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 slo-py-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.


Introduction
The characteristics of strong ionospheric disturbances associated with the 2011 Tohoku-Oki earthquake (M9.0) occurring at 0546:18 UTC on 11 March 2011 in Japan have been analyzed by the high-resolution GPS total electron content (TEC) observation of GPS receiver network, GEONET [1] [2] [3] [4], the ionograms of four ionosonde stations in Japan [5] [6], and the SuperDARN Hokkaido HF radar [7] [8]. The initial ionospheric disturbances observed by GPS TEC measurements showed small impulsive TEC enhancements about 7 min after the earthquake onset and concentric waves appearing to propagate in the radial direction with a velocity 138 -3457 m/s [3]. Ionospheric disturbances were detected at 06:00 UTC at four ionosonde stations in Japan and an irregular distorted echo trace was observed at Kokubunji closest to the epicenter and 400 km away [5] [6]. The SuperDARN Hokkaido HF radar observed northward-propagating disturbances (343 -136 m/s) due to atmospheric gravity waves (AGWs) exiting near the epicenter after the passage of the 6.7 -1.8 km/s waves [8]. The preseismic ionospheric anomalies detected before the earthquake are investigated with a method of CoRelation Analysis (CRA) from phase delays of signals observed at densely arranged ground-based stations of Global Navigation Satellite Systems for the 2011 Tohoku-Oki earthquake [9], the 2016 Kumamoto earthquake [10] and the 2016 Taiwan earthquake [11]. The preseismic ionospheric electron enhancements before large earthquake have been reported for short-term earthquake prediction [12] [13] [14]. Preseismic ionospheric anomalies in Taiwan were examined to reveal that the ionospheric TEC and NmF2 surrounding the epicenters of the events decreased abnormally within 5 days before large earthquakes [15] [16] [17] [18].
Ionospheric disturbances associated with the 2011 Tohoku-Oki earthquake have been studied in many excellent papers. The oblique ionospheric sounding data, namely oblique ionograms have not been previously examined for studying the characteristics of anomalous radio propagation associated with the super strong earthquake. It is new to focus on both before and after the strong earthquake, and to study the ionospheric disturbances of strong earthquake as seen by oblique ionograms. In this paper the characteristics of anomalous radio propagation in the frequency 1 -30 MHz before and after the 2011 Tohoku-Oki earthquake are investigated by oblique ionograms obtained for the pass of relatively medium distance 1068 km between Wakkanai and Kokubunji which is closest to the epicenter. This paper focuses on the trace shape and trace distortions around the maximum observable frequency in the oblique ionogram trace which shows a signature of the anomalous radio propagation. Vertical and oblique ionospheric sounding system in Japan is shown briefly. Then the characteristics appearing in oblique ionograms before and after the 2011 Tohoku-Oki earthquake are shown and discussed in three types of oblique ionogram traces.  , and +120 s for Wakkanai, Kokubunji, Yamagawa, and Okinawa, respectively [5]. Therefore both of vertical and oblique ionograms are available at approximately same time. The sweep-frequency rate of these four ionosondes is 2 MHz/s. The transmitter and receiver frequency of ionosonde are varied together over a range of 1 -30 MHz. In this paper we used oblique ionograms obtained for a medium distance 1068 km path between Wakkanai and Kokubunji which is closest to the epicenter of the 2011 Tohoku-Oki earthquake (38.10˚N, 142.86˚E). Figure 2 shows the geometry of the one-hop F2 propagation for the high-angle ray path and low-angle ray path as the oblique ionospheric sounding, and the vertical ionospheric sounding at Wakkanai and Kokubunji. The sweep-frequency ionospheric sounding technique with High-frequency (3 -30 MHz) radio waves was developed to probe the vertical structure of the ionosphere and to study the ionospheric dynamics. Ionosondes are operated routinely for the ionospheric weather monitoring, providing ionospheric variations in near real-time and from a global network of observatories. It is also possible to investigate the characteristics of anomalous radio propagation by the oblique ionospheric sounding method receiving sweep-frequency pulsed radio waves from other vertical ionospheric Open Journal of Earthquake Research

Characteristics of Oblique Ionograms after the 2011
Tohoku-Oki Earthquake    We found that the characteristic of strong disturbances triggered by the earthquake shows very clear signature of wavy-shape-trace appeared in the ob-lique ionogram. Our result about the wavy-shape-trace seen by oblique ionogram for the pass from Wakkanai to Kokubunji is consistent with the existing physical theory of postseismic phenomena to cause due to AGWs exited near the epicenter according to the above analysis.

Characteristics of Oblique Ionograms before the 2011 Tohoku-Oki Earthquake
The oblique ionograms within 2 hours before the 2011 Tohoku-Oki earthquake are shown in Figures 6(a)   March 2011 are presented for Wakkanai and Kokubunji in Figure 9. The hourly values of foF2 are manually scaled with reading accuracy 0.1 MHz which has been established internationally [20]. The difference between the monthly median of foF2 and the hourly value of foF2 is from +0.  Figure 10 shows the conceptual diagrams for the "nose" of  the relatively medium distance oblique ionograms which were observed before and after the 2011 Tohoku-Oki earthquake. Normal oblique ionogram in the daytime shows (a) parabolic-shape-trace. Anomalous oblique ionograms after and before the earthquake show (b) wavy-shape-trace and (c) slopy-shape-trace respectively. Point-to-point oblique ionograms are traces of group path versus frequency for oblique HF radio propagation between two fixed points separated by some distance. The "nose" of oblique ionogram with the parabolic-shape-trace shows two propagation path of low angle ray and high angle ray at the same frequency between two fixed points of Wakkanai and Kokubunji as shown in Figure 2. The slopy-shape-trace is considered a signature under some disturbed

Summary and Conclusions
We have investigated the characteristics of anomalous HF propagation before and after the 2011 Tohoku-Oki earthquake that appeared in the oblique ionograms for the pass of relatively medium distance 1068 km from Wakkanai to Kokubunji. An oblique ionogram with a wavy-shape-trace was observed at 06:30 UTC on 11 March 2011 after the 2011 Tohoku-Oki earthquake. This wavy-shape-trace of oblique ionogram is considered northward-propagating disturbances due to AGWs excited near the epicenter. The propagation velocity of this disturbance is estimated to be 130 m/s. We found that the characteristic of strong ionospheric disturbances triggered by the earthquake appearing in the oblique ionogram shows very clear signature of the wave-shape-trace.
An oblique ionogram with a slopy-shape-trace was observed at 04:45 UTC on 11 March 2011 one hour before the 2011 Tohoku-Oki earthquake. The slope ratio of virtual height h' to sweep-frequency f (Δh'/Δf) is about 25 (km/MHz) for the oblique ionogram at 04:45 UTC. Characteristics of the other oblique ionograms before the earthquake showed normal oblique ionograms with a parabolic-shape-trace. The difference between the monthly median of foF2 and the hourly value of foF2 at 04:45 UTC was +0.85 MHz. The ionospheric condition around 04:45 UTC before the earthquake was no disturbances at quarter hourly level on 11 March 2011. A steep slopy-shape-trace before one hour of the earth-quake is necessary to investigate further its physical characteristics and the frequency of occurrence before the strong earthquake. The conceptual diagrams classified into three types of (a) parabolic-shape-trace, (b) wavy-shape-trace, and (c) slopy-shape-trace for the "nose" of the relatively medium distance oblique ionograms were shown and discussed. As the oblique ionogram is possible to reveal the ionospheric variations along the propagation pass over the ocean above tectonic plates, it is useful for a study of the preseismic ionospheric anomaly.