Low Q Zone in Izu-Oshima Volcano Island, Detected by Attenuation of Volcanic Tremor

The Q-structure of Izu-Oshima Volcano Island has been studied based on the propagation characteristics of the isolated-type eruption tremor originated from the central crater, Mihara-crater. This kind of tremor has been known to occur at the time of a small Strombolian eruption at the crater. Temporary observation was carried out at thirty-nine points, using one portable seismometer. The frequency detected of the tremor is within the range lower than 30 Hz, and its maximum amplitude exists roughly at 3 Hz wave component. The propagation mode of higher than 5 Hz seismic wave of the tremor looks like that of the body wave. The high frequency body wave generated is considered to have been by the explosion sound at the crater or by high frequency vibration of the materials at the crater due to the volcanic explosion. Large attenuation of certain wave components has been observed on the eastern flank of the volcano. On the other hand, notable large power of 10 to 12 Hz wave component is observed to the WNW and ESE just outside of Mihara-crater. The former phenomenon seems to be by the influence of the coarse materials which form the eastern area of the volcano, which is the low Q-structure region. The latter phenomenon seems to be by the influence of new pyroclastic materials stratified in the atrio of Mihara-crater.