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

Abstract

The results of simultaneous monitoring of subionospheric LF propagation over two different paths prior to a very strong Tokachi-oki earthquake (near the east coast of Hokkaido Island on September 25, 2003) with magnitude 8.3 are first presented. Nighttime amplitude fluctuations of the Japanese Time Standard Transmitter (JJY, 40 kHz) signal received at Moshiri (Japan, geographic coordinates 44 deg N, 142 dig E) and at Petropavlovsk-Kamchatski, Russia (53 deg N, 158 deg E) were analyzed. As a possible precursory signature synchronous intensification of quasi-periodic 16-day variations of the dispersion in the signals received at both observation stations was observed 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.