Very exceptional cases of VLF/LF ionospheric perturbations for deep oceanic earthquakes offshore the Japan island

It is so far believed that ionospheric perturbations as detected by subionospheric VLF/LF (very low frequency/low frequency) propagation, are generated above and around the earthquake (EQ) epicenter. This paper presents very rare cases, which are in complete contrast to the above fact. We have found that in extremely rare cases when EQs happened (i) in the Pacific Ocean or (ii) offshore the Soya cape (Hokkaido) both with very large depths (300-400 km), corresponding ionospheric perturbations take place far away from the EQ epicenter and above the regions with considerable seismic intensity at the time of each EQ. Two EQs happened in the Torishima area of lzu islands (magnitude 7 and depth 400 km), and corresponding seismic intensity was observed in the Tokyo and lbaraki districts. Our VLF data have indicated that the ionospheric perturbation takes place over such regions with high seismic intensity. Another group is two EQs (magnitude 5) offshore the Soya cape of Hokkaido, and the spatial distribution of seismic intensity at the time of each EQ is just around Aomori prefecture. VLF data have indicated the ionospheric perturbations taken place over the same Aomori area, which is in complete coincidence with the spatial distribution of seismic intensity. As a conclusion, these exceptional examples are, in principle, very similar to the concept of 'selectivity' (or sensitive zone) of geoelectric measurement by the Greek group, and we try to interpret these cases in the context of lithosphere-atmosphere-ionosphere coupling. (c) 2014 Elsevier Ltd. All rights reserved.