Abstract
During the Mw9 Tohoku-Oki earthquake, gradual increases in both ionosphere total electron content (TEC) and geomagnetic declination signals were observed, starting from ~40 minutes before the mainshock, followed by impulsive enhancements ~10 minutes after the mainshock. There have been many studies on pre-seismic TEC enhancements, including their characteristics, debates regarding whether TEC anomalies are real signals or artefacts, and the explainable models, and many studies have reported that the impulsive TEC enhancement was caused by a tsunami-induced neutral atmospheric gravity wave. Since TEC and geomagnetic declination anomalies were synchronized so that their origin should be attributed to the same seismic activities, any models must explain both anomalous phenomena, but not the case considered herein. Compared with the corresponding TEC anomalies, we re-examined the characteristics of geomagnetic variation just before and after the mainshock, focusing on the generation process of the impulsive enhancement immediately after the mainshock. We showed that the observed anomaly could be explained if there are quasi-static electric currents of 20 - 30 kA generated near the epicentre area. The possible mechanism of the current generation is discussed in terms of the ionization process in the atmosphere near the sea surface.
Highlights
There have been many studies on pre-seismic total electron content (TEC) enhancements, including their characteristics, debates regarding whether TEC anomalies are real signals or artefacts, and the explainable models, and many studies have reported that the impulsive TEC enhancement was caused by a tsunami-induced neutral atmospheric gravity wave
Methods, if any, can be used to identify precursor signals are of considerable concern from the viewpoint of active disaster prevention. Such a case occurred for the 2011 Mw9.0 Tohoku-Oki earthquake; that is, anomalous ionospheric disturbances that appeared above the rupture zone immediately before and after the mainshock were revealed by high-resolution GPS total electron content (TEC) observation
Note that synchronous with change in the ionospheric TEC signals, a similar anomaly appeared in geomagnetic declination change (ΔD); i.e., preseismic positive increases starting from ~40 minutes before the mainshock up to ~0.4 arcmin at stations closer to the epicentre, followed by an impulsive ΔD increase ~10 minutes after the mainshock [6], whether the ΔD anomalies were due to a space ionospheric magnetic storm [7] or a pre-seismic activity [8] is still being discussed
Summary
Note that synchronous with change in the ionospheric TEC signals, a similar anomaly appeared in geomagnetic declination change (ΔD); i.e., preseismic positive (eastward) increases starting from ~40 minutes before the mainshock up to ~0.4 arcmin at stations closer to the epicentre, followed by an impulsive ΔD increase ~10 minutes after the mainshock [6], whether the ΔD anomalies were due to a space ionospheric magnetic storm [7] or a pre-seismic activity [8] is still being discussed We believed that both the pre-seismic anomalies in TEC and ΔD satisfy the validation criteria in the “guidelines for the submission of earthquake precursor candidates” presented by Wyss [9], except for an understanding of the underlying causal mechanism [6] [10]. The corresponding ionospheric VTEC data used to compare with the geomagnetic data are reported elsewhere in detail [6]
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