Observation of co-seismic electromagnetic waves associated with the Geiyo earthquake in 2001

Many observations have been conducted to clarify the relationship between electromagnetic (EM) phenomena and earthquake (EQ) activities. In our observation system, EM waves associated with EQs have been received in the very high frequency (VHF) band. We have newly developed a dual frequency observation method with a highly sensitive synthesized FM tuner. Our observation method can clarify and judge whether a received signal is a broadband EM wave or an FM broadcasting wave. We had the Geiyo EQ (M/sub J/=6.4, d=60 km) and the aftershock of the Geiyo EQ (M/sub J/=5.2, d=50 km) in 2001. Our observation system detected co-seismic broadband EM waves associated with the Geiyo EQ in the VHF band. The observed EM waves reached a peak right after the EQ and slowly returned to a normal level about three hours later. We also describe a new phenomenon: a small fluctuation of about 2 dB occurred about one day before the Geiyo EQ, then, the fluctuation just disappeared at the occurrence of the earthquake.