15 FRONTIER RESEARCH ON EARTH EVOLUTION, VOL. 1 Polynesia broadband ocean bottom seismic observation project Hajime Shiobara 1,2 , Yoshio Fukao 1,2 , Daisuke Suetsugu 1 , Hiroko Sugioka 1 , Toshihiko Kanazawa 2 and Kiyoshi Suyehiro 3 1 Research Program for Mantle Core Dynamics, Institute for Frontier Research on Earth Evolution (IFREE) 2 Earthquake Research Institute (ERI), University of Tokyo 3 Japan Marine Science and Technology Center (JAMSTEC) Outline of this project We plan to deploy eight seismometers on the seafloor in French Polynesia in January of 2003, and observe ground motion caused by earthquakes for two years, until the end of FY2004. The ocean bottom observation sites are to be selected so as to supplement the distribution of the current land-based seismic stations (Fig. 1). The purpose of the observation is to explore the deep Earth structure beneath French Polynesia by analyzing seismic wave records. We will also study the loca- tion and mechanism of earthquakes in French Polynesia as a part of our project. This project has been started as a collabo- ration with Dr. Guilhem Barruol of Universite de la Polynesie Francaise, who conducts the PLUME project that started in 2002 and deployed seismometers on oceanic islands of French Polynesia (Barruol et al., 2001); Dr. Dominique Reymond of Laboitoire de Geophysique, who is operating a permanent seismic network in French Polynesia; and Dr. Alain Bon- neville of Laboratoire de Géosciences Marines Institut de Physique du Globe de Paris, who is an expert in marine geo- physical observation. French Polynesia is a unique place in view of Earth Sci- ence, as it is considered to be the site of a large-scale uprising of magma from the bottom of the Earth's mantle at a depth of 2900km. However, the detailed structure of the crust and the mantle under the region remains to be investigated (Fig. 2). So far, seismic observations have been performed on ocean- ic islands in French Polynesia to study underground structure, and earthquake location and mechanism. The oceanic islands, however, are not distributed uniformly in the region, which limits the accuracy and resolution of the obtained structure model. Our ocean bottom seismic stations can be deployed almost uniformly, enabling us to improve the quality of the model. We expect that our observations at the ocean bottom, along with French observations on the oceanic islands, will enable us to obtain a detailed view of the deep Earth structure of the region and elucidate the process of uprising magma. The planned seismic observation is expected to be useful in obtaining an underground structure model (surface to 2900km in depth) with much higher accuracy than before. This will improve our understanding of the cause of magmatic activity, and the dynamics of the Earth’s interior behind this activity. The ocean bottom observation will also improve the accuracy of earthquake location and mechanism, giving an improved idea of seismic activity and tectonics. Our observations may even detect unknown seismic activity in French Polynesia, because this is the first scientific project involving seismic observation on the sea bottom in the region. Observation plan The deployment and operation of the broadband ocean bot- tom seismometers (BBOBS) consist of the following phases: 1) Deploy eight BBOBSs owned by the ERI with a research vessel (R/V Yokosuka, JAMSTEC) to start the observation in January 2003. The data is stored in disks in the BBOBS. 2) Recovery and re-deployment of BBOBSs will be done during the winter of 2003~ 2004, probably by using a French vessel, and we will retrieve accumulated data from the disks to computers, for data analysis. BBOBSs to be re-deployed are owned by the IFREE, and they will be put at the same sites. 3) Final recovery of BBOBSs will be done during the win- ter of 2004~2005, and the accumulated data will be retrieved from the disks to computers for data analysis. The project is completed. The BBOBS (Fig. 3) has been developed at the ERI since 1999 based on the OBS with a geophone (e.g., Kanazawa et al., 2001). A broadband sensor (CMG-3T/EBB for OBS, Guralp) is installed on an active leveling unit developed at the ERI. The data is digitized by a 20 bit ADC with 128Hz, and recorded on 2.5 inch HDDs, with a total capacity of more than 40GB. These, with about 80 Li cells (DD size), are fixed inside a tita- nium sphere housing (D=65cm). The maximum recording peri- od of the BBOBS is about 1.5 year, and it is deployed by free falling from the sea surface, and pops up by its buoyancy in the recovery. The anchor is released by a forced electric corrosion of two thin titanium plates after receiving a command from an acoustic transponder on the ship. Planned data analysis and expected results P-wave and S-wave tomography P-wave and S-wave tomography for whole mantle structure is an important component of the planned data analysis. The purpose of the study is to improve the resolution of the pre- sumed plumes beneath the South Pacific. A checkerboard res- olution test using events from 1995~1997 indicates that a sub- stantial improvement in the resolution should be expected from the proposed observations (Fig. 4). Surface wave tomography Surface wave tomography will be performed to obtain a 3D upper mantle shear-wave velocity model with high resolution. The surface wave tomography supplements the above-men- tioned body wave studies, which have less resolution in the upper mantle than in the lower mantle beneath the South