Multi-channel seismic (MCS) reflection experiments were carried out in 2005 to investigate a typical oceanic island arc system by obtained crustal structures in the Izu-Ogasawara region, by using R/V Kairei of Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Swath-bathymetry, geomagnetic and gravity data are simultaneously observed during these cruises. In 2005, we collected a total of 3720km of MCS data: 1739.65km in KR05-06 and 1980.35km in KR05-16 cruises. The clear seismic reflection profiles of the subducting Pacific plate, forearc tectonic features and backarc basin structures, for the investigation of the crustal growth, showing that all of the MCS data collected are of good quality by onboard processing.

In order to investigate formation processes of the across-arc sea mount chains in the Izu-Bonin arc, we have carried out two active source seismic cruises by using JAMSTEC R/V Kairei in 2006. The wide-angle seismic data were acquired across the across-arc seamount chains (Nishi-shichito ridge) in KR0605 and along the across-arc seamount chain (Minami-Iojima ridge) in KR0601. In addition to those two wide-angle profiles, multichannel seismic data were also acquired along two profiles which are parallel to the strike of the across arc seamount chains. The wide-angle seismic data indicate thinning of crust toward the south across the Nishi-shichito ridge, and relatively flat and thin crust beneath the Minami-Ioujima ridge. The multichannel data show that the basement of the Shikoku basin can be traced beneath the western flank of the across-arc seamount chain. This suggest that a boundary between the Shikoku basin crust and the Oligocene Izu arc may be situated around the eastern end of the across-arc seamount chains.

We carried out a deep wide-angle seismic experiment using a large airgun array and total 110 ocean bottom seismographs (OBSs) in the middle Izu-Ogasawara arc area, which was conducted by R/V Kaiyo of Japan Agency for Marine-Earth Science and Technology (JAMSTEC) from October 5 to November 3, 2005 (KY05-11 cruise). Objectives of this cruise are to know a velocity structure of the across the northern Izu arc, especially velocity variation between an old Eocene arc beneath the forearc and a new active arc, and a relationship between the structural heterogeneity with the velocity variation and the active backarc rifting. These are important keys to clarify nature of the oceanic arc growth. An airgun-OBS seismic line was set from a trench slope break adjacent to the Izu-Ogasawara trench to the western Shikoku Basin through the forearc basin, the volcanic front, the Sumisu rift, the Miocene rear arc, the eastern Shikoku Basin and the Kinan seamount chain. We shot a large airgun array with total volume 12,000 cu. in. and recorded the seismic signals on OBSs with four components and a 24-channel hydrophone streamer. In this paper, we summarize information of the seismic experiments and introduce OBS data and reflection data.

In February 2006, intensive seismic survey was carried out in Sagami Bay area, central Japan, during the KY06-01 cruise using R/V Kaiyo of JAMSTEC (Japan Agency for Marine-Earth Science and Technology). This cruise follows the previous KY05-06 cruise survey, in which we conducted single-channel seismic survey in the same area. Used tools are a 18 channel streamer with two 355 in³ GI guns, and a single-channel streamer with a cluster gun. We obtained 21 profiles with GI-guns in the eastern Sagami Bay, 20 profiles with GI-gun in the western Sagami Bay and 20 profiles with cluster gun near Hatsushima cold seep communities in the western Sagami Bay. Very clear acoustic images were captured with different frequencies on the same lines. We present here the seismic profiles obtained during the KY06-01 cruise.

We conducted a multi-beam bathymetry, gravity, and magnetic survey of the Horeki seamount in the Izu-Bonin arc, the western Pacific. On the basis of gravity anomaly, the flanks of the Horeki seamount are consistent with higher density anomaly. On the other hand, low density anomaly is distributed parallel to the long axis in the top part of the seamount. The prominent circular low Bouguer gravity anomaly, appeared in the northern part of flat-topped summit, indicates light density material filling the flat-topped summit. From the aspect of magnetic anomaly, the main body of the seamount is magnetized in positive. The magnetization indicates the volcanism constructed the main body is most robust in Gauss normal magnetic epoch (3.6-2.6 Ma). The negative magnetization is in the eastern part of the small ridges located to the north of the seamount, therefore the volcanism of these ridges is estimated to be active in Matuyama reverse epoch (2.6-0.8 Ma). In contrast, the western part of the northern ridges is magnetized in positive, and suggested to be constructed in Brunhes normal epoch. The southern part of the seamount flanks, where small volcanic cones are well developed, is magnetized in slightly positive with low magnitude. The remnant magnetization of the seamount flanks may be nearly canceled each other out by the small cone's magnetization. These volcanic cones may have erupted in various ages or in different epoch from the seamount's main body.