Fractal analysis for VHF electromagnetic noises and the identification of preseismic signature of an earthquake

Multi-fractal analysis under the self-organized criticality (SOC) hypothesis is performed for the VHF electromagnetic noises observed before an earthquake. A rather strong (M=7.2) earthquake took place in the off-sea of Sendai, and we have utilized the corresponding VHF noise (at 49.5 MHz) data at several stations in the Tohoku area. Among the three observing stations used for analysis, the VHF data only at one station (Kunimi) have exhibited significant precursors; that is, a significant increase in multi-fractal complexity at the last stage of the rupture. The use of fractal analysis enables to sort out the lithospheric effect following the SOC, among the few possible noise sources at VHF. The change observed only at this station might be related with the geological structure around this station because there are fault regions around the station.