Potential tsunamigenic faults of the 2011 off the Pacific coast of Tohoku Earthquake
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 13 April 2011
Accepted: 20 May 2011
Published: 27 September 2011
Faults related to the tsunamigenic 2011 Tohoku-Oki Earthquake (Mw 9.0) were investigated by using multichannel seismic reflection data acquired in 1999 and submersible seafloor observations from 2008. The location of the fault system interpreted in the seismic reflection profile is distributed around the area with largest slip and tsunami induction of the 2011 event. Cold-seep communities along the trace of the branch reverse fault and a high scarp associated with the trace of a normal fault suggest current activity on these faults. We interpret the fault system in the seismic profile as a shallow extension of the seismogenic fault that may have contributed to the resulting huge tsunami.
Here, we identify a series of faults from a seismic reflection profile obtained in 1999 near the hypocenter (JMA, 2011) and seafloor observations of the fault trace made in 2008 by the manned submersible Shinkai 6500. Because the surveyed area includes the region where the largest vertical displacement is predicted to have occurred (Ueno and Satake, 2011; Shao et al., 2011; Fig. 1(a)), the shallow faults here are likely to be directly related to the tsunami characteristics.
2. Seismic Reflection Data
Multi-channel seismic reflection data acquired by R/V Kairei (JAMSTEC) in 1999 (Line MY102 of KR99-08 cruise; Tsuru et al., 2002) were analyzed for an investigation of fault geometry and to select dive points for seafloor observations (Fig. 1(b)). In the seismic survey, the sound source was an array of ~200-L (12,000 cubic inch) airguns fired every 50 m. The receiver array was a 156-channel, 4-km streamer, and the record length was 13.5 s.
3. Geological Interpretation
On the reflection profile (Fig. 2), we identified three predominant faults branching from the plate boundary fault: (A) a backstop reverse fault acting as a boundary between a seaward accreted sequence and a landward less-deformed Cretaceous sequence (von Huene et al., 1994; Tsuru et al., 2002), (B) a branch reverse fault constructing the significant seafloor slope break (Fig. 2(d)), and (C) a steeply dipping normal fault branching from a plate boundary fault and extending towards a seafloor ridge (Fig. 2(c)). However, potential underplating structures are observed landward of the backstop reverse fault defined by Tsuru et al. (2002) (Fig. 2(b)).
Displacement along the steeply-dipping normal fault has offset a Cretaceous sequence surface by ~800 m (Fig. 2(c)). The plate boundary fault and steep normal fault appear to bound a pop-up structure. The seafloor ridge associated with the normal fault displacement is well identified on the seafloor topography (Fig. 1(b)) and is as long as several tens of kilometres parallel to the trench axis. Therefore, the normal fault should play an important role in the plate convergent margin off Miyagi.
4. Submersible Seafloor Observations
In May 2008, we used the manned submersible Shinkai 6500 (YK08-06) to visit two points along this seismic line: the seafloor trace of the (B) branch reverse fault (Fig. 2(d)) and a ridge associated with displacement along the (C) branch normal fault (Fig. 2(c)). Because of an insufficient depth capability of Shinkai 6500, we could not dive to the seafloor trace of the (A) backstop reverse fault located at a depth of ~7000 m as well as plate boundary fault (~7500 m).
A scarp ~150 m high marks the trace of the normal fault (Figs. 1(c), 2(c), 3(b)), and continuously exists along the ridge (~2 km dive track of the Shinkai). The slope angle of the scarp is nearly vertical and overhanging in places (Fig. 3(b)). We sampled rocks from the steep scarp and estimated the depositional age as 0.51–0.85 Ma from both calcareous nannofossils (e.g., Okada and Bukry, 1980) and fossil diatoms (e.g., Yanagisawa and Akiba, 1998). Because there is a fresh scarp surface without any manganese coating and because we observed a dead clam there, this scarp may have been generated by recent earthquake activity.
5. Summary and Discussions
Faults related to the tsunamigenic 2011 Tohoku Earthquake were investigated by using seismic reflection data and submersible seafloor observations. The fault system identified in this study is located at the seaward edge of the rupture area (Yagi, 2011) or consistent with the maximum rupture area (JMA, 2011; Shao et al., 2011) (Fig. 1(a)). The largest tsunami was inferred to have been induced near the interpreted fault (Ueno and Satake, 2011) where the largest vertical static displacement is expected (Shao et al., 2011). Therefore, the interpreted fault system may be shallow extensions of the seismogenic fault that also slipped during the earthquake.
Seafloor displacement during the earthquake estimated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC, 2011) demonstrated that the seafloor between the trench and the normal fault is significantly deformed in a seaward direction (50 m horizontal displacement) and uplifted (7 m vertical displacement). Therefore, the normal fault seems to act as a landward boundary of a significant displacement region. This observation suggests that the geological unit between the normal fault and the plate boundary fault should be uplifted (moves to seaward direction), as inferred from the fault interpretations on seismic profile (Fig. 2(b)).
Because displacement along the plate boundary fault near the trench is much larger than the deeper fault landward of our survey area (e.g., Fujii et al., 2011), the geological unit above the plate boundary fault is in a tensile state of stress. Due to the tensile stress state, the normal faults should be ruptured during this earthquake event.
If the steeply-dipping normal faults slipped during the earthquake, in addition to the plate boundary fault, they can induce the huge tsunami even as a result of a smaller displacement. The potential underplating unit landward of the backstop reverse fault may also have caused uplift that contributed to the tsunami.
We thank T. Sasaki (University of Tokyo) for the bathymetric map. We are grateful to two reviewers for their useful comments. The bathymetric data in Fig. 1(b) were acquired by R/V Kairei, Yokosuka, and Mirai (JAMSTEC). The seismic data were acquired by R/V Kairei (JAMSTEC). This study is supported by Grant-in-Aid for Scientific Research on Innovative Areas (21107003).
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