Seismic exploration of the deep structure beneath the westsern Pacific

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Seismic exploration of the deep structure beneath the western PacificThis project aimed at revealing the structure of the deep mantle beneath the western Pacific, the largest subduction zone over the globe.To achieve this goal, we planned(1)to install a broadband station at kamehckoe(kamchatka), (2)to version up the Ponape station, Micronesia, and(3)tomake a tomographic study to obtain a seismic image of the mantle beneath the wastern Pacific.Following is a brief report on the results of our research project.(1)Installation of a broadband station at KamehckoeIn 1993 we made various preparation for the installation of a seismic station, including the acquiement and set up of the equipments, negotiation with Russian researchers, and the construction of a vault by them. In 1994 we set up the broadband instruments inside the vault and the operation started for the two components of the instruments.In 1995 the seismometer of the remaining component started in operation. Now all the three co … More mponent seismometers are operated in normal condition.(2)Version up of the observation system at PonapeIn 1993 the installation of a broadband station was completed at Ponape by a different fund.In 1994 the observation system was up-dated thanks to the replacent of telephone lines with optical cables throughout the island.In 1995 the data transmission system was improved.On the whole, we were able to obtain the data with a good quality throughout the project period.(3)Seismic tomographic study of the Earth's interiorWhole mantle P wave tomography(1993) : We made a whole mantle P wave tomography by increasing the data as 5 times as large as those used in Fukao et al.(1992)and obtained a sharper image of the stagnant slabs in the mantle transition zone.We also measured the travel time differences between P and PP waves by a waveform correlation method on broadband seismograms.Travel time differences were found to be consistent with the tomographic model where the resolution in the model is good.measurement of group velocities of surface waves(1994) : We used the newly obtained broadband data at Ponape and Titi-jima to measure the group velocities of Rayleigh waves.We found that in terms of group velocity the oldest ocean floor(160 Ma)between Mariana and Ponape is fastest and the youngest ocean floor between Mariana and Titi-jima is slowest among the sea floors of the Pacific Ocean.PcP travel time tomography for the core/mantle boundary(1995) : We obtained a laterally heterogeneous structure in the vicinity of the core/mantle boundary(CMB) by a tomographic inversion of P and PcP travel time data. We focused on extracting the significant features of the CMB rather than presenting a new model.We found the low-velocity anomalies beneath southeastern Asia and the high-velocity anomaly under the Arctic region. Less