IONOSPHERIC DISTURBANCES ASSOCIATED WITH GREAT EARTHQUAKE OF HOKKAIDOSOUTHWEST COAST, JAPAN OF JULY 12, 1993

A great earthquake of M 7.8 occurred near the Okushiri island off the Hokkaido southwest coast in northern Japan at 2217 JST (UT + 9 h), Jul y 12, 1993. Systematic decreases of the ionospheric F-2-layer critical frequency, f(0)F(2) below its monthly median, were observed at Wakkan ai, Kokubunji and Yamagawa, Japan from 3 days before the earthquake on set to 3 days after it. The virtual height of the ionospheric F-layer, h'F increased above its monthly median for nights of July 9, 10 and 1 1 before the earthquake onset at Kokubunji; while the h'F decreased be low its monthly median for nights of July 12, 13 and 14 after the eart hquake onset. Local geomagnetic activities near the three ionospheric stations were quiet or moderate with no geomagnetic storm from July 5 to July 16. The ionospheric disturbances observed were independent of solar-terrestrial events. Possible generation mechanisms are discussed on the ionospheric disturbances associated with the earthquake. (C) 1 998 Elsevier Science B.V.