3. Future Plan of Ocean Hemisphere Research Center (OHRC)

As ERI plans to have an institutional reorganization of its research centers and divisions at the end of the 2009 fiscal year (i.e., March, 2010), which coincides with the end of the universityfs 6-year term, it is our plan to keep OHRC in the present form till then even after the end of its originally planned 10-year period. In the following, we will discuss what our main research focus will be during the extended term of our center (5 years or so). It is our hope that the review committee will evaluate this plan, and gives us advices and/or recommendations, so that we may utilize the resources that we possess in maximum for better understanding of the earthfs internal processes.
Seismic and electromagnetic joint ocean-bottom mobile array research:
Through the OHP project, OHRC has established permanent geophysical networks in the western Pacific region, which are now largely maintained by IFREE/JAMSTEC in cooperation with OHRC. In the post-OHP era, our focus has shifted toward more mobile array type observational science, such as one currently performed in the Stagnant Slab project (see Section 2-1-2). In the project, the long-term ocean bottom broadband seismic and electromagnetic observations are conducted together as a team effort for the first time to map out the integrated image of the mantle transition zone beneath the Philippine Sea. We believe that it is OHRCfs great strength to be able to conduct such science using our own instruments, and this should be pursued further for the coming decade to make major contribution in the mantle dynamics research. New systems for marine seismic, EM and geothermal observations are under development by individual research program in OHRC. Deployment of these new instruments is expected to lead us to another breakthrough in terms of the quality of observation data.
Although at present we have no definite plan for the next big project after SSP, one of the possible target areas may be the northwestern Pacific, seaward of the Japanese subduction zone, where land-based seismic tomography studies with limited resolution have provided images of a low velocity anomaly around a depth of 400km. Recent geophysical and geological observations have also found fresh volcanoes in the old lithosphere and seismic activities in the middle of the northwestern Pacific. Marine geophysical observations in this region will contribute to improve the resolution of the mantle image and to elucidate the relation among these phenomena. Seismic observations using BBOBSs, electromagnetic observations using OBEMs, and heat flow measurements should be carried out in such regions. The seismic and EM tomography can provide seismic velocity/attenuation and electrical conductivity structure models, respectively, while the heat flow measurements will provide boundary conditions of thermal modeling. Integrated analysis of these data allows us to separate effects of thermal and compositional heterogeneities of the mantle, and helps understanding of the origins of peculiar activities observed.
Following the Stagnant Slab project which ends in FY2008, to pursue the line of research, we hope to get funding by applying for large funding sources such as Monbu Kagakushofs Grants-in-Aid for Scientific Research for gSpecially Promoted Researchh (gtokubetsu suishin kenkyuh).
Land-based mobile broadband seismic array research:
While the ocean-based approach provides the OHRC's unique contribution to the mantle dynamics research, we also plan to strengthen our land-based approach, which provides high quality and large quantity data set. One of such projects is now about to be launched.
NorthEast China Extended SeiSmic Array (NECESSArray) is a passive broadband seismic experiment aiming to reveal the seismic structure beneath the northeastern China, where the Sino-Korea craton and unusual volcanism in the continent are tectonically quite interesting. The structure of the upper mantle is the main target of NECESSArray to clarify the tectonic roots of this interesting and seismologically unexploited area and its relation to the stagnant slab beneath this region. This project may be considered, to some extent, as a land-based component of the Stagnant Slab project; or at least it will greatly complement the achievements of SSP. NECESSArray runs under the collaboration with the China Earthquake Administration (led by Dr. Y. Chen) and USA scientists (Drs. S. Grand, F.-L. Niu, J. Ni), and OHRC will take the leading role of the Japanese side. 140 mobile broadband stations are planned to be deployed for two years, and with 140 permanent stations of Chinese regional seismic networks, NECESSArray will consists of 280 stations (Fig. 3.1), the scale and density comparable to one foot of the USArray; thus it is a frontier and challenging observational seismology conducted in the best accessible area to study deep subduction processes. Moreover, the experiment will play an important role to image deep mantle structures beneath the western Pacific where the largest cold and hot superplumes are mutually interacting. The investigation in northeast China will be eventually integrated to more institutional project including not only the NECESSArray but also EM, GPS and gravity observations, geochemical and petrological studies, and computer simulations, which are currently conducted rather independently.

Fig. 3.1. Station of the planned temporary NECESSArray (black squares) are shown together with the permanent stations (some are planned) in China.
As to a future project in this context, we consider a mobile broadband array experiment in Vietnam (see Section 2-2-2; Fig. 2.2.10), where we currently operate 6 stations, as a potential target. Vietnam is located near the antipode of the south America where deep seismicity is high, and thus this array will greatly improve our knowledge of the deepest part of the inner core which is least known in this planet.
It is our plan as a center to spend resources available to OHRC mainly to these lines of research, while keep conducting other research activities discussed in Section 2 of this report. For the purpose, we plan to apply for funding supports from various external agencies, and also hope to receive financial and personnel support from the institute.