Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanSEISMOGENIC FAULTS OF THE 2011 GREAT EAST
JAPAN EARTHQUAKE: INSIGHT FROM SEISMICDATA AND SEAFLOOR OBSERVATIONS
Takeshi TSUJI1, Yoshihiro ITO2 Kiichiro KAWAMURA3, Toshiya KANAMATSU4,
Takafumi KASAYA5, Masataka KINOSHITA6, Toshifumi MATSUOKA7, andYK11-04E and YK11-06E Shipboard Scientists
1

Assistant Professor, Graduate School of Engineering, Kyoto University,
Kyoto, Japan, tsuji@earth.kumst.kyoto-u.ac.jp
2
Assistant Professor, Graduate School of Science, Tohoku University,
Miyagi, Japan, yito@aob.gp.tohoku.ac.jp
3
Senior Research Scientist, Fukada Geological Institute,Tokyo, Japan, kichiro@fgi.or.jp
4Senior Research Scientist, Institute For Research on Earth Evolution, JAMSTEC,
Kanagawa, Japan, toshiyak@jamstec.go.jp
5Research Scientist, Institute For Research on Earth Evolution, JAMSTEC,
Kanagawa, Japan, tkasa@jamstec.go.jp
6
Principal Research Scientist, Institute For Research on Earth Evolution,
JAMSTEC, Kanagawa, Japan, masa@jamstec.go.jp
7
Professor, Graduate School of Engineering, Kyoto University,
Kyoto, Japan, matsuoka@earth.kumst.kyoto-u.ac.jp

ABSTRACT:
Faults related to the 2011 Tohoku earthquake were investigated using seismic data andsubmersible seafloor observations conducted before and after the earthquake. Seismic
profiles image several normal faults within continental crust. The seafloor observations
after the earthquake demonstrate that open fissures are developed along the fault traces
which could not be observed before the earthquake. Due to large displacement along theplate interface near the trench, geological unit above the plate interface is extension stressstate, and the normal faults and open fissures should be ruptured during this earthquake.
Key Words: Great East Japan earthquake, seismic reflection analysis, submersible
seafloor observations, fault distributions, tsunami mechanisms

INTRODUCTION
The March 2011 Tohoku-Oki earthquake (Mw 9.0) ruptured a wide area along the plate interface
off the Pacific coast of Tohoku, Japan (e.g., Ide et al., 2011). The tsunami induced in the 2011281

earthquake was extremely huge (Fujii et al., 2011). However, the estimated tsunami source area
(significant seafloor uplifted area) was not so wide (Hayashi et al., 2011; Fujii et al., 2011; Fujiwara etal., 2011; Fig. 1). From the offshore tsunami records, a large plate boundary slip was estimated to beoccurred off Miyagi near the trench (Maeda et al., 2011). To reveal mechanisms of the huge tsunami
generation, the shallow fault distributions and mechanisms are important to know. Because
long-period slip near the trench contributing to the huge tsunami generated almost no energy (Koper etal., 2011), it is difficult to reveal the rupture process near the trench only from onshore seismometerdata. Here we identify a series of faults near the trench from a seismic profiles and observe dynamicchange of the seafloor trace of fault system from submersible observations before the earthquake(YK08-06 in 2008) and after the earthquake (YK11-04 and YK11-06 in 2011). Because our survey
area includes the region where the largest vertical displacement is predicted to have occurred (Fujii etal., 2011; Fig. 1), the shallow faults discussed here are likely to be directly related to the tsunami
characteristics.

Fig. 1 (a) Index map for 2011 Tohoku Earthquake in the Japan Trench (JCG and JAMSTEC, 2011).
Hypoceneter of the mainshock is shown by red circle. Red, yellow and green areas are uplifted regions
estimated from tsunami inversion (Fujii et al., 2011). Red lines are the locations of seismic profiles
used in this study. Blue arrows indicate dynamic seafloor displacement revealed using seafloor
observatories (Sato et al., 2011; Kido et al., 2011). (b) Enlarged map around the diving points (stars).
The diving points are on the seismic transects, because we identified fault distribution on the seismic
profiles. Red dashed lines indicate the seafloor trance of the normal faults.

FAULT DISTRIBUTION ON SEISMIC PROFILES
Around the huge tsunami source area off Miyagi (Fig. 1), multi-channel seismic reflection data
were acquired by R/V Kairei of JAMSTEC (Tsuru et al., 2002). We analyzed this data for an282

investigation of fault geometry (Tsuji et al., 2011) and conducted submersible observations for the
deployment of seafloor observatories for the interpreted fault traces from 2008 (Fig. 2; Ito et al., 2011).
We applied conventional seismic processing for the multi-channel seismic data, including trace editing,multiple suppression, deconvolution, velocity analysis, stacking, and post-stack migration. We thenobtained the depth-domain profile (Fig. 2) by using stacking velocity (Tsuji et al., 2011). Due tolimitation of the streamer length, it was difficult to determine seismic velocities accurately in the
deeper lithology.
On the reflection profile acquired at the huge tsunami source area (MY102 in Fig. 1 and Fig. 2),
large normal fault maybe connecting to the plate interface and extending toward a seafloor ridge isidentified (Tsuji et al., 2011) in addition to several branching faults and backstop interface interpreted
as a boundary between seaward accreted sequence and landward less-deformed Cretaceous sequence(von Huene et al., 1994; Tsuru et al., 2002). The displacements along the normal fault offset a
Cretaceous sequence surface by ~800 m (Fig. 2c). Because seafloor ridge is also observed due tonormal fault displacements (~150m; Fig. 2c), the offset could be generated by accumulation of many
earthquake ruptures along the fault. Although we cannot clearly observe the reflection signal fromdeeper part of the normal fault, the ~800 m total displacements would be possible when the normal
fault connects to the plate interface. The seafloor ridges generated by the normal fault are wellidentified on the seafloor topography and continued parallel to the trench axis (red dashed lines in Fig.
1b). These ridges further become the transition point of seafloor slope angle; seafloor seaward side of
the ridge is steep slope compared to the landward side. Therefore, the normal fault should play animportant role at the plate convergent margin off Miyagi.
Landward side of the backstop interface, reflection events suggesting underthrusting sequence
were observed beneath the interpreted Cretaceous sequence (Fig. 2). This interpreted underthrusting
sequence could be less-consolidated sequence, because the P-wave velocity of this unit is lower thanthat of hanging wall side. Since the soft sediment is covered by the consolidated Cretaceous sequence,
the overburden should generate overpressure within this unit. Because of high pore pressure as well as
coseismic high pressure pulse (Ranero et al., 2008), these is a possibility that hydrofracturing isoccurred within this unit.
On the reflection profile located at northern part of rupture area (Lines SR101 and MY101 in Fig.
1), the fault distribution within the wedge is much different from the profile of Line MY102. Backstop
interface as well as subducting sediment can be clearly imaged on these profiles. Furthermore,seaward-dipping normal faults are intensively developed at the seafloor. These normal faults may begenerated by gravitational flow. The seafloor topography (Fig. 1b) further indicates several large
submarine slides have generated in this northern region (Sasaki, 2004).
DYNAMIC SEAFLOOR DEFORMATIONThe seafloor deformation revealed by acoustic GPS (Kido et al., 2011) and positioning of Ocean
Bottom Seismometers (Ito et al., 2011) demonstrated that dynamic horizontal seafloor displacement
across the normal fault is much changed; the station seaward of the normal fault (~70m) is much
moved to seaward direction, compared to the landward side of the fault (~30m) (red arrows in Fig. 2b).
The large extension stress between these two stations (>10-3; Ito et al., 2011) should be possible whennormal fault was ruptured because the dip angle of the normal fault is not so steep (< 50 degree).
Tsunami waveform inversion demonstrated the seafloor uplifting was generated near the trench (Fujii
et al., 2011; Maeda et al., 2011). Furthermore, the location of normal fault is in good agreement withthe landward edge of the large displacement region (Fig. 1). These observations suggest that thewedge-shaped unit between the normal fault and plate boundary fault (seaward side of the normal
fault) is significantly moved to seaward direction (Tsuji et al. 2011; Ito et al. 2011). Many
normal-fault-type aftershocks in the overriding plate (Asano et al., 2011) also support thisinterpretation of the normal fault displacement. In addition to the large normal fault located at theseafloor slope break, small normal faults are densely distributed especially at the landward side. These
normal faults also worked during this earthquake and contributed to the extension.

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Fig. 2 (a) Original seismic reflection profile with amplitude gain control (AGC). (b) Composite
seismic reflection profile with fault interpretations. Red arrows indicate the seafloor displacement
derived from seafloor observatories (Ito et al., 2011; Kido et al., 2011). (c) Detailed profile around thenormal fault and ridge structure. Displacement of steeply dipping fault (red dots) offsets the sedimentbasement surface (yellow dots). (d) Detailed profile around the seafloor trace of reverse fault (reddots). The fault can be identified as a clear reflection.

SEAFLOOR OBSERVATIONS BEFORE AND AFTER THE EARTHQUAKEBefore the earthquake, we used the manned submersible Shinkai 6500 to visit seafloor trances offault system identified on the reflection profiles (starts in Fig.1; Tsuru et al., 2002; Tsuji et al., 2011).
After the earthquake (in June and August 2011), we revisited these previous dive points using Shinkai
6500 and deep-tow camera (YK11-04E and YK11-06E). By comparing the seafloor fault traces before
and after the earthquake, we reveal dynamic change of seafloor geometry and environment associatedwith the 2011 Tohoku earthquake.

Huge landward-dipping normal fault
At the seafloor trace of the normal fault on the seismic line displayed in Fig. 2 (Line MY102), a
scarp ~150 m high was observed continuously along the ridge (Tsuji et al., 2011). The slope angle of284

the scarp is nearly vertical and overhanging in places. Because there is a fresh scarp surface without
manganese coating, this scarp may have been generated by recent tectonic activities (Tsuji et al. 2011).
We observed and sampled dark consolidated rocks attached to the cliff. This material differed greatly
from the poorly consolidated rock composing the cliff in its depositional age. The depositional age ofthe consolidated rock is 2.4-2.0 Ma (Table 1) and is much older than the reference unconsolidated rock
(0.85-0.51 Ma; Tsuji et al., 2011). Furthermore, the physical properties of the consolidated rock aremuch different from those of the unconsolidated reference rock (Table 1). We suggest that theconsolidated rock represents a deeper lithology that was carried to the surface along the fault plane,
perhaps during oblique fault displacements. The relation between depositional age and depth suggeststhat the consolidated rock was uplifted from 100–200 m below the seafloor. However, the P-wave
velocity of this rock is much greater than the value expected from the depositional depth (Tsuru et al.,2002). Therefore, the rock adhering to the fault scarp may have been consolidated by dynamic fault
activity such as heating or compaction.
When we dived to the same places after the earthquake, diatomaceous soft sediment layers as wellas greenish fluffs cover the seafloor at the fault trace. This thick soft sediment seems to be deposited
during or after the 2011 earthquake, because the seafloor before the earthquake was covered by gravels
collapsed from the scarp. The soft sediments should be derived from landward seafloor slope, because
the ridge structure works as a bank for submarine flow sediment (Fig. 2).
The bacterial mats (cold-seeps) were identified at the seafloor trace of southern continuation of thenormal fault. Although we have never dived to this position before the earthquake, these bacterial matscould be generated by the dynamic activities of the normal fault in the 2011 earthquake. Furthermore,
the heat flow values measured at the seafloor traces of the normal fault (>100mW/m2) are muchhigher than the background values in this region measured before the earthquake (20~40 mW/m2;
Yamano et al., 2008). These observations suggest that intensive fluid flow along the fractures has
recently occurred associated with the normal fault activities in the earthquake.

Reverse fault
At the previously interpreted reverse fault (Figs. 2b and 2d), chemosynthetic communities
observed along the fault trace before the earthquake (Fig. 3; Tsuji et al., 2011). The post-earthquake
seafloor observations (YK11-04E in 2011) demonstrate that several open fissures (1~3 m in width) aredeveloped along the interpreted fault (Fig. 3). These fissures were almost arranged in N-S direction(parallel to trench and fault strike). Because these fissures were not observed before the earthquake,they could rupture during this earthquake event. We have some small earthquakes for the three years(2008 to 2011), but the most presumable reason to form these fissures is the 2011 Tohoku earthquake.
We further observed bacterial mattes at a rim of the fissures, suggesting existence of intensive seepalong the fissure. Furthermore, dead Calyptogenas were stuck at the open cracks or collapsed underfalling rocks, suggesting intensive seafloor sediment flow.
Before the earthquake, we interpreted this fault as reverse type because of its geometry on theseismic profiles (Fig.2; Tsuji et al. 2011). However, we could not find any phenomena for reverse fault
type displacement from the post-earthquake seafloor observations.

Table 1. Physical properties and depositional ages of consolidated rock adhering to the fault scarp andreference rock making up the scarp. Velocities were measured at atmospheric pressure.
Porosity
P velocity
S velocity
Depositional age
18%
3.6 km/s
2.2 km/s
2.4–2.0 Ma
Consolidated rock
(YK08-06, 1071-R2)

Reference rock70%

1.6 km/s(YK08-06, 1071-R1)

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---

0.85–0.51 Ma

Fig. 3 Seafloor pictures of reverse fault before and after the earthquake. The location of this seafloor isindicated in Figs. 1b and 2a.

Backstop interface
At the backstop interface located at northern edge of the rupture area after the earthquake, we
further observed open fissures and bacterial mats. These open fissures were not observed before theearthquake. Because the seafloor seaward side of some fissures is lower than the landward side andbecause seaward dipping normal faults are observed on the seismic profile acquired at the northernpart of rupture area, these fissures could be generated by seafloor gravitational failure. Heat flow
values measured after the earthquake at this position (40 – 60 mK/m) are not much higher than the
background value in this region measured before the earthquake. Therefore, the earthquake rupture
may not propagate to the seafloor along the backstop interface at the northern edge of the rupture area.
Seaward-dipping normal fault
When we dived to seafloor trace of seaward-dipping normal fault where submersible surveys
were conducted before the earthquake, we also observed open fissures as well as bacterial mats. Heatflow value measured here (~61 mK/m) was lower than that of the landward dipping normal fault
region.

DISCUSSION AND CONCLUSIONSIn the seafloor observations after the earthquake, we observed open fissures in almost all seafloor
observation points, suggesting intensive extension near the seafloor associated with large displacement
along the plate interface. From the seafloor observations as well as fault distributions on the reflection
profiles, we estimate mechanisms of huge tsunami generation (see Fig. 4).
Rupture generated at the plate interface deeper than our seismic profile (hypocenter of thisearthquake is ~25 km in depth). In the frequently generated interplate earthquakes, the rupture does
not propagate to the trench (Yamanaka and Kikuchi, 2004). Such coseismic slip along the deep plateinterface induces smaller seafloor uplift and smaller tsunami (Fig. 4a). In the 2011 Tohoku earthquake,
however, the displacement along the plate interface propagated to the trench (Fig. 4b). The reason of
rupture propagation to the trench is not clearly understood. The plate boundary slip near the trench islarger than that of deeper fault maybe because of the effect of the dynamic overshoot (Ide et al., 2011).
In such a case, geological unit above the plate interface is in extension state (Fig. 4b). Due to the286extension, normal faults could be ruptured (Fig. 4c; Tsuji et al., 2011). Many open fissures observed
by submersible seafloor observations could also reflect the extension state above the plate interface.
Because the normal fault as well as fissures may relax the extension state above the plate interface,
they further accelerated the significant plate boundary slip near the trench (50-80m; Fig. 4c). Because
the seafloor slope seaward of the normal fault is steeper than landward side, the significant horizontal
movement of the steep seafloor near the trench generates significant seafloor uplift and huge tsunami.
Whereas, the submarine sliding of sediment masses (Kawamura et al., 2011) could generate hugetsunami. The seaward-dipping normal faults identified on the profile at the northern side of the rupturearea should be developed maybe due to the submarine slide, because the seafloor slope angle is very
steep. This event also moved the geological unit to seaward direction and caused tsunami. In thismechanism, large displacement along the plate interface near the trench is not necessary for tsunami
generation. Therefore, this mechanism could generate large tsunami (seafloor level change) even in thenorthern edge of the rupture area.

Fig. 4 Schematic image of fault ruptures during the earthquake. Red arrows indicate the fault
displacements. Black arrows indicate the seafloor movements which generate tsunami. Blue arrows
indicate the stress state within the sedimentary sequence.

ACKNOWLEDGMENTS
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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