Subionospheric LF monitoring of ionospheric perturbations prior to the Tokachi-oki earthquake and a possible mechanism of lithosphere-ionosphere coupling

We first present the results of simultaneous monitoring of subionospheric LF propagation over two di#erent paths prior to a very strong Tokachi-oki earthquake (near the east coast of Hokkaido Island on September ,/, ,**-) with magnitude 2.-. Nighttime amplitude fluctuations of the Japanese Time Standard Transmitter (JJY, .* kHz) signal received at Moshiri (Japan, geographic coordinates .. N, +., E) and at Petropavlovsk-Kamchatski, Russia (/N, +/2 E) were analyzed. As a possible precursory signature we observed synchronous intensification of quasi-periodic +0-day variations of the dispersion in the signals received at both observation stations before the earthquake. The strongest deviations observed as a rule were depletions of signal amplitude probably connected with an increase in loss in the ionosphere by the enhancement of turbulence. This is due to dissipation of internal gravity waves (IGW) at the lower ionospheric heights. A scenario of interconnection between seismo-activity, atmospheric gravity waves and planetary waves, is proposed to explain the observed association with strong earthquakes. key words: subionospheric LF propagation, earthquakes, ionospheric perturbations

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