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

Nature Geoscience volume 1, pages 771–776 (2008)Cite this article

Abstract

The Philippine Sea and Pacific plate slabs both subduct beneath Tokyo, and so their configuration and seismic potential have been subject to intensive study1,2,3,4,5,6,7,8. Previous work suggests that the Philippine Sea slab extends up to 100 km northwest of Tokyo and subducts to a depth of 90 km beneath the Kanto basin, where it is folded against the underlying Pacific slab. Here we evaluate seismic data in three dimensions and delineate a distinct 25-km-thick and 100-km-wide body beneath the Kanto basin that has hitherto been considered to be part of the Philippine Sea slab. We find that several of its characteristics—such as its high seismic velocity and the presence of a double seismic zone—point instead to an affinity with the Pacific slab, implying that the Philippine Sea slab penetrates to depths of only 35–40 km beneath the Kanto basin. We propose that the body is a fragment of Pacific lithosphere that formed by the collision of two intersecting seamount chains with the Japan Trench 2–3 million years ago. We suggest that such slab fragments may not be uncommon, especially where seamount subduction deforms trenches and near triple junctions.

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Figure 1: Japan Trench seamounts and Kanto seismicity.
Figure 2: Interpretations of Kanto tectonics.
Figure 3: Schematic evolution of the proposed fragment.
Figure 4: Perspective views of seismicity, slabs and volcanoes.
Figure 5: Role of outer-rise earthquakes in fragmentation.

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Acknowledgements

We are indebted to J. Nakajima and A. Hasegawa for tomographic expertise and thank W. Thatcher, J. Lin, T. Seno, Y. Okada and M. Billen for reviews and discussion. We are grateful for financial support from Swiss Re.

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Authors and Affiliations

  1. Active Fault Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Higashi, 1-1, Tsukuba, 305-8567, Japan

    Shinji Toda

  2. US Geological Survey, MS 977, Menlo Park, California 94025, USA

    Ross S. Stein & Stephen H. Kirby

  3. AMEC Geomatrix, Inc., 2101 Webster Street, Oakland, California 94612, USA

    Serkan B. Bozkurt

Authors
  1. Shinji Toda
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  2. Ross S. Stein
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  4. Serkan B. Bozkurt
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Correspondence to Shinji Toda.

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Supplementary Movie (for high-speed connection) — the Movie can also be downloaded from http://sicarius.wr.usgs.gov/fragment/ (SWF 62190 kb)

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Supplementary Movie (degraded for low-speed connection) (SWF 24031 kb)

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Toda, S., Stein, R., Kirby, S. et al. A slab fragment wedged under Tokyo and its tectonic and seismic implications. Nature Geosci 1, 771–776 (2008). https://doi.org/10.1038/ngeo318

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