory effects of an earthquake (EQ) could disturb the ionosphere
The physical mechanism on how pre-EQ activities in the lithosphere can affect the ionosphere, i.e., lithosphere-atmosphere-ionosphere coupling (LAIC) process, is not well understood, but a few hypotheses have been proposed: (1) chemical channel, in which radon emanation from the lithosphere is the main player [9,10,11]; (2) atmospheric gravity wave (AGW)/acoustic wave (AW) channel that is excited by any perturbations with ground or atmospheric oscillations [12,13,14,15]; (3) electrostatic channel that is based on the generation of stress-activated positive holes that are near the ground surface [16,17] (stress changes affect several properties, including dielectric constants [18] andorientation [19] of the electric dipoles that formed due to point defects [20,21])
As compared with the first chemical and the third electrostatic channel, much observational evidence has been mainly accumulated in the subionospheric very low frequency/low frequency (VLF/LF) data to support the second AGW hypothesis [12], and we think that the AGW channel is the most promising candidate as the LAIC mechanism
Summary
It is recently agreed that precursory effects of an earthquake (EQ) could disturb the ionosphere (see recent books by [1,2,3,4]). The physical mechanism on how pre-EQ activities in the lithosphere can affect the ionosphere, i.e., lithosphere-atmosphere-ionosphere coupling (LAIC) process, is not well understood, but a few hypotheses have been proposed: (1) chemical (and associated electric field) channel, in which radon emanation from the lithosphere is the main player [9,10,11]; (2) atmospheric gravity wave (AGW)/acoustic wave (AW) channel that is excited by any perturbations with ground or atmospheric oscillations [12,13,14,15];. As compared with the first chemical and the third electrostatic channel, much observational evidence has been mainly accumulated in the subionospheric VLF/LF data to support the second AGW hypothesis [12], and we think that the AGW channel is the most promising candidate as the LAIC mechanism
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