Seismic properties in the asthenosphere beneath the petit-spot region inferred from BBOBS data

The petit-spot is a term for submarine volcanoes recently discovered by Hirano et al. [2006]. They are young (0-10 Ma) volcanic micro-knolls in very old (~130 Ma) NW Pacific plate (about 500 km offshore from the Japan Trench). Although the estimated activity distributes widely in time (0-10 Ma) and in space (over 600 km), the volume of each volcanic edifice is small (several orders of magnitude less than the previously known seamounts and knolls). The mechanisms of melt production and magma eruption process of the new type of volcanism are still unknown. Hirano et al. [2006] proposed that small fraction of melt came from the asthenosphere through fractures in the lithosphere which were induced by flexure around the outer-rise. Local seismicity located in petit-spot region may relate to the magma eruption process. In order to investigate nature of the petit-spot, we promote a comprehensive survey based on geophysics, geochemistry, geology, and numerical modeling, which is called petit-spot multidisciplinary project. As a part of the petit-spot multidisciplinary project, seismological observation using Broad-Band Ocean Bottom Seismometers (BBOBSs) is conducted. We deploy three BBOBSs in the petit-spot region with about 100 km spacing. We arrange BBOBS array to enclose both Yukawa-kaikyu and recent seismicity around the petit-spot. The sites are equipped with Guralp CMG-3T sensors recorded at 100 Hz. The BBOBSs were deployed in May 2007 by R/V KAIREI (JAMSTEC) and were recovered July 2008 by R/V YOKOSUKA (JAMSTEC). All the BBOBSs were successfully recovered with high quality data. We establish two main goals of this observation, precise earthquake location of local events, and estimation of seismic properties in the asthenosphere beneath the petit-spot. The first and second one will constrain the mechanism of the magma eruption and the melt production, respectively. In this study, we focus on the melt production process. We plan to measure seismic attenuation and travel-time anomalies in the asthenosphere beneath petit-spot region to infer physical properties of the region. The results are expected to constraint especially on the melt production process of the petit-spot. In this presentation, we will introduce the overview of the earthquake observation in the petit-sport region, example of BBOBS data, and preliminary results of the estimation of seismic properties of the asthenosphere beneath the petit-spot region.