T. Horie

A possible effect of ionospheric perturbations associated with the Sumatra earthquake, as revealed from subionospheric very-low-frequency (VLF) propagation (NWC-Japan)

The observation of subionospheric very‐low‐frequency (VLF) propagation between the transmitter NWC (Australia) (frequency 19.8 kHz) and our three receiving sites in Japan (Chofu, Chiba, and Kochi) are used to find a possible precursor of ionospheric perturbations to the Sumatra earthquake in late December 2004. Night‐time fluctuations in VLF propagation data at all three sites have exhibited significant enhancements from 4 days before the earthquake, which is likely to be a precursor to the earthquake.

A statistical study on the correlation between lower ionospheric perturbations as seen by subionospheric VLF/LF propagation and earthquakes

The subionospheric VLF/LF propagation is extensively used to investigate the lower ionospheric perturbation in possible association with earthquakes. An extensive period of data over seven years from January 2001 to December 2007 and a combination of different propagation paths in and around Japan are used to examine the statistical correlation between the VLF/LF propagation anomaly (average nighttime amplitude, dispersion and nighttime fluctuation) and earthquakes with magnitude greater than 6.0. It is then found that the propagation anomaly exceeding the 2σ (standard deviation) criterion indicating the presence of ionospheric perturbation is significantly correlated with earthquakes with shallow depth (<40km). Finally some comments on the mechanism of seismo-ionospheric perturbations are discussed.