Research ArticleGEOPHYSICS

Upper and lower plate controls on the great 2011 Tohoku-oki earthquake

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Science Advances  20 Jun 2018:
Vol. 4, no. 6, eaat4396
DOI: 10.1126/sciadv.aat4396
  • Fig. 1 Seismic stations and earthquakes used in this study.

    (A) Distribution of the 382 seismic stations (blue squares) used in this study. The red beach ball denotes the focal mechanism of the great 2011 Tohoku-oki earthquake (Mw 9.0), whereas the pink beach balls denote focal mechanisms of other megathrust earthquakes (Mw ≥ 7.0) during 1917 to 2017. The white line marks the downdip limit of interplate seismicity (2). The yellow dashed lines denote depth contours of the UBP. (B) Tectonic settings of the study region (blue box). (C) Map view showing the distribution of 4760 local earthquakes used in this study. The blue crosses denote 2537 events that occurred beneath the seismic network (9). The red circles show 1788 events that occurred under the Pacific Ocean and were relocated using sP depth phase data (7, 48). The green circles denote 435 suboceanic events located using OBS data (49). The three black lines denote locations of the vertical cross sections in Fig. 5. (D) East-west and (E) north-south vertical cross sections of the 4760 earthquakes shown in (C). The red triangles in (A) to (C) denote active arc volcanoes. The red star in (B) and (C) denotes the mainshock epicenter of the 2011 Tohoku-oki earthquake (Mw 9.0).

  • Fig. 2 Forearc anomalies in the Tohoku subduction zone.

    (A) Residual topography and (B) residual gravity derived from Bassett et al. (13). (C) P-wave velocity (Vp) tomography along the UBP and (D) average 2D Vp image of the overriding plate (ORP) obtained by this study. The magenta line marks the FSB proposed by Bassett et al. (13). The other labeling is the same as that in Fig. 1.

  • Fig. 3 Anomalies in the source area of the 2011 Tohoku-oki earthquake.

    (A) Residual topography and (B) residual gravity in the Tohoku forearc (13). (C) P-wave velocity (Vp) variations along the UBP and (D) average Vp variations of the ORP obtained by this study. Gray lines in (A) to (D) show profiles along the UBP depth contours of 10 to 30 km (black bold dashed lines in Fig. 2) with an interval of 1 km. The horizontal axes show the distance of points along the profiles from the FSB (the magenta line). The blue line denotes the mean of each ensemble of profiles (gray lines). The red star denotes the epicenter location of the great 2011 Tohoku-oki earthquake (Mw 9.0).

  • Fig. 4 Vp tomography and characteristics of the 2011 Tohoku-oki earthquake.

    The colors in (A to C) show Vp tomography along the UBP obtained by this study. Black bold contour lines in (A) denote the back-slip rate (5). In (B), the yellow stars mark epicenters of very low frequency earthquakes (VLFEs) (24); the red rectangle denotes a slow slip event (22) preceding the 2011 Tohoku-oki earthquake; the black rectangles show locations of coseismic strong ground motions (39); the magenta contour line marks the site of coseismic high-frequency P-wave radiation with a relatively low seismic moment during the 2011 Tohoku-oki earthquake (40); and the black short lines near the trench indicate seafloor traces of normal faults (34). The blue and red contour lines in (C) denote the coseismic slip (1) and the afterslip (43), respectively. The other labeling is the same as that in Fig. 2.

  • Fig. 5 Vertical cross sections of Vp tomography (left) and corresponding cartoons (right).

    The east-west vertical cross sections are along the three profiles shown in Fig. 1C. The normalized residual topography (blue line) and gravity (green line) along each profile are shown atop each cross section. The red and blue colors in (A to C) denote low and high Vp perturbations, respectively, whose scale is shown beside (A). The white bold and dashed lines in (A) to (C) denote the UBP and the forearc Moho, respectively. The red star denotes the mainshock hypocenter of the 2011 Tohoku-oki earthquake (Mw 9.0). The black and yellow stars indicate other megathrust earthquakes (Mw 7.0 to 8.0) during 1917 to 2017 and the VLFEs (24) within a 40-km width of each profile, respectively. Note that the hypocenters of the VLFEs and the megathrust earthquakes are set on the UBP, because their accurate focal depths are unclear for most of them. The reverse triangle denotes the Japan Trench axis. HF, high-frequency. In the right panels, the red, green, and blue lines denote low-, normal-, and high-Vp anomalies atop the subducting Pacific plate, respectively, according to the tomographic results of this study. The gray dashed line denotes the forearc Moho.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/6/eaat4396/DC1

    fig. S1. Seismograms and ray paths of sP depth phases.

    fig. S2. Results of the first CRT for Vp tomography.

    fig. S3. Results of the second CRT for Vp tomography.

    fig. S4. Results of the third CRT for Vp tomography.

    fig. S5. Results of the first RRT for Vp tomography.

    fig. S6. Results of the second RRT for Vp tomography.

    fig. S7. Results of the third RRT for Vp tomography.

    fig. S8. Correlation coefficients between four different anomalies beneath the Tohoku forearc.

    fig. S9. Along-trench variations of the subducting sediment thickness revealed by active-source seismic profiles.

    fig. S10. Structural heterogeneity beneath the Tohoku forearc and characteristics of the 2011 Tohoku-oki earthquake.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Seismograms and ray paths of sP depth phases.
    • fig. S2. Results of the first CRT for Vp tomography.
    • fig. S3. Results of the second CRT for Vp tomography.
    • fig. S4. Results of the third CRT for Vp tomography.
    • fig. S5. Results of the first RRT for Vp tomography.
    • fig. S6. Results of the second RRT for Vp tomography.
    • fig. S7. Results of the third RRT for Vp tomography.
    • fig. S8. Correlation coefficients between four different anomalies beneath the Tohoku forearc.
    • fig. S9. Along-trench variations of the subducting sediment thickness revealed by active-source seismic profiles.
    • fig. S10. Structural heterogeneity beneath the Tohoku forearc and characteristics of the 2011 Tohoku-oki earthquake.

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