Capability of TEC correlation Analysis and Deceleration at Propagation Velocities of Medium-Scale Traveling Ionospheric Disturbances: Preseismic Anomalies before the Large Earthquakes

Data analysis method (CRA, hereafter) to correlate multiple TEC anomaly signals has detected pre-seismic anomalies before the 2011 Tohoku-Oki earthquake (Iwata & Umeno 2016), the 2016 Kumamoto earthquake (Iwata & Umeno 2017) and the 2016 Tainan earthquake (Goto et al. 2019). However, a critical argument said that those anomalies detected by CRA would not be pre-seismic anomalies published by Journal of Geophysical Research-Space Physics (126), 2021 (JGR-SP (126), hereafter). In this paper, we would point out its incorrect use of statistical anomalies in evaluating CRA as the following points: CRA is shown to increase the signal-to-noise ratio (SNR) to amplify pre-seismic TEC’s small anomaly signals with synchronizing and correlating multiple GNSS receivers’ data. We proved again that pre-seismic anomalies certainly exist before the 2011 Tohoku-Oki earthquake and the 2016 Kumamoto earthquake with additional data analysis. In particular, as a temporal anomaly, deceleration at propagation velocities of medium-scale traveling ionospheric disturbances (MSTID, hereafter) before the 2016 Kumamoto earthquake captured by CRA (Iwata & Umeno 2017) is elucidated as pre-seismic anomalies. Furthermore, we proposed a physical model to predict that 35 m/s change at MSTID propagation velocities estimated by TEC’s CRA requires 0.58 × 10-3 V/m electric field in the F Layer ionosphere. Contrary to the claim with the incorrect use of statistical anomalies in JGR-SP (126), TEC’s correlation anomalies detected by CRA (Iwata & Umeno 2016 and Iwata & Umeno 2017) clearly provided supporting evidence that physical pre-seismic anomalies really exist.

Characteristics of Anomalous Radio Propagation before and after the 2011 Tohoku-Oki Earthquake as Seen by Oblique Ionograms

The characteristics of anomalous radio propagation in the frequency 1 - 30 MHz before and after the 2011 Tohoku-Oki earthquake are firstly revealed by using oblique ionograms for the pass from Wakkanai to Kokubunji which is closest to the epicenter. An oblique ionogram with a wavy-shape-trace was observed at 06:30 UTC on 11 March 2011 after the 2011 Tohoku-Oki earthquake. The velocity of northward-propagating disturbance caused this wavy-shape-trace is estimated to be 130 m/s. This wave-shape-trace shows very clear signature appearing in the oblique ionograms as the characteristic of strong ionospheric disturbances triggered by the earthquake. An oblique ionogram with a steep slopy-shape-trace was observed at 04:45 UTC on 11 March 2011 one hour before the 2011 Tohoku-Oki earthquake. This slopy-shape-trace is investigated as the signatures of preseismic ionospheric anomaly. This anomalous oblique ionogram with a slopy-shape-trace is examined with the slope ratio of virtual height to sweep-frequency, and the difference between monthly median foF2 and hourly value foF2 at Wakkanai and Kokubunji. These features appearing in oblique ionograms suggest that it is useful for studying the signatures of preseismic ionospheric anomaly.