Anomalous sporadic-E layers observed before M 7.2 Hyogo-ken Nanbu Earthquake: Terrestrial gas emanation model

Abstract

Before the M 7.2 Hyogo-ken Nanbu earthquake of January 17, 1995, anomalous f(sub 0)E(sub s) increases above 7 MHz were observed at Shigaraki about 120 km northeast of the epicenter and at Kokubunji about 500 km east of it in the daytime on January 15, 1995. Since there were very quiet geomagnetic conditions and no solar event on January 15 and the f(sub 0)E(sub s) is normally below 5 MHz in the Japanese winter, the anomalous f(sub 0)E(sub s) increases above 7 MHz observed on January 15 may be presumably an ionospheric seismic precursor. An upward tornado-type seismic cloud with rapid air currents occurred between altitudes of 680 m and 2,000 m over the epicenter region, and anomalous radon ion density increases of about 100 times the normal radon ion density were also observed at epicentral distance of about 200 km before this earthquake. The epicentral area of the anomalous f(sub 0)E(sub s) increase occurrence before this earthquake is about the same as that of the terrestrial gas emanation from active faults before great earthquakes (C.-Y. King; J. Geophys. Res., 91B, 12,269, 1986). The radon ions carried up to cold high altitudes produce positively charged ice crystals in the topside cloud and the bottom-side cloud has negative charges. An electrostatic field is set up in the lower ionosphere by positively charged cloud-to-ground lightning discharges. A temporal electrostatic field above the air breakdown electric field causes ambient electron heating and ionization of neutrals in the lower ionosphere. Times for producing the anomalous f(sub 0)E(sub s) increases computed by the quasi-electrostatic heating and ionization process at altitude of 100 km are comparable with seismic flash duration before great earthquakes.