A slab fragment wedged under Tokyo and its tectonic and seismic implications

Shinji Toda, Ross Stein, Stephen Kirby and Serkan Bozkurt, Nature Geoscience, doi:10.1038/ngeo318, November 2008
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News & Views article about Toda et al:
Seismology: Breaking the slab, by Meghan S. Miller (Rice University)

Press-friendly Figure A (click here for the figure in JPEG format) (Editable AI file format)
Topography and bathymetry of the Kanto tectonic ‘triple junction’ viewed from the south. Topography and bathymetry are vertically exaggerated by a factor of four. The Kanto plain marks the land surface of the deep Kanto basin. The main Japanese island of Honshu is in the upper left. The Philippine Sea plate descends west-northwestward beneath Tokyo the along the Sagami and Suruga troughs. The broad Izu volcanic ridge and the young age of the Philippine Sea plate make the plate buoyant, uplifting of the peninsulas south of the Kanto Plain, which form the southern margin of the Kanto Basin. The much older Pacific plate descends to the northwest along the Japan and Izu trenches. A chain of Pacific seamounts is visible entering the Japan Trench on the right, forming the east margin of the Kanto basin. The authors argue that the buoyant seamounts interfere with the descent of the Pacific plate and so distort the trench into a broad arc.

Press-friendly Figure B (click here for the figure in JPEG format) (Editable AI file format)
Perspective views of the Kanto fragment with the Pacific and Philippine Sea plate slabs, volcanoes and seismicity. The top panel shows view of southwest Japan (the coastline is grey), together with the volcanic front (yellow pyramids), their presumed magma conduits (red vertical lines) tend to lie along the 120-140 km (75-85 mi) depth contour (blue dashed lines) of the Pacific plate. The blue ball is Tokyo. The bottom panel shows a close-up view of the proposed Kanto fragment with microseismicity colored by depth. Toda et al argue that the Kanto fragment broke off the Pacific plate 2-3 million years ago, and is being pushed northwestward under Tokyo. Note the seismicity streak on the upper surface of the proposed fragment, perhaps indicative of its motion with respect to the overlying crust. These are frames from the Movie.

Press-friendly Figure C (click here for the figure in JPEG format) (Editable AI file format)
Anonymous contemporary woodcuts of Tokyo (formerly, Edo) before and after the great 11 November 1855 magnitude~7.3 Ansei-Edo earthquake. Toda et al contend that this quake struck as a result of the movement of the Kanto fragment against the Pacific plate slab below, about 70 km (45 mi) deep. The Japanese government estimates that a repeat of this quake today would cost $1 trillion, destroy about 500,000 buildings and take up to 10,000 lives. Woodcut source, Documenting Disaster: Natural Disasters in Japanese History, 1703-2003, Foundation for Museums of Japanese History, Chiba, 2003.

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