Electromagnetic precursors to the 2004 Mid Niigata Prefecture earthquake

This paper summarizes the electromagnetic precursory phenomena for the 2004 Mid Niigata Prefecture earthquake on October 23, 2004. Three different kinds of seismogenic perturbations have been observed; (1) lithospheric DC/ULF emissions, (2) atmospheric radio noise in the VHF band and (3) ionospheric disturbances. The DC/ULF emissions as observed at Nakatsugawa (in Gifu Prefecture) are detected on October 2-6, and the direction finding result has indicated the possible observed azimuth very close to the epicentral region. Then, we have observed the atmospheric radio noises in the VHF band on October 15-18, which are found to have the azimuth from Chofu very close to the epicentral zone. Finally, nearly at the same time (about one week before the earthquake) we have observed significant ionospheric perturbations on the subionospheric VLF/LF propagation characteristics by means of the shift in terminator time and enhancement in signal fluctuation for one particular path from the JJY (in Fukushima Prefecture) to Kochi. These perturbations not only in the lithosphere, but also in the atmosphere and ionosphere, are likely to be precursors to the 2004 Mid Niigata Prefecture earthquake.