著者

植田 義夫 小野寺 建英 大谷 康弘 鈴木 晃 中田 節也

出版者

特定非営利活動法人日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.46, no.4, pp.175-185, 2001-08-30

被引用文献数

1

---

The Myojin-sho volcano is one of the active submarine volcanos in the northern part of the Izu-Ogasawara arc about 400 km south of Tokyo. This volcano is a somma edifice of the Myojin-sho caldera, 6.5 km×8 km in diameter and 1000 m deep. The topography, seismic profiler, magnetic and gravity surveys around the Myojin-sho caldera were conducted by the Hydrographic Department, Japan (JHD) in 1998 and 1999. The geophysical structures of the caldera were derived, and the possible cause of the caldera formation is discussed. The residual gravity anomalies were calculated from the observed free-air anomalies by subtracting the gravity effect of 2-layer subbottom model structure, which amounts to 10 m Gals in a localized zone from the caldera to the northern somma. Bouguer gravity anomalies with the assumed density of 2.0 and 2.4 g/cm^3 also show the positive anomaly over the same zone, which is accompanied by the acoustic and magnetic basement depression. Moreover, it seems that the sediment volume nearby Myojin-sho caldera cannot compensate the volume loss of caldera (20 to 41 km^3). These features insist that the Myojin-sho caldera is caused by the collapse of the pre-caldera edifice rather than the explosion. The origin of the high gravity caldera may be ascribed to the magma pocket causing the depression, instead of the high density erupted material filling the caldera floor. The magnetization intensity of 4.8-5.3 A/m at the Myojin-sho volcano is derived from the magnetic anomaly, which may claim that the Myojin-sho volcano consists of andesitic to basaltic rock rather than dacitic rock. On the other hand, magnetization of the central cone of Takane-sho volcano is estimated to be 1.1-1.9 A/m, which is consistent with the fact that dacite pumices were sampled.