Supporting information may be found in the online version of this article.

2012gl054148fs01.tifTIFF image5489KFigure S1. Depth-migrated MCS profile SUMAE05 with the top of the oceanic crust (TOC) picks (red dots) superimposed. Location of the profile is indicated in the figure ¡°2012gl054148-fs02.tif¡±.
2012gl054148fs02.tifTIFF image622KFigure S2. Locations of the acquired MCS data profiles (purple lines) in our survey (SO198-2) and the positions of the TOC picks from these profiles (black dots) and from the MCS profiles (red dots) published by Franke et al. [2008].
2012gl054148fs03.tifTIFF image1153KFigure S3. OBS A09 (a) in-line and (b) off-line profile sections with seismic phases labeled. (inset) Locations of OBS A09 and the profile sections. Pap: P-wave refracted in the accretionary prism; Pg': P-wave refracted in the backstop block; Pn: P-wave refracted in the oceanic mantle; PcP: P-wave reflected at the top of the oceanic crust; PmP: P-wave reflected at the oceanic Moho surface.
2012gl054148fs04.tifTIFF image1042KFigure S4. OBS A30 (a) in-line and (b) off-line profile sections with seismic phases labeled. (inset) Locations of OBS A30 and the profile sections. Ps': P-wave refracted in the shallow sediment layer; Pg': P-wave refracted in the backstop block.
2012gl054148fs05.tifTIFF image2226KFigure S5. Velocity cross-sections extracted from the 3-D initial velocity model used for the final travel-time inversion. (a) vertical cross-sections perpendicular to the trench; (b) vertical cross-sections parallel to the trench; (c) Location of these cross-sections.
2012gl054148fs06.tifTIFF image604KFigure S6. The distribution of the residuals between the travel-times calculated from the initial model and the picked travel-times.
2012gl054148fs07.tifTIFF image561KFigure S7. The distribution of the final residuals of the travel-times used in the inversion. Blue lines indicate the uncertainty level assigned to the travel-times according to their source-receiver offset.
2012gl054148fs08.tifTIFF image559KFigure S8. The root-mean-squares residual of the travel-time inversion over successive iterations. Convergence is reached after 22 iterations.
2012gl054148fs09.tifTIFF image641KFigure S9. 3-D ray-paths traced through the final velocity model inverted from the first-arrival travel-time tomography (every 50th ray-path plotted).
2012gl054148fs10.tifTIFF image1113KFigure S10. Checkerboard tests. Recovered velocity anomaly patterns at different depth for anomaly size 40, 30, 20, and 10 km (from left to right), respectively.
2012gl054148fs11.tifTIFF image883KFigure S11. Checkerboard tests. Recovered velocity anomaly patterns in vertical cross-sections perpendicular to the trench (a) and parallel to the trench (b) for anomaly size 40 km × 5 km. (c) Locations of these cross-sections.
2012gl054148fs12.tifTIFF image2649KFigure S12. The velocity variations relative to the averaged along-strike velocities at fixed depths from the top of the oceanic crust, projected on a horizontal plane. The purple line, approximating the zero velocity variations, outline the anomalous velocity zone. Note the consistent low velocity anomaly in the subducting plate as it approaches the 2004–2005 rupture segment boundary. Red star: the 2004 Mw 9.2 earthquake; Green star: the 2005 Mw 8.7 earthquake; Yellow stars and Black dots: seismicity during 1964–2007 (from EHB catalogue). Depth bel. inf.= depth below the top of the oceanic crust.

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