Evidence for a slab fragment wedged under Tokyo and its tectonic and seismic implications

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1 Supplementary Information to accompany Evidence for a slab fragment wedged under Tokyo and its tectonic and seismic implications Shinji Toda, Ross S. Stein, Stephen H. Kirby, and Serkan B. Bozkurt This section contains Supplementary Figures Macmillan Publishers Limited. ll rights reserved.

2 a 1 Principal stress Kanto seismic corridor σ2 6 σ3 σ σ1h PC PHS Top of fragment 5 b 1 (4-6 (4-6 depth) depth) & strike-slip Kanto Kanto seismic seismic corridor corridor 6 Kanto fragment No. Tokyo Bay cluster PC PHS Base of fragment (6-7 (6-7 depth) depth) Supplementary Figure 1. Principal stresses indicate fragment motion. Stress tensor inversion from National Research Institute of Earth Science and Disaster Prevention (NIED) focal mechanisms on the upper (a) and lower (b) surfaces of the proposed Kanto fragment. Principal compression axes are oriented NW on the top of the fragment (b) parallel to the PHS motion. This is because the fragment is being pushed northwest by the leading edge of the PHS slab. In contrast, compression axes are oriented E-W at the base of the proposed fragment because the fragment is being pushed to the west by the PC slab. The 23 July 25 M=6. shock at 68 depth struck in the North Tokyo cluster; its mechanism is identical to the principal stress orientation shown here. 28 Macmillan Publishers Limited. ll rights reserved.

3 EUR Pacifc plate contours 3 4 Volcanic front Ishida (1992) Depth of seismicity () 16 a 37 b 7 Tokyo Tokyo B S Reinterpreted with updated seismicity PC S PH C P Mantle wedge c 2 Distance () PHS seismic zone B Distance () Suruga 12 2 B 5 B South of Sagami trough 4 C P e sthenosphere 4 g a mi PHS seismic zone 16 d EUR 4 PH B Ishida (1992) interpretation r uga 2 Sa Su B PH S Depth () Distance () Supplementary Figure 2. Relationship between seismicity and plate configuration. (a) Earthquakes at 3-2 depth, which effectively removes EUR and shallow PHS seismicity, together with the volcanic front. s also shown in the Movie, the front is seen to lie above deep earthquakes, as in other Japanese island arcs, and so the PHS does not control the position of the volcanic front. (b) Map showing formerly drawn contours of PC (Ishida, 1992) which differ from the earthquake depths in (a). (c) Cross-section and interpretation from Ishida (1992). (d) Updated and more precisely located seismicity, together with our interpretation. (e) Cross-section south of the Sagami trough, where the PHS slab cannot be descending, showing seismicity similar to section -. Such a seismicity pattern is also observed in the Suruga wing of the PHS (Toda and Matsumura, 26). 28 Macmillan Publishers Limited. ll rights reserved.

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5 Japan trench Kanto Plain Seamounts Suruga trough Sagami trough Nishishichito-Shichito-Iojima volcanic ridge Izu trench Supplementary Figure 4. The Kanto triple junction viewed from the south. Topography and bathymetry are vertically exaggerated by a factor of four. The PHS subducts to the northwest along the Sagami and Suruga troughs. The broad Nishishichito-Shichito-Iojima volcanic ridge and the ~25 Ma age of the Philippine sea plate make it highly buoyant, in our judgment causing uplift of the Boso and Miura peninsulas (labeled in Fig. 1b) south of the Kanto Plain, which form the southern margin of the Kanto Basin. The ~14 Ma PC subducts to the west, and a chain of Pacific seamounts is visible entering the Japan Trench on the right. (The seamounts are more visible when the figure is viewed on screen rather than on paper.) lso see Figure 1a for observed seamounts, and Figure 1b for inferred subducted seamounts. 28 Macmillan Publishers Limited. ll rights reserved.

6 Interpreted Magnetic nomalies Kashima - Daiichi seamount chain Kanto basin no data mm/yr 1 2 Izu- Bonin ridge 29 mm/yr? Japan Group seamounts no data Supplementary Figure 5. Magnetic anomalies reveal subducted Kashima-Daiichi seamount chain. Interpreted magnetic anomalies associated with seamount subduction, strike-slip faulting in the overlying EUR, the Izu-Bonin ridge impact into Honshu, and the Kanto basin. (Makino, M., N. Isekzaki, T. Yamazaki, T. Ishihara, Y. Okubo, and T. Nakastsuka, Geological tlas of Japan, 2nd Ed., Geol. Surv. Japan, sakura Pub. Co., 1992). 28 Macmillan Publishers Limited. ll rights reserved.

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