The Electromagnetic Observation and Inter Station Transfer Function Analysis before and after the 2002 Dyke Intrusion at Hachijo Island

There are many volcanoes and large earthquakes or earthquake swarms in Izu‑Bonin Arc which stretches to the south in the Pacifi c Ocean. We have many electromagnetic observation systems in this area. In this study we mention about the data observed at Hachijo Island. In August 2002 a dyke intruded in the N‑S direction at a depth of 3 km beneath the Hachijo‑Fuji one of the volcanoes in Hachijo Island. The 3 components magnetic observation was performed in Hachijo Island by Hydrographic & Oceanographic Department of the Japan Coast Guard. To investigate the possible changes of electric conductivity beneath the Hachijo‑Fuji we analyzed this 3 component magnetic data before and after the dyke intrusion with the method of Inter Station Transfer Function （ISTF）. We used data at Kakioka Magnetic Observatory as a reference station. The ISTF analysis provides the information on the difference of the electrical underground structure between the observation station （Hachijo Island） and reference station （Kakioka）. We calculated monthly mean ISTF elements from 1998 to 2005. We used continuous wavelet transform instead of the conventional Fourier transform to promote accuracy in the calculation. As a result of analysis we could not fi nd obvious changes of ISTF components before and after the dyke intrusion in August 2002. The magnetic observatory was far （about 10 km from Hachijo‑Fuji） to detect the infl uence of the dyke intrusion.