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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy Sn are 8.2 km/s and 4.7 km/s, respectively. The velocity variations are about 5% for P-wave and about 3.5% for S-wave. From this result, a seismic anisotropy of the uppermost mantle is suggested, and the fast direction seems to be perpendicular to the magnetic lineations. Therefore the anisotropy below the WP-2 is suggested to originate in a preferred orientation of olivine crystals in the uppermost mantle. The upper mantle velocity structure is inferred from travel times of earthquakes recorded by the WP-2 and the previous studies of the northwestern Pacific basin. To explain late first arrivals from the earthquakes with epicentral distances smaller than 2200 km, a low velocity zone below a depth of 30 km and a rapid increase of velocity at a depth of 210 km are needed. To perform receiver function analysis, we selected 16 events with a magnitude greater than 6, that have epicentral distance ranging from 30 degrees to 90 degrees, and have high signal-to-noise (S/N) ratio from the WP-2 data. A bandpass filter of 8-360 seconds was applied for the receiver function analysis. A velocity model named the WP-2 model which is modified Iasp91 model where crustal layers are replaced by the results of the airgun experiment was constructed. After the theoretical Ps-P times are calculated using the WP-2 model, travel times for large amplitudes of the receiver function corresponding to 410 km and 660 km discontinuties were read. Averaging the read times, conversion depths of 416 km and 666 km were obtained. Keywords: seafloor seismic observatory, lithosphere, seismic anisotropy

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS016 Poster presentation 2353 Underwater observation with new tool makes it different:Concept is not new, but the result is worth nugget Mr. Kiyoyuki Kisimoto Institute of Geology and Geoinfomation Geological Survey of Japan, AIST IASPEI Masato Joshima, Kiyokazu Nishimura Whenever locally collected long-term observational data is to be analyzed to extract global phenomena, correction/estimation of local/regional effects on the data is always important. Especially in case of the ocean bottom observations with multi-disciplinary collaboration, careful site selection procedure or presurvey is essential. Recent advancement of the swath bathymetric surveys provides us more and more accurate and high resolution topographic data and imagery of the seafloor. This is good news, however, we still have little data of even the shallowest part of the subbottom of the deep seafloor. Various kinds of deep submergence vehicles (DSVs) are now available for detailed ocean bottom experiments, such as ROV, AUV and HOV. In our presentation we show several new findings obtained by our latest tool called DAI-PACK (Deep-sea Acoustic Imaging Package) system, which is a portable piggy-back system to be accommodated on various DSVs. The DAI-PACK has two sensors, i.e. (1) deep-sea side-scan sonar and (2) deep-sea sub-bottom profiler and all components except sensors are packaged in an aluminum pressure sphere, which has been successfully installed as a piggy-back payload on several vehicles. The DAI-PACK is an off-line (stand-alone) system to avoid the interference with the DSVs. We have produced good results so far (1) detailed side-scan imagery of a hydrothermal area of deep-sea caldera in Izu-Bonin ridge, (2) subbottom layer profiles of very thinly sedimented off-ridge volcanoes near the southern East Pacific Rise, which is useful to infer the eruption dates, (3) underwater observation of 2004 Sumatra earthquake epicentral region using ROV, (4)mapping of the depth change of Gas-Hydrate sub-layers near the seabottom, which led to find the hydrate-ice exposure on the bottom surface near coast of Japan in 2006. The DAI-PACK system is a very useful site-survey tool for many ocean bottom observations. Keywords: acoustic imaging, subbottom profiling, dai pack

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