type:招待論文

There are two kinds of methods to estimate precursory phenomena of earthquakes using electromagnetic wave. One is to measure direct wave from near the ground surface of the epicenter. ULF/ELF electromagnetic waves are used because propagation loss is very low for these waves. The other is to measure the change of the intensity or phase of electromagnetic wave from a transmitting station reflected at the ionosphere. These fluctuations are arrested as a change of ionosphere that is disturbed by the energy from the epicenter. VLF/VHF electromagnetic waves are used in order to observe electromagnetic wave from transmitting station. These two kinds of techniques are very different from each other in terms of observation place, observation system, and analysis methods. The subionospheric VLF signals (amplitude and phase) transmitted from NWC VLF transmitter (f=19.8 kHz) in Australia have been monitored at Kasugai (Geographic lat. 35°16' N , long. 137°01' E, Geomagnetic lat. 25°07' N, near Nagoya) in order to find out ionospheric disturbance associated with earthquakes. We have also carried out our observation of ULF/ELF electromagnetic waves below 50Hz at Nakatsugawa (Geographic lat. 35°25 ' N, long. 137°33' E, Geomagnetic lat. 25° 19' N) in Gifu Prefecture. In addition, we will show the precursory phenomena at the specific frequency 223Hz on the Off-West of Fukuoka earthquakes. In this frequency band, we tried to observe the direct wave from the earth or reflected wave from the ionosphere. We will report the relationship between precursor of the earthquake and excitation of these waves.