HR: 13:45h
AN: V43F-01    [Abstracts]
TI: Geophysical features of hydrothermal system in Suiyo Seamount, Izu-Ogasawara Arc, Western Pacific
AU: * Seama, N
EM: seama@kobe-u.ac.jp
AF: Research Center for Inland Seas, Kobe University, 1-1 Rokkodai, Nada, Kobe, 657-8501 Japan
AU: Nishizawa, A
EM: azusa@jodc.go.jp
AF: Hydrogr. & Oceanogr. Dep., JCG, Tsukiji, Tokyo, 104-0045 Japan
AU: Kawada, Y
EM: kawada@eps.s.u-tokyo.ac.jp
AF: Dept. of Earth & Planetary Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033 Japan
AB: Surface geophysical, airgun-OBS (ocean bottom seismograph), and deep-towed surveys by R/V Kairei KR01-15 cruise, and ten OBS array observation from Aug. 5 to Oct. 9, 2002 were conducted to characterize geophysical features of a hydrothermal system in Suiyo Seamount as a part of Archaean Park Project. Geomagnetic results from surface and deep towed magnetometers suggest 1) the seamount was formed during Brunhes Epoch (after 0.78Myr), 2) low magnetization area exists on its summit and is limited only in eastern side with its diameter of 600 meters. 2-D ray tracing results, using four OBS data with a GI gun across the seamount in NE-SW direction, indicate that P wave velocity of 2.2-4.2 km/s (2km thickness) locates all over the observation line. Two-month OBS observation reveals the seismic activity, which is characterized by dominance of earthquakes with S-P times of 1-2 sec. The hypocenter determination indicates that the locations of these earthquakes are focused within 3 km in diameter just beneath the Suiyo volcanic cone at a depth of 3-7 km. We propose a model to explain these geophysical observations; the former main conduit of Suiyo Seamount is in the middle of cooling after the last eruption, and its heat drives the hydrothermal system in Suiyo Seamount. The low magnetization area probably results from high alteration of the rock through the hydrothermal activity. The area with P wave velocity of 2.2-4.2 km/s (2km thickness) suggests higher porosity area where a major hydrothermal circulation exists. The hypocenter locations below a depth of 3 km probably correspond to the locations close to the former main conduit boundary where high thermal stress is expected due to changes in temperature and high thermal gradient. The area without seismicity (less than 3 km in depth) has probably released the thermal stress already through the hydrothermal circulation. This scenario is also supported by our numerical simulation; a magma injection in the conduit with its diameter of 1 km maintains its heat during a few thousands years, although the surface hydrothermal activity cools the uppermost part of the conduit.
DE: 3015 Heat flow (benthic) and hydrothermal processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting