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The 2011 Tohoku-Oki Earthquake: Displacement Reaching the Trench Axis

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Science  02 Dec 2011:
Vol. 334, Issue 6060, pp. 1240
DOI: 10.1126/science.1211554

Abstract

We detected and measured coseismic displacement caused by the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (MW) 9.0] by using multibeam bathymetric surveys. The difference between bathymetric data acquired before and after the earthquake revealed that the displacement extended out to the axis of the Japan Trench, suggesting that the fault rupture reached the trench axis. The sea floor on the outermost landward area moved about 50 meters horizontally east-southeast and ~10 meters upward. The large horizontal displacement lifted the sea floor by up to 16 meters on the landward slope in addition to the vertical displacement.

The large tsunami that followed the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (MW) 9.0] is believed to have been caused by a fault rupture extending to a shallow part of the subduction zone at the Japan Trench. This is indicated by various seismic and geodetic inversion procedures (1, 2); however, an accurate up-dip limit of the coseismic displacement has not yet been determined. We report repeated multibeam bathymetric surveys across the trench in the rupture zone before and after the earthquake to estimate its up-dip limit and quantify sea-floor displacement.

In 1999 and 2004, multibeam bathymetric data were acquired simultaneously during active-source seismic surveys (3, 4). After the earthquake, from 22 to 23 March 2011, we carried out a bathymetric survey along the same track (Fig. 1, A and B) (5). The relative differences among these bathymetric data are minimal on the seaward side of the trench despite potential errors of several meters in vertical displacement and ~20 m in horizontal displacement (5).

Fig. 1

Changes in sea-floor elevation between bathymetric data before and after the 2011 Tohoku-Oki earthquake. (A) Location map with bathymetric survey track shown as yellow line. Coseismic horizontal displacement is estimated over the landward slope indicated by solid portion of yellow line. Cross shows the epicenter. (B) Multibeam bathymetry collected in 2011. Red triangles mark the trench axis; the blue triangle marks the landward slope break. Change in sea-floor elevation by subtracting the 1999 bathymetric data from the 2011 data (C), the 2004 data from the 2011 data (D), and the 1999 data from the 2004 data (E). The yellow star marks location of probable submarine landslide.

There were, however, large relative differences landward extended up to the trench axis, suggesting the earthquake fault rupture reached the trench axis. A comparison of the bathymetry before and after the earthquake shows a sharp contrast in sea-floor elevation at the trench axis, and the sea floor is shallower throughout the landward side. Notably, on the outermost landward slope, the 40-km-wide area between the slope break and the trench axis, the difference between the 1999 and 2011 data shows that the sea floor is 16 ± 9 (±SD) m shallower on average (Fig. 1C). A comparison of the 2004 and 2011 data (Fig. 1D) shows the same trend, although the change was somewhat smaller (11 ± 8 m). Furthermore, upward and downward changes in sea-floor elevation of ±50 m are evident at the axial sea floor (Fig. 1, C and D), which are likely due to a submarine landslide (fig. S1E). A comparison of the 1999 and 2004 data obtained before the 2011 earthquake indicates no clear difference between the two sides of the trench axis [the average sea-floor elevation is 0 ± 7 m (Fig. 1E)].

The observed sea-floor elevation change on the outermost landward slope corresponds to a sum of vertical displacement and additional uplift for the sloping sea floor due to horizontal displacement. We estimated the horizontal displacement by calculating the offset distance to maximize the cross correlation of bathymetry (5). The estimated displacement is 56 m relative to the 1999 data, and 50 m relative to the 2004 data, toward the east-southeast. After restoring the horizontal displacement, the average elevation change became 10 ± 7 m in comparison between the 1999 and 2011 data (7 ± 7 m between 2004 and 2011). We interpret these to represent vertical displacement from the fault motion along the subducting plate and uplift from other unknown processes such as inelastic deformation. Overall, the sea floor on the outermost landward slope moved ~50 m east-southeast toward the trench and ~7 to 10 m upward between 1999 and 2011.

Our results are consistent with results of coseismic displacements determined at Global Positioning System (GPS)/acoustic sea-floor geodetic stations (6, 7) and other ocean-bottom instruments (8). Although our estimate of the average vertical displacement may be larger because of coseismic displacement, the earthquake probably caused little change on the seaward side. Combined with these geodetic studies, our study demonstrates the coseismic displacement increased toward the trench and reached the trench axis. This large coseismic horizontal displacement and the steeply sloping sea floor produced large additional uplift by ~4 to 6 m in addition to the vertical displacement (5). This uplift was likely an important factor contributing to the generation of the massive pulsating pattern of tsunami waves (2).

Supporting Online Material

www.sciencemag.org/cgi/content/full/334/6060/1240/DC1

Materials and Methods

Fig. S1

Table S1

Reference (9)

References and Notes

  1. Material and methods are available as supporting material on Science Online.
  2. Acknowledgments: We thank the crew of R/V Kairei and the technicians of Nippon Marine Enterprises for their dedication. The 2011 survey, part of the program launched following the earthquake, was supported by a Grant-in-Aid for Special Purposes of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) and a contribution from JAMSTEC.
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