Tomographic analysis of the seismic velocity structure beneath oceans has always been difficult because offshore events determined by onshore seismic networks have large uncertainties in depth. In order to use reliable event locations for our computations, we have developed a method to use the hypocentral depths determined by the NIED F-net with moment tensor solutions using long-period (20-50 s) waves from offshore events away from onshore seismic networks. We applied seismic tomographic method to events occurring between the years 2000 and 2015 to generate a tomographic image of the Japanese Islands and the surrounding using travel time data picked by the NIED Hi-net, hypocenteral information for onshore earthquakes from the Hi-net, and hypocenter information for offshore events from the F-net. The seismic velocity structure at depths of 30-50 km beneath the Pacific Ocean off the east coast of northeastern Japan and onshore Japan was clearly imaged using both onshore and offshore event date. The boundary between high and low P-wave velocities (Vp) is clearly seen at the Median Tectonic Line beneath southwestern Japan at depths of 10 and 20 km. We discuss how the high-Vp lower crust and low-Vp upper crust beneath central Japan and towards the Sea of Japan are responsible for the failed rift structures formed during the opening of the Sea of Japan. Due to consequent shortening, the crustal deformation has been concentrated along the failed rift zone. Resolution of shallow structures beneath the ocean is investigated using S-net data, confirming the possibility of imaging depths of 5-20 km. In future studies, application of S-net data will be useful in evaluating whether the failed rift structure, formed during the late Cretaceous to early Tertiary, continues towards the shallow regions beneath the Pacific Ocean.

1. [1] K. Obara, K. Kasahara, S. Hori and Y. Okada, “A densely distributed high-sensitivity seismograph network in Japan,” Hi-net by National Research Institute for Earth Science and Disaster Prevention Review of Scientific Instruments, Vol.76, 021301-doi:10.1063/1.1854197, 2005.
2. [2] Y. Okada, K. Kasahara, S. Hori, K. Obara, S. Sekiguchi, H. Fujiwara, and A. Yamamoto, “Recent progress of seismic observation networks in Japan -Hi-net, F-net, K-NET and KiK-net-,” Research News Earth Planets Space, Vol.56, xv-xxviii, 2004.
3. [3] T. Kanazawa, “Japan Trench earthquake and tsunami monitoring network of cable-linked 150 ocean bottom observatories and its impact to earth disaster science,” 2013 IEEE Int. underwater technology symposium (UT). IEEE, pp. 1-5, 2013.
4. [4] K. Uehira, T. Kanazawa, M. Mochizuki, H. Fujimoto, S. Noguchi, T. Shinbo, K. Shiomi, T. Kunugi, S. Aoi, T. Matsumoto, S. Sekiguchi, Y. Okada, M. Shinohara, T. Yamada, “Outline of Seafloor Observation Network for Earthquakes and Tsunamis along the Japna Trench (S-net),” EGU General Assembly 2016, EGU2016-13832, 2016.
5. [5] M. Matsubara, K. Obara and K. Kasahara, “Three-dimensional P- and S-wave velocity structures beneath the Japan Islands obtained by high-density seismic stations by seismic tomography,” Tectonophysics, Vol.454, pp. 86-103, doi:10.1016/j.tecto.2008.04.016, 2008.
6. [6] D. Zhao, Z. Huang, N. Umino, A. Hasegawa, and H. Kanamori, “Structural heterogeneity in the megathrust zone and mechanism of the 2011 Tohoku-oki earthquake (Mw 9.0),” Geophys. Res. Lett., Vol.38, L17308, doi:10.1029/2011GL048408, 2011.
7. [7] Z. Huang and D. Zhao, “Mechanism of the 2011 Tohoku-oki earthquake (Mw 9.0) and tsunami: Insight from seismic tomography,” J. Asian Earth Sci., Vols.70-71, pp. 160-168, 2013.
8. [8] M. Matsubara and K. Obara, “The 2011 Off the Pacific Coast of Tohoku earthquake related to a strong velocity gradient with the Pacific plate,” Earth Planets Space, Vol.63, pp. 663-667, doi:10.5047/eps.2011.05.018, 2011.
9. [9] Y. Asano, T. Saito, Y. Ito, K. Shiomi, H. Hirose, T. Matsumoto, S. Aoi, S. Hori, and S. Sekiguchi, “Spatial distribution and focal mechanisms of aftershocks of the 2011 off the Pacific coast of Tohoku Earthquake,” Earth Planets Space, Vol.63, pp. 669-673, 2011.
10. [10] E. Fukuyama, M. Ishida, D. S. Dreger, and H. Kawai, “Automated seismic moment tensor determination by using on-line broadband seismic waveforms,” Zisin (J. Seism. Soc. Japan), Vol.51, pp. 149-156, 1998 (in Japanese with English abstract).
11. [11] M. Matsubara, N. Hirata, H. Sato, and S. Sakai, “Lower crustal fluid distribution in the northeastern Japan arc revealed by high resolution 3D seismic tomography,” Tectonophysics, Vol.388, pp. 33-45, doi:10.1016/j.tecto.2004.07.046, 2004.
12. [12] M. Matsubara, H. Hayashi, K. Obara, and K. Kasahara, “Low-velocity oceanic crust at the top of the Philippine Sea and Pacific plates beneath the Kanto region central Japan, imaged by seismic tomography,” J. Geophys. Res., Vol.110, B12304, doi:10.1029/2005JB003673, 2005.
13. [13] D. Zhao, A. Hasegawa, and S. Horiuchi, “Tomographic imaging of P and S wave velocity structure beneath northeastern Japan,” J. Geophys. Res. Vol.97, pp. 19,909-19,928, 1992.
14. [14] G. Nolet, “Seismic tomography,” D. Reidel Publishing Company, 386pp., 1987.
15. [15] M. Ukawa, M. Ishida, S. Matsumura, and K. Kasahara, “Hypocenter determination method of the Kanto-Tokai observational network for microearthquakes,” Res. Notes Natl. Res. Cent. Disaster Prev., Vol.53, pp. 1-88, 1984 (in Japanese with English abstract).
16. [16] M. Matsubara, K. Obara and K. Kasahara, “High-V_p/V_s zone accompanying non-volcanic tremors and slow slip events beneath southwestern Japan,” Tectonophysics, Vol.472, pp. 6-17, doi:10.1016/j.tecto.2008.06.013, 2009.
17. [17] M. Matsubara, H. Sato, T. Ishiyama, and A. D. Van Horne, “Configuration of the Moho discontinuity beneath the Japanese Islands derived from three-dimensional seismic tomography,” Tectonophysics, Vols.710-711, pp. 97-107, doi:10.1016/j.tecto.2016.11.025, 2017.
18. [18] S. Nishimoto, M. Ishikawa,M. Arima, T. Yoshida, and J. Nakajima, “Simultaneous high P-T measurements of ultrasonic compressional and shear wave velocities in Ichino-megata mafic xenoliths: Their bearings on seismic velocity perturbations in lower crust of northeast Japan arc,” J. Geophys. Res., Vol.113, B12212, doi:10.1029/2008JB005587, 2008.
19. [19] H. Sato, “Imaging of active fault systems by seismic reflection profiling,” Report for the Coordinating Committee for Earthquake Prediction, Vol.94, pp. 397-402, 2014 (in Japanese).
20. [20] T. Iwata, A. Hirai, T. Inaba, and M. Hirano, “Petroleum system in the Offshore Joban Basin, northeast Japan,” J. Japanese Assoc. Petro. Tech., Vol.67, No.1, pp. 62-71, 2002 (in Japanese with English abstract).
21. [21] M. Osawa, S. Nakanishi, M. Takahashi, and H. Oda, “Structure, tectonic evolution and gas exploration potential of offshore Sanriku and Hidaka provinces, Pacific Ocean, off northern Honshu and Hokkaido, Japan,” Japanese Assoc. Petro. Tech., Vol.67, No.1, pp. 38-51, 2002 (in Japanese with English abstract).
22. [22] H. Ando, “Geologic setting and stratigraphic correlation of Cretaceous to Paleocene Yezo forearc basin in Northeast Japan,” Japanese Assoc. Petro. Tech., Vol.70, No.1, pp. 24-36, 2005 (in Japanese with English abstract).
23. [23] Geological Survey of Japan (Eds.), “Gravity Database of Japan,” DVD edition, Digital Geoscience Map P-2, Geological Survey of Japan, AIST, 2013.
24. [24] T. Nakatsuka and Okuma, S., “Aeromagnetic anomalies database of Japan,” Digital geoscience map P-6, Geological Survey of Japan, 2005.
25. [25] D. Bassett, D. T. Sandwell1, Y. Fialko, and A. B. Watts, “Upper-plate controls on co-seismic slip in the 2011 magnitude 9.0 Tohoku-oki earthquake,” Nature, Vol.531, pp. 92-96, doi:10.1038/nature16945, 2016.
26. [26] S. Aoi, N. Yamamoto, W. Suzuki, K. Hirata, T. Kunugi, H. Nakamura, T. Kubo, T. Maeda, and S. Suzuki, “Real-time tsunami inundation forecast system using S-net data,” 16^th World Conf. on Earthquake Engineering, 517, 2017.
27. [27] N. Yamamoto, S. Aoi, K. Hirata, W. Suzuki, T. Kunugi, and H. Nakamura, “Development of real-time tsunami inundation forecast method using a dense offshore observation network,” 16^th World Conf. on Earthquake Engineering, 1952, 2017.
28. [28] G. P. Hayes, D. J. Wald, and R. L. Johnson, Slab1.0, “A three Dimensional model of global subduction zone geometries,” J. Geophys. Res., Vol.117, B01302, doi:10.1029/2011JB008524, 2012.
29. [29] Ministry of Education, Culture, Sports, Science and Technology (MEXT) and National Research Institute for Earth Science and Disaster Resilience, “Report for Long-period earthquake ground morion mapping project,” p. 191, 2013.
30. [30] N. Hirata, S. Sakai, S. Nakagawa, M. Ishikawa, H. Sato, K. Kasahara, H. Kimura, and R. Honda, “A new tomographic image on the Philippine Sea Slab beneath Tokyo – Implication to seismic hazard in the Tokyo metropolitan region –,” EOS, Trans., American Geophysical Union, T11C-06, 2012.
31. [31] L. Prawirodirdjo and Y. Bock, “Instantaneous global plate motion model from 12 years of continuous GPS observations,” J. of Geophysical Research, 109:B08405, doi:10.1029/2003JB002944, 2004.
32. [32] G. M. Steblov, M.G. Kogan, R. W. King, C. H. Scholz, R. Burgmann, and D. I. Frolov, “Imprint of North American plate in Siberia revealed by GPS,” Geophyscial Research Letters, Vol.30, No.18, doi:10.1029/2003GL017805, 2003.
33. [33] P. Wessel and W.H.F. Smith, “New version of generic mapping tools released,” EOS Trans. AGU 79 329, 1995.
34. [34] M. Matsubara, “Software for Viewing 3D Velocity Structures Beneath Whole Japan Islands,” Report of the National Research Institute for Earth Science and Disaster Prevention, Vol.76, pp. 1-9, 2010.