MIS33-01

Room:102B

Time:May 26 09:00-09:15

Monitoring of seismic activity around the source region of the Tohoku-oki earthquake byocean bottom seismometers
SHINOHARA, Masanao1∗ ; YAMADA, Tomoaki1 ; NAKAHIGASHI, Kazuo2 ; MACHIDA, Yuya3 ; SHIMBO, Takashi4 ;MOCHIZUKI, Kimihiro1 ; SHIOBARA, Hajime1 ; MURAI, Yoshio5 ; HINO, Ryota6 ; ITO, Yoshihiro7 ;SATO, Toshinori8 ; UEHIRA, Kenji4 ; YAKIWARA, Hiroshi9 ; OBANA, Koichiro3 ; KODAIRA, Shuichi3
1

Earthquake Research Institute, the University of Tokyo, 2 Kobe University, 3 Japan Agency for Marine-Earth Science and Technology, 4 National Research Institute for Earth Science and Disaster Prevention, 5 Hokkaido University, 6 Tohoku University,
7
Disaster Prevention Research Institute, Kyoto University, 8 Chiba University, 9 Kagoshima University
The 2011 off the Pacific coast of Tohoku earthquake occurred at the plate boundary between the Pacific plate and the landward
plate on March 11, 2011, and many aftershocks followed the mainshock. To obtain a precise aftershock distribution is importantfor understanding of mechanism of the earthquake generation. In order to study the aftershock activity of this event, we carriedout extensive sea floor aftershock observation using more than 100 ocean bottom seismometers (OBSs) just after the mainshock.
Four days after the mainshock, we started to deploy seventy-two OBSs in the source region. Consequently, we observed theaftershocks at 121 sites including the pre-installed OBS sites in total (1st term). The observation area covered the source regionof the mainshock with OBS interval of 25 km. Some OBSs were recovered in late April and deployment of OBSs to the same
position were carried out (2nd term). In June, almost of the deployed OBS were recovered and we concentrated observations
with OBSs in off-Miyagi and off-Boso regions (3rd term). Observations in both areas were terminated in September 2011. Thereis a possibility that spatial and temporal changes of seismic activity occur due to the recovery process of plate coupling. To detectspatiotemporal changes of the seismic activity, we deployed 40 long-term OBSs (LT-OBSs), which have observation duration of
one year, in the whole source region, and started monitoring of seismic activities in the source region (4th term). In April 2012,
other 40 LT-OBSs were deployed in the southernmost source region to increase spatial density of the network. In October andNovember 2012, all the LT-OBSs on seafloor were recovered, and spatial high dense network by using 40 LT-OBSs was deployedwith OBS interval of approximately 20 km in the off-Fukushima region (5th term) in November 2012. After one-year seafloor
observation, the network off Fukushima was retrieved. In September 2013, we deployed 30 LT-OBSs in off-Miyagi and off-Iwate
regions to monitor seismic activity (6th term). These LT-OBSs were successfully recovered in October in 2014, and we continue
seafloor seismic observation in the off-Miyagi region with 18 sites from October 2014.
We selected events whose epicenter is located below the OBS network from the JMA earthquake catalog, and P and S-wave
arrival times were picked from the OBS data. Hypocenters were estimated by a maximum-likelihood estimation technique with
one dimensional velocity structures. Thickness of sedimentary layer, which changes at each OBS site was evaluated and theestimated travel times by the location program were adjusted. From the observations in the 1st and 2nd terms, a precise aftershock distribution for approximately three months were obtained. The aftershocks form a plane dipping landward in the wholearea. Comparing our results to velocity structures by marine seismic surveys, there is no aftershock along the plate boundary
in the region off Miyagi, where a large slip during the mainshock is estimated. A plate coupling in this region may change due
to occurrence of the mainshock. Activity of aftershocks within the landward plate above the source region is high and many
aftershocks within the landward plate have normal fault type or strike-slip type mechanism. Within the subducting oceanic plate,most of earthquakes has normal fault type or strike-slip type mechanism. Using hypocenter distribution by long-term observationfrom the autumn 2011, we compare locations of the hypocenters with those of the aftershock just after the mainshock. In theaftershock distribution, the low-seismicity region is recognized at the plate boundary in the off-Fukushima region. The long-term
observations show the seismicity is not low in the identical region. On the other hand, seismic activity along the plate boundary
in off-Miyagi region was still low until the end of the long-term observation carried out from 2011 to 2012.

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MIS33-02

Room:102B

Time:May 26 09:15-09:30

What long-term seafloor observations told us about the 2011 Tohoku-Oki Earthquake
HINO, Ryota1∗
1

Tohoku Univ.

A number of important aspects of the 2011 Tohoku-Oki earthquake (Mw 9.0) were clarified by the seafloor observation above
the rupture area of the earthquake. The most important observations were the extraordinarily large coseismic displacements,
putting strong constraint on the processes of the fault rupture and tsunami generation. Continuous monitoring of ground motion
using seismometers and pressure gauges clarified that gradual acceleration of the aseismic slip took place not only in the vicinityof the hypocenter of the eventual mainshock but also in the updip side of it. In addition, the seafloor instruments detected several
unexpected phenomena associated with the earthquake. One of the instruments was displaced by about 1 km and detected largepressure and temperature excursions beginning three hours after the mainshock occurrence. These observations gave a strong
evidence for the occurrence of tsunami-generated turbidity current in the area. Sediments trapped by the outer-shells of these
instruments also helped to identify the origin and path of the flow. Including these non-seismological/geodetic ones, pre- co- andpost-seismic observations are invaluable to characterize the massive and infrequent event and are still under careful inspection.
We continued seismological and geodetic observation after the earthquake to know postseismic activity. The obtained data
indicate that the postseismic crustal deformation field show very complex spatial pattern as compared to those observed by theonshore network. The complexity is caused by large viscoelastic relaxation induced by the huge coseismic slip and makes it
difficult to identify the elastic deformation associated with the afterslip along the megathrust, although it is the most important
information to understand the behavior of the fault. The situation requires us to enhance the abilities of seafloor monitoring todetect the slip activities on the fault. Detecting slow-slip transient slips is one of the solutions and we started an array of arrays
observation including broad-band seismographs to detect and locate slow-slip events and low-frequency tremors, which can happen in the transient process regaining interplate coupling. Another observation we started is direct-path acoustic ranging acrossthe trench axis. Slip rate of the shallow fault can be measured by monitoring the change in distance between the benchmarks on
the incoming and overrding plates.

Keywords: 2011 Tohoku-Oki Earthquake, ocean bottom seismology, seafloor geodesy, temperature monitoring

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MIS33-03

Room:102B

Time:May 26 09:30-09:45

What happened at the northern Japan Trench (around 39.5 N) during the 2011 Tohoku
earthquake ?
SATAKE, Kenji1∗ ; FUJII, Yushiro2
1

Earthquake Res. Inst., Univ. Tokyo, 2 IISEE, Buliding Res InstFor the 2011 Tohoku earthquake, many models of slip distribution based on seismic waves, GPS and tsunami data have been
proposed. The maximum slip is located at around 38 N off Miyagi prefecture, although the amount and the down-dip location(depth) are different among the models. The coastal tsunami height was largest around Miyako city at 39.5 N, which is roughly
100 km north of the maximum slip. Satake et al. (2013, BSSA) made inversion of tsunami waveforms recorded on bottompressure, GPS and costal wave and tide gauges to estimate spatial and temporal distribution of coseismic slips. Their result
indicates that very large slips (maximum 69 m) occurred near the trench axis about 3 minutes after the earthquake origin time
(rupture initiation), and it propagated toward north along the trench. They concluded that the delayed slip near the trench axis
was the main cause of the largest tsunami in Iwate prefecture.
Because seismic wave analyses indicate that the rupture processes were at most 3 minutes, the delayed slip may not be seismic(faulting) origin. A recent paper by Tappin et al. (2014, Marine Geology) claimed that the cause of the large tsunami along
the Iwate coast is a submarine landslide. Their analysis indicates that the submarine landslide occurred at 135 seconds after theorigin time at around 39.5 N along Japan Trench, with a length of 40 km, a width of 20 km, a slope thickness of 2 km, a verticaloffset (rotation) of 100 m. The total landslide volume was estimated as 500 km3 .
Along the Japan Trench off Iwate, a large slip occurred during the 1896 Sanriku earthquake. The estimated slip amount is 10
to 20 m, and the slip extended further north of the 2011 delayed slip. If coseismic slip occurred in 1896 and 2011, the total slip
amount would be 20 to 30 m. Although this is smaller than the largest slip of the 2011 Tohoku earthquake, it is enigmatic that aslip larger than the plate convergence (˜8 m/century) occurred within 100 years.
In order to identify the cause of the 2011 tsunami source off Iwate, whether it was a fault slip or a submarine landslide,submarine surveys such as detailed bathymetry or subsurface structure are expected.

Keywords: Tohoku earthquake, Japan Trench, tsunami, submarine landslide

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MIS33-04

Room:102B

Time:May 26 09:45-10:00

Regional distribution of seafloor displacement detected by bathymetric surveys after the2011 Tohoku-oki earthquake
FUJIWARA, Toshiya1∗ ; KODAIRA, Shuichi1 ; DOS SANTOS FERREIRA, Christian2 ; FUJIE, Gou1 ;KAIHO, Yuka1 ; KANEDA, Yoshiyuki3 ; KASAYA, Takafumi1 ; NAKAMURA, Yasuyuki1 ; NO, Tetsuo1 ;SATO, Takeshi1 ; STRASSER, Michael4 ; TAKAHASHI, Narumi1 ; TAKAHASHI, Tsutomu1 ; WEFER, Gerold2
1

Research and Development Center for Earthquake and Tsunami, JAMSTEC, 2 MARUM, Center for Marine Environmental
Sciences, University of Bremen, 3 Disaster Mitigation Research Center, Nagoya University, 4 Geological Institute, ETH ZurichAfter the 11 March 2011 Tohoku-oki earthquake (M W 9.0), we have carried out multibeam bathymetric surveys in the rupturezone. Some survey tracks were aligned along the tracks obtained before the earthquake across the Japan Trench, and we analyzed
the difference in bathymetry before and after the earthquake in the area near the trench. For the analysis, apparent offsets of theabsolute values of depth soundings and the uncertainty of ship position were examined on the seaward side because the seaward
was thought to have suffered little change from the earthquake. Although the results may incorporate errors of several meters invertical displacement and about 20 m in horizontal displacement [e.g. Fujiwara et al., MGR 2014], the extraordinary coseismic
seafloor displacement caused by the 2011 earthquake was detected by the bathymetric surveys. For the survey track crossing
the trench axis at 38o 05’N, off the coast of Miyagi Tohoku, near the epicenter, there were large relative differences landward
extended up to the trench axis, suggesting the earthquake fault rupture reached the trench axis [Fujiwara et al., Science 2011;
JpGU 2012]. The seafloor after the earthquake 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 shows the seafloor is shallower than
10 m on average. This uplift was likely an important factor contributing to the generation of the huge tsunami. The observed
seafloor elevation change on the outermost landward slope corresponds to a sum of vertical displacement and additional uplift
for the sloping seafloor due to horizontal coseismic displacement. We estimated the horizontal displacement by calculating theoffset distance to maximize the cross-correlation of bathymetry. The estimated displacement is approximately 50 m in the eastsoutheast toward the trench. Furthermore, locally upward and downward changes in seafloor elevation of ±50 m are evident
at the axial seafloor of the survey track. This morphological structure is interpreted as compressional (thrust-up structure) withreverse faults branching from an interface in the subducting sedimentary layer, which is interpreted as the coseismic master fault.
The structure where subsidence was observed is interpreted to have formed by slumping, which may have caused gravitational
instability [Kodaira et al., Nature Geosci. 2012; Strasser et al., Geology 2013]. The size of this deformed morphological structureis 3 km across the trench and 13 km along the trench within a confined area. For the track crossing the trench axis at 38o 35’N,
about 50 km north of the 38o 05’N track, bathymetric data comparison shows the same trend and there are relative differences
landward extended up to the trench axis. However, change in seafloor elevation along this track is of smaller magnitude than thatalong the 38o 05’N track and shows direct evidence indicating smaller coseismic displacement along this track. The horizontal
seafloor displacement seems to be difficult to resolve accurately due to the estimation errors. For the track crossing the trenchaxis at 37o 25’N, off the coast of Fukushima, about 70 km south of the 38o 05’N track, change in seafloor elevation along thistrack is even smaller than that along the 38o 35’N track. Also, we didn’t observe significant deformed structures at the axial
seafloor outside of the extra-large displacement area at 38o 05’N.
Keywords: 2011 Tohoku-oki earthquake, tsunami, Japan Trench, multibeam bathymetry, seafloor displacement

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MIS33-05

Room:102B

Time:May 26 10:00-10:15

Spatial extent of sedimentation triggered by the 2011 Tohoku earthquake from short-lived
radioisotope data, Japan Trench
MCHUGH, Cecilia1∗ ; KANAMATSU, Toshiya2 ; SEEBER, Leonardo3 ; CORMIER, Marie-helene4 ;BOPP, Richard5 ; IKEHARA, Ken6 ; USAMI, Kazuko6
1

Queens College, C.U.N.Y., USA, 2 JAMSTEC, Japan, 3 Lamont-Doherty Earth Observatory, USA, 4 University of Rhode Island,
USA, 5 Rensselaer Polytechnic Institute, USA, 6 Geological Survey of Japan, AIST

Extensive work has been conducted along the Japan Trench since the 2011 Tohoku megathrust earthquake and tsunami andmuch has been learnt as a result of these studies that can be applied regionally and globally to other subduction systems. In 2013,
the Japan Agency for Marine-Earth Science and Technology conducted expeditions NT13-02 and NT13-19 to the 2011 Tohoku
Mw 9.0 megathrust earthquake and tsunami source, with R/V Natushima in 800-5,900 m water depth. The goal was identifying
earthquake-triggered deposits and mapping their spatial and temporal distribution, as a strategy to recognize the sedimentarysignature of Tohoku-like events and measure recurrence intervals for seismic hazard assessment. Twenty-four piston cores, 3 to
6 m long, were recovered during the NT13-19 expedition along a 300 km-long portion of the mid-slope terrace. This elongatedstructure is parallel to the strike of the Japan Trench, and located landward of the frontal prism where deformation is most intense.
Faults, sometimes forming steep scarps, define small (5km long) confined basins that were targeted for coring.
Very high activities in xs210 Pb and concentrations of 137 Cs were measured in the upper half-meter of the cores. Detection of
Cs and enrichment of 137 Cs provided a Fukushima signature that was found in the upper 15 cm of several cores. Together
with x-ray fluorescence elemental analyses, these radioisotopes provide evidence for multiple pluses of sedimentation triggeredby the Tohoku 2011 earthquake and possibly some of its aftershocks, and of older earthquakes that occurred as far back as thelast hundred years.
134

Widespread shaking by the 2011 earthquake induced synchronous fluidization and resuspension of near bottom sediments for
˜250 km along the strike of the Japan Trench. Sediment thickness seems to depend on its proximity to the zone of maximum
megathrust slip, but could also depend on local topography and supply of unstable sediment. The sediment deposited as a resultof the earthquake shaking is homogeneous and lacks bioturbation. The earthquake also generated turbidity currents as evidenced
by sand-rich beds. Proximal to the area of maximum megathrust slip, and presumably disruption on the upper plate, the earthquake caused brecciation, dewatering and minor slumping of sediments. Re-suspended sediments were deposited on the seafloor
for at least 30 days after the earthquake and likely for much longer.

Keywords: Sedimentation generated by Tohoku 2011 earthquake, spatial distribution and pulses of sedimentation, short lived radioisotopes track sedimentation, Mid-slope terrace, Japan Trench, detection of Fukushima signature in sediments, sedimentation
relative to maximum megathrust slip

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MIS33-06

Room:102B

Time:May 26 10:15-10:30

Generation process of earthquake-related turbidity currents along the mid slope terrace
on the Japan Trench inner slope
USAMI, Kazuko1∗ ; IKEHARA, Ken1 ; KANAMATSU, Toshiya2 ; MCHUGH, Cecilia3
1

Geological Survey of Japan, AIST, 2 JAMSTEC, 3 Queens College, C.U.N.Y

We conducted radiocarbon age determinations for bulk organic carbon of piston core sediment samples, recovered from themid slope terrace on the Japan Trench inner slope, 36.6-40.8 N, 142.8-144.2 E, water depth 4203-6217 m, during the NT13-19
and YK14-E01 cruises. The obtained conventional radiocarbon ages are assumed to be almost 2000 years older than actual ages
based on the ages of the core top samples and of the intercalated age-known tephras.
Many deep-sea turbidites were intercalated in the cores. They are considered to be seismo-turbidites regarding the sedimentary
structures, grain compositions and surrounded topographic conditions. Age differences between hemipelagic and turbidite muds
suggest that remobilization of surface sediments by the earthquake is the important process for turbidite deposition on the terrace.
Keywords: earthquake, Japan Trench, turbidite, tephra, 14C age

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MIS33-07

Room:102B

Time:May 26 10:30-10:45

Sub-bottom environmental change around the Sanriku coastal area after the 3.11 Tohoku
Earthquake and tsunami.
SAKAMOTO, Izumi1∗ ; YOKOYAMA, Yuka1 ; IIJIMA, Satsuki1 ; INOUE, Tomohito1 ; ARAKAWA, Takuya1 ;YAGI, Masatoshi1 ; NEMOTO, Kenji1 ; FUJIMAKI, Mikio2
1

School of Marine Science and Technology, Tokai Univ., 2 COR

The recent 2011 Tohoku tsunami strongly affected the coastal area of the Pacific coast of Tohoku. Tokai University investigatedthe Tohoku coastal area as a part of Tohoku Ecosystem-Associated Marine Sciences (TEAMS). Our purpose will be to get theknowledge of distribution of rabble, bottom sediment environment and tsunami information in the coastal local area.
We researched using acoustic equipments (Multi narrow beam echo sounder : MNB, Sub bottom profiler : SBP and Side scansonar : SSS), bottom sampler and ROV.
Some gigantic crescent marks were observed around shallow bottom area (-10m to -30m in depth) from the Toni and Okiraibay. These marks were estimated that they formed by erosion caused around the rubble at the down flow stage of tsunami. We
are trying to observe these marks by MNB again for changing forms in a secular variation.
In the Hirota bay, NW-SE trending high reflector zone with 1.5km in length is formed at the front of the mouth of Kesen river,by SSS survey. We have also the bottom sediment sampling by grab sampler in 50m interval (total 100 samples), along the highreflector zone, on twice a year from 2012 to 2014. As the result of grain analysis for the bottom samples, gravels an coarse sandsediment consist the high reflector zone, and become coarse year by year. However, inner and central areas of the Hirota bay
become fine. Large-scale restoration construction is developed at a coastal region around the Sanriku area. It’s expected that thesea-bottom environment changes with these restoration construction.

Keywords: tsunami, sub-bottom environmental change

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MIS33-08

Room:102B

Time:May 26 11:00-11:15

A Physical Oceanographic Study of the Sanriku Coastal Seas
TANAKA, Kiyoshi1∗ ; MICHIDA, Yutaka1
1

Atmosphere and Ocean Research Institute, The University of Tokyo

There are a dozen or more small bays on the Sanriku ria coast, located in the northeastern part of Japan. The bays have lengthsof several to a few tens of kilometres, and depths of several tens to a few hundred meters, and they are well known important
areas of inshore fisheries, where a variety of seaweeds and shellfish are farmed. The farming is characterized by a non-feeding
type.
After the 2011 off the Pacific coast of Tohoku Earthquake, the Atmosphere and Ocean Research Institute (AORI) of the University of Tokyo launched a research program to clarify the factors controlling the dynamics of the ecosystems in the Sanriku coastalseas, which will clarify what is needed to restore the area’s fishing industry. The program consists of multidisciplinary researcheswith scientists in the fields of physics, chemistry, and biology, and it has been supported by Tohoku Ecosystem-Associated Marine Sciences (TEAMS) from Ministry of Education, Culture, Sports, Science, and Technology of Japan (MEXT).
We have investigated the seawater circulation in this area, especially from a view-point of physics (physical oceanography):
Shipboard surveys were conducted many times in Otsuchi, Kamaishi, and Hirota Bays, using a ship-mounted acoustic Doppler
current profiler (ADCP) and a conductivity-temperature-depth profiler (CTD profiler). Moreover, a variety of monitoring instruments, such as current profilers, thermometers, wave sensor, and so on, have been deployed in Otsuchi Bay (see Figure).
Furthermore, oceanic observations were also conducted outside the bays, using a research vessel.
As a result, a large amount of hydrographic data were collected to successfully provide clear images of the seawater circulationin the Sanriku coastal seas: In summer, for example, a prominent baroclinic circulation with a three-layer structure extends overthe greater part of the ria bay, and its flow directions change on time scales of several to a few tens of hours.
In addition, we continue to find a way to solve not only academic problems, but also social problems simultaneously andsynergistically, together with the local community. This is because the hydrographic observations over the inshore fishery areas
cannot be made without support from fishermen and their cooperative associations. At the same time, there are many fisheryproblems that cannot be solved without an academic approach; for example, the physical oceanography is needed to reveal theseawater circulation that conveys nutrients into the ”non-feeding” sea farming areas.

Keywords: Seawater Circulation, Sanriku, Physical Oceanography, Tohoku Ecosystem-Associated Marine Sciences

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MIS33-09

Room:102B

Time:May 26 11:15-11:30

Whait kind of disturbance have Great East Japan Earthquake and Tsunamis given?
KITAZATO, Hiroshi1∗
1

Japan Agency for Marine-Earth Science and Technology

March 11, 2011, Megaearthquake and Tsunamis took place at Tohoku Region. Marine Ecosystems have totally damaged due
to Earthquake and Tsunamis. Tohoku Region is a rich in marine biodiversity. Thus, the region is famous for fisheries. Together
with Tohoku University and AORI, University of Tokyo, JAMSTEC is going to make research on disturbances and recovary
processes of marine ecosystems off Tohoku. The project is supported by MEXT. This project aims to make crear disturbancesof marine ecosystems by Earthquake and Tsunamis and to monitor recovery processes from ecosystem disturbances. We plan totransfer and/or share research results to fisheries, local governments and citizens at Tohoku Areas. In my talk, I try to introduce
about Tohoku marine sciences. I also show what kind of disturbances have taken place at bottom ecosystems of Tohoku Region.
Keywords: Great East Japan Earthquake, Tsunami, Marine Ecosystems, Sediment liquification, Turbidite, Repopulation

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MIS33-10

Room:102B

Time:May 26 11:30-11:45

Bioturbation in shallow marine deposits along Sanriku Coast after the 2011 tsunami disturbance
SEIKE, Koji1∗
1

Atmosphere and Ocean Research Institute, University of Tokyo

The huge tsunami waves induced by the 2011 M9.0 Tohoku-Oki Earthquake severely affected shallow marine ecosystems
along the Pacific coast of northeastern Japan (Seike et al., 2013). This study focuses on sedimentary features (physical and biogenic sedimentary structures) of shallow marine deposits along Sanriku Coast, i.e., Funakoshi and Onagawa bays, northeasternJapan after the 2011 tsunami disturbance. Core samples were observed using X-ray radiography, computed tomography scanning, and grain size analysis to identify temporal changes in the physical and biogenic sedimentary structures following the 2011
tsunami disturbance. At Funakoshi Bay, Iwate Prefecture, sediment coring was conducted in September of 2014. The seafloor
sediments of this bay were composed of laminated sandy deposits (tsunami-induced deposits). The upper section (between thesurface and a depth of 20 cm) was totally mixed (bioturbated) by burrowing activity of the heart urchin Echinocardium cordatum ,and contained no physical sedimentary structures. At Onagawa Bay, Miyagi Prefecture, sediment coring was conducted between
October 2012 and April 2013 (three observations). The seafloor sediments of this bay consisted of two lithological layers. The
upper section was composed of muddy sediments whereas the lower part of the cores (below a depth of 8 cm) consisted oflaminated sandy deposits (tsunami-induced deposits). In 2012 and 2013 observations of the bay, burrows produced by benthic
animals were seen only in the upper mud layer. In contrast, in 2014 observation, abundant burrows were seen in both the upper
mud and lower sand layers. These results from Funakoshi and Onagawa bays indicate that recolonization of large and deepburrowing animals began within three years of the 2011 tsunami. Also, the intense sediment mixing by large burrowing animals
will homogenize the seafloor sediment.
Seike K, Shirai K, Kogure Y (2013) Disturbance of Shallow Marine Soft-Bottom Environments and Megabenthos Assemblages
by a Huge Tsunami Induced by the 2011 M9.0 Tohoku-Oki Earthquake. PLoS ONE 8(6): e65417. doi:10.1371/journal.pone.0065417

Keywords: tsunami, bioturbation, burrow, Sanriku, sediment, shallow marine

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MIS33-11

Room:102B

Time:May 26 11:45-12:00

Long term monitoring of bottom environments off Ohtsuchi Bay
OGURI, Kazumasa1∗ ; FURUSHIMA, Yasuo1 ; TOYOFUKU, Takashi1 ; KASAYA, Takafumi1 ; WATANABE, Shuichi1 ;FUJIKURA, Katsunori1 ; KITAZATO, Hiroshi1
1

JAMSTEC

The 2011 off the Pacific coast of Tohoku Earthquake induced heavy disturbances on sea floor environments in wide areas fromcoastal zone to the hadal trenches in the northerastern Japan (Kawagucci et al,. 2012; Noguchi et al., 2012; Arai et al., 2013;
Oguri et al., 2013). On the sea floor, strong turbid layers were developed presumably by strong shaking or intensified hydrodynamic surges by tsunami (Toyofuku et al., 2014). To investigate environmental changes or the recovery from the disturbances,
we conducted long term monitoring at 300m and 998m in water depth off Ohtsuchi bay.
For the monitoring, two monitoring stations were constructed to mount ADCP-CTD-DO-turbidity sensors (Aanderaa RDCP600),
combinations of LED light, HDTV camera and lithium-ion battery (handmade) on a titanium frame. The hydrodynamic andchemical data were acquired every one hour interval. The still images were taken every day and the 4.5 minutes videos wererecorded every week, respectively. The station was deployed at 300 m depth from 2013/3/12. Although this station was unexpectedly captured by a trawling boat, the data and the photographs recorded for five and half months were recovered. At 998 m
depth, the other station was deployed from 2012/8/12, and it was safely recovered during R/V Natsushima NT13-21 cruise.
At 300 m depth, dominated current direction was NNE to SSW. The 25 hours averaged current intensity was 0-30 cm/sec.
Decrease of water temperature from 8 to 2 oC was observed on May/2013. Salinity decrease from 33.3 to 32.8 was also synchronized with the temperature variation. These changes seemed to reflect contribution of Oyashio water. Dissolved oxygen (DO)ranged from 290˜250 µM, but it was suddenly decreased to ˜100 µM, and recovered in a few hours to a few days. Turbidity
showed increased trend in a short time on April to May/2013, reflecting phytoplankton blooming observed by remote sensing.
This period, strong turbidity by marine-snow was also recorded by the camera. Photographs taken at the sea floor recorded highdensity habitats of brittle star. A few fishes and other organisms (sea anemone etc.), were also observed.
At 998 m depth, dominated current direction was also NNE-SSW. The averaged intensity was 0˜15 cm/sec. Water temperature
was 3 oC and the salinity was 34, which ranged almost constant values throughout the deployment. DO was 25˜29 µM, indicating
just in the DO minimum zone. Turbidity was increased after Feb/2013, but it seemed to reflect biofauling, because bottom water
was not turbid from the photographs.
On 2012/12/7, strong earthquake (M=7.3) supposed as an aftershock of the Tohoku earthquake occurred. The camera recorded
the turbulences of sea floor and benthic habitats. Turbidity was intensified just after the earthquake and brittle stars were buried.
However, the turbidity and the activity of the benthic habitats were recovered quickly and the effect of the earthquake seemed tobe small for the organisms.

Keywords: sea floor, long term monitoring, earthquake, benthic habitat

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MIS33-12

Room:102B

Time:May 26 12:00-12:15

Unexpected type of biodiversity disturbances of benthic ecosystem of off Shimokita after
3.11
TOYOFUKU, Takashi1∗
1

JAMSTECOn March 11th, 2011 the Mw 9.0 2011 T?hoku-Oki earthquake resulted in a tsunami which caused major devastation in coastal
areas. Along the Japanese NE coast, tsunami waves reached maximum run-ups of 40 m, and travelled several kilometers inland.
Whereas the devastation was clearly visible on continental areas, the underwater impact was much more difficult to assess. Here,
we report unexpected results during a research cruise targeting the marine floor off Shimokita (NE Japan), five months after
the disaster. The geography of the studied area is characterized by smooth coastline and a gradually descending shelf slope.
Although high-energy tsunami waves caused major sediment reworking in shallow-water environments, shelf ecosystems werecharacterized by surprisingly high benthic diversity and showed no evidence of mass mortality. Conversely, just beyond theshelf break, the benthic ecosystem was dominated by a low-diversity, opportunistic fauna indicating an ongoing colonization ofmassive sand-bed deposits.
Keywords: tsunami

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MIS33-13

Room:102B

Time:May 26 12:15-12:30

Impacts on the deep-sea ecosystem off Sanriku from the mega-earthquake and tsunami of
2011: Research by the TEAMSFUJIKURA, Katsunori1∗ ; KITAZATO, Hiroshi1
1Japan Agency for Marine-Earth Science and Technology

The Great East Japan Earthquake of March 11, 2011 generated a massive tsunami wave that severely damaged coastal areas ofJapan. The earthquake and tsunami of the Great East Japan Earthquake also caused extensive damage to the marine ecosystem
including deep-sea off Sanriku region. It means that local fisheries received the devastating damage from this catastrophe. The
deep-sea fishing is one of the most important fisheries in this region. In order to help understand and utilize marine ecosystems and fisheries including deep-sea fisheries, JAMSTEC has conducted multidisciplinary researches under the project, Tohoku
Ecosystem-Associated Marine Sciences: TEAMS as a decadal program beginning in FY 2011 with the Tohoku University and
the Tokyo University. JAMSTEC subjects are:- to estimate the influence of debris on ecosystems and fisheries,- to reveal the ecology of organisms living on the seafloor in offshore areas,- to explain how the seafloor environment will change,- to reveal the state of pollution in the sea by monitoring levels of PCB,
- to create habitat and ecosystem maps,- to share TEAMS activities and results known to the public (Database).
For progress of these subjects, we have carried out investigations and research mainly in offshore waters using a range of tools
and equipment, such as research ships, ROVs and IT technology.
We will present progress activities of TEAMS by the JAMSTEC and would like to discuss how to contribution for reconstruction of local fisheries from science aspects.

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MIS33-14

Room:102B

Time:May 26 14:15-14:30

Effect of the 2011 Tohoku Earthquake on meiofauna inhabiting the landward slope of theJapan Trench off Sanriku
KITAHASHI, Tomo1∗ ; WATANABE, Himiko2 ; JENKINS, Robert3 ; NOMAKI, Hidetaka4 ; SHIMANAGA, Motohiro5 ;FUJIKURA, Katsunori4 ; KOJIMA, Shigeaki1
1

Atmosphere and Ocean Research Institute, The University of Tokyo, 2 Faculty of Science, Kumamoto University, 3 School of
Natural System, College of Science and Engineering, Kanazawa University, 4 Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 5 Center for Marine Environment Studies, Kumamoto University
Deep-sea floors contain high levels of biodiversity, which is sometimes comparable to tropical rain forests, despite the apparent uniformity of its environment (Hessler & Sanders 1967; Grassle 1989). It is explained that small-scale disturbances, such aspredation and near-bottom currents, frequently occur and cause a patchy distribution of different successional stages across anarea (Rex 1981; Levin et al. 2001). The effect of small-scale disturbance on deep-sea assemblages has been studied for several
taxa (Kaminski 1985; Hall 1994; Paterson & Lambshead 1995; Thistle 1998). On the other hand, turbidity current, which is abottom-flowing current laden with suspended sediments generated by tsunamis and earthquake-induced landslides, is an example
of a catastrophic disturbance (cf. Harris 2014) and expected to cause a large effect on meiofaunal assemblages. However, little
is known how the deep-sea benthic meiofauna respond to such a catastrophic disturbance.
On 11 March 2011, an earthquake of Mw9.0 known as the 2011 off the Pacific coast of Tohoku Earthquake occurred off thecoast of Miyagi Prefecture. Sedimentation caused by the turbidity current was documented over an extensive area (Ikehara et al.
2011; Arai et al. 2013). Therefore, the 2011 Tohoku Earthquake probably impacted not only the shallow-water (e.g. Kanaya
et al. 2012; Seike et al. 2013) but also the deep-sea ecosystems. This study evaluated the effect of large-scale disturbance on
the deep-sea benthic assemblages inhabiting the landward slope of the Japan Trench using meiofauna, which is most abundantmetazoan in deep sea, especially on benthic copepods (harpacticoid copepods).
Sediment samples were collected on the landward slope of the Japan Trench off Sanriku during 3 cruises. Two cruises wereconducted from late July to early August 2011 (4.5 months after the 2011 Tohoku Earthquake); specifically the KT11-17 cruiseof the R.V. Tansei Maru and the YK11-E06 cruise of the R.V. Yokosuka. The third cruise, KT12-18 of the R.V. Tansei Maru, wasconducted in late July 2012 (1.5 years after the earthquake). Meiofaunal specimens were extracted from sediments and they werethen sorted into higher taxa and counted. We compared the total meiofaunal densities obtained from this study to those before
the earthquake in the study area (Shirayama & Kojima 1994). Benthic copepod (harpacticoid) specimens were further identified
at the genus level.
During the present study, it is indicated that major disturbances to deep-sea sediments mainly influenced the vertical distribution, but meiofaunal densities remained similar through the earthquake or quickly recovered within 4.5 months of the event.
In addition, harpacticoid community structure did not change in 1 year after the earthquake even between before and after theearthquake. These results suggest that the strong resilience of meiofauna inhabiting the landward slope of the Japan Trench,where high-magnitude earthquakes frequently occur, against disturbance. In the presentation, we will explain the results andbackground of this study in more detail.
Keywords: earthquake, disturbance, meiofauna, harpacticoid, community structure

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MIS33-15

Room:102B

Time:May 26 14:30-14:45

Impact of Tohoku Earthquake on Macrobenthic Fauna: Sanriku Waters
TAMURA, Mai1∗ ; JENKINS, Robert2 ; KITAHASHI, Tomo3 ; NOMAKI, Hidetaka4
1

School of Natural System, College of Science and Engineering, Kanazawa University, 2 School of Natural System, College of
Science and Engineering, Kanazawa University, 3 Atmosphere and Ocean Research Institute, The University of Tokyo, 4 Japan
Agency for Marine-Earth Science and Technology
Sanriku experienced the impacts of the Tohoku earthquake, Mw 9.0, and associated tsunami on March 11, 2011. This study
investigates the effect of the Tohoku earthquake on the macrobenthos of the Sanriku area (depth range: 120m to 5600m). Core
samples were taken 4.5 months after the earthquake struck. Turbidites were prominent in the core samples (Ikehara et al., 2014).
To begin with, the macrobenthos in the cores were examined. This was followed by a comparison of this macrobenthic faunawith those before the earthquake (Kojima and Ohta, 1989). The decreasing macrobenthos abundance with water depth at Sanriku
is a common observation around the globe (Rex et al., 2006). Post earthquake examinations of the Sanriku small macrobenthos(0.5mm to 1mm) show increased abundance in depths greater than 2000m. In addition, although the thickness of turbidites are
not related to the water depth, observations from the current study show that as the thickness of the layer increases the abundanceof macrobenthos decreases. There was an absence of large Nematoda (>1mm) in areas covered by more than 3cm thick turbidite
layers. While large Arthropoda and mollusks (>1mm) were absent from areas covered by 5cm thick turbidite layers. It is inferredthat these may have been transported by erosion or buried by the turbidities.

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MIS33-16

Room:102B

Time:May 26 14:45-15:00

Dynamics of the brittle star population in the continental slope off Sanriku, Northeast
Japan
YAMAKITA, Takehisa1∗ ; OGURI, Kazumasa1 ; YOKOOKA, Hiroyuki2 ; KEN, Ikehara3 ; FUJIWARA, Yoshihiro1 ;FURUSHIMA, Yasuo1 ; KASAYA, Takafumi1 ; KAWATO, Masaru1 ; TSUCHIDA, Shinji1 ; WATANABE, Shuichi1 ;YAMAMOTO, Hiroyuki1 ; KITAZATO, Hiroshi1
1Japan Agency for Marine-Earth Science and Technology(JAMSTEC), 2 IDEA Consultants, Inc., 3 National Institute of Advanced
Industrial Science and Technology (AIST)

Ophiuroidea are the dominant species at the sea bottom of the upper bathyal zone from 200 m to 600 m off the Sanriku region.
The region has been estimated to have 373 individuals, with 124 g per m2 (Fujita and Ohta, 1989). Brittle stars potentially play animportant role in submarine matter flow and in the food chain in this region. Because of their importance in ecosystem functions,brittle stars have gained attention, particularly for large-scale conservation planning and sustainable fishery (Yamakita et al., 2015a). To reveal the temporal dynamics of brittle stars in the Sanriku region, the number of brittle stars and their size composition
were observed. We used an interval video system for long-term sea-bottom observation platforms (lander systems) at 300- and1000-m-deep sites off Otsuchi Bay. Furthermore, we collected literature, pictures, and videos using a remotely operated vehicle(ROV) and conducted a geological survey to evaluate the spatial distribution of brittle stars before and after the earthquake.
Three ophiuroid species (Ophiura sarsii Lutken, 1855; Ophiura leptoctenia Clark, 1911; and Ophiophthalmus normani Lyman, 1879) were observed, and the dominant species differed with site. O. sarsii was the dominant species at the 300-m site, andO. normani dominated at the 1000-m site with apparently lower density.
At the 300-m study site, a trend of decreasing population number and increasing body size was observed. At the 1000-m study
site, a sporadic change in the body size and population number was observed despite the stable environmental conditions.
The growth rate of the shallow brittle star community in our study corresponded to that reported previously. Increase in turbidity and burial of organisms were the probable causes of the decrease in population number at the deeper site. Some of theturbidity was related to another medium-sized earthquake. Higher-resolution images were needed for the detection of the recruitment process and hidden environmental changes at the deeper site. Although there were no obvious changes in the distributionof species before and after the earthquake except for an increase in marine debris in the deep-sea valley (Yamakita et al. 2005 b),
the preliminary result showed a difference in the size histogram. Further analysis and collection of additional samples are needed
to examine these changes.

Fujita, T., & Ohta, S. (1989). Spatial structure within a dense bed of the brittle starOphiura sarsi (Ophiuroidea: Echinodermata)in the bathyal zone off Otsuchi, Northeastern Japan. Journal of the oceanographical Society of Japan, 45, 289-300.
Yamakita, T., Yamamoto, H., Nakaoka, M., Yamano, H., Fujikura, K., Hidaka, K., et al. & Shirayama, Y. (2015a). Identification of important marine areas around the Japanese Archipelago: Establishment of a protocol for evaluating a broad area using
ecologically and biologically significant areas selection criteria. Marine Policy, 51, 136-147.
Yamakita, T., Yamamoto, H., Yokoyama, Y., Sakamoto, I., Tsuchida, S., Lindsay, D., et al. & Kitazato, H. (2015b). Distribution of the marine debris on seafloor from the primary report of five cruises after the Great East Japan Earthquake 2011 IN:
Marine Productivity: Disturbance and Resilience of Coastal Socio-Ecosystems. Eds. Ceccaldi, H. et al. Springer in press

Keywords: Population dynamics, Spatial distribution, Ophiuroidea, The 2011 Great East Japan Earthquake, Long-term seabottom observation platforms lander

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MIS33-P01

Room:Convention Hall

Time:May 26 18:15-19:30

Extensive fault rupture reached Japan trench is landslide
NAKATA, Takashi1∗ ; GOTO, Hideaki2 ; TOKUYAMA, Hidekazu3
1

Professor Emeritus, Hiroshima University, 2 Hiroshima University, 3 Kochi University

Analysis on topographical change caused by the 2011 off the. Pacific coast of Tohoku Earthquake was carried out by using
anaglyph images based on multi-narrow beam data collected by JAMSTEC and Japan Coast Guard. Apparent and distinctive
change is not widely observed in spite of the large earthquake mainly because of rather coarse DEM grid (150 m ), except along
the seafloor of Japan trench between N 38.0 and N 38.2 where JAMSTEC claims based on topographical and structural changes
after the earthquake that displacement over 50 m of the earthquake source fault reached the trench axis. However, detailed
observation on anaglyph images reveals that such changes were caused by re-activation of pre-existing landslides located along
the toe of a gigantic slope failure located near the epicenter. Mounds appeared along the trench axis probably rotating mass oflandslide.

Keywords: Japan trench, submarine landslide, submarine active fault, 2011 Earthquake

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MIS33-P02

Room:Convention Hall

Time:May 26 18:15-19:30

Seabed topography and subbottom images from 200 m to 3,000 m in water depth, off
Miyagi and Iwate prefectures
NITTA, Sayaka3 ; KASAYA, Takafumi1 ; MIURA, Seiichi1∗ ; KAWAMURA, Kiichiro2
1

Yamaguchi University/Present:OYO, 2 JAMSTEC, 3 Yamaguchi University/JAMSTEC

The 2011 Tohoku earthquake of Mw 9.1 occurred on March 11, 2011 at ˜24 km in depth and ˜130 km southeast from Ojika
Peninsula. We have detected the recent dislocation and crustal movement due to the Tohoku earthquake using seafloor topography
investigation, earthquake exploration and ocean GPS system and so on (e.g., Sato et al., 2011). The dislocation and movements
should be recorded into sediment layers. We need to disclose recent tectonic activities in this area, but there are only a few study
examples being geologic structural studies in the sediment layers (e.g., Nitta et al., 2013; AGU abstract).
We analyzed in detail recent deformation structures around Tohoku area using mainly a subbottom profiler (following SBP)
system. The ?SBP provided sediment structures within ˜100 m below seafloor around off Miyagi and off Iwate. The water
depths were from 200 m to 3,000 m. Total SBP survey lines in this study was 101 lines. For the analyses, we also used multinarrow-beam (MBES) data to describe deformational seabed topography and single channel seismic data to describe large-scale
deformational structures within ˜1000 m below seafloor.
As a result, we disclosed below three points.
?1) From the MBES data, the many lineaments were observed in the south area from 38:45N. This is regarded as a creep
deformation.
2) From the SBP data analysis, we observed deformational structures having the cover layers of various thickness. The relative
active deformational structures having thin cover layers are located mainly in the south area of 38:05N being SE bulge.
?3) From the MCS data, we detected subsidence and uplifted areas. The uplifted areas correspond to the topography. In
addition, we observed several faults of 600 m long.
Based on the above-mentioned result, we concluded our study results as follows.
?1) Based on a geographic characteristics using the MBES and SBP data, we divided into three deformational domains (Domain A, B, C).?
2) Domain A is located ˜50 km SE from Ojika Peninsula. It is ˜300 km2. Judging from the cover layer thickness, We assume
that Domain A is continuously in active and also in active now.?
3) Domain B is located on the east of Domain A. It is distributed in ˜500 km2. This domain is similar to Domain A. A foldbelt exists between Domain A and B. This implies that Domain A moves faster speed than Domain B.?
4) Domain C is located on the north of Domain B and is distributed in more than ˜900 km2. Judging from the deformational
structures and thickness of the cover layers, this area is in active now, but it was stopped at a previous preriod.?
5) According to Arai et al. (2013), there are a long-term subsidence area due to tectonic erosion at the east edge of Domain B.
We assume that Domain B moves with the subsidence. Domain A would move to downslope with Domain B movement.

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MIS33-P03

Room:Convention Hall

Time:May 26 18:15-19:30

Stratigraphy of seismo-turbidite assisted by paleomagnetic secular variation in 2011 Tohokuoki earthquake rupture zone
KANAMATSU, Toshiya1∗ ; USAMI, Kazuko2 ; IKEHARA, Ken2 ; MCHUGH, Cecilia3
1

JAMSTEC, 2 AIST, 3 City University of New York

Turbidite sequences trapped in the lower slope terrace at 4000-6000 m water depth were collected for 300 kilometers along theJapan Trench in order to reconstruct earthquake occurrences of the Tohoku region Northern Japan. The major lithology obtainedin the cores is diatomaceous hemipelagic clay interbedded with turbidite layers. Hr-FP tephra patches/layers derived from the
Honshu arc during 500-600 years A.D. (Usami et al., 2014) were identified in the cores. We measured NRMs in 23 of the coresso far. The magnetizations are generally stable to A.F. demagnetizations. The variation of declination shows a systematic shift
within 60 degrees. A comparison of the data to references, which are archeomagnetic and sediment paleomagnetic data during
the past 3000 years, show a good agreement of the data to the references. The secular variations of the cores hopefully willcontribute to date the seismo-turbidite stratigraphy.
Keywords: The 2011 off the Pacific coast of Tohoku Earthquake, Japan Trench, turbidite, paleomagnetic secular variation

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MIS33-P04

Room:Convention Hall

Time:May 26 18:15-19:30

Possibility for the occurrence of tsunami-generated turbidity currents: Insights from the2011 Tohoku-Oki Earthquake
ARAI, Kazuno1∗ ; NARUSE, Hajime2 ; YOKOKAWA, Miwa3 ; IRINO, Tomohisa4 ; IKEHARA, Ken5 ; SAITOH, Yu6 ;HAYASHIDA, Akira7 ; KANAMATSU, Toshiya8
1
7

Saitama University, 2 Kyoto University, 3 Osaka Institute of Technology, 4 Hokkaido University, 5 AIST, 6 Kochi University,
Doshisha University, 8 JAMSTEC

In this study, characteristics of the turbidites and turbidity currents associated with the 2011 Tohoku-Oki Earthquake andTsunami were investigated. As a result, this study proposes a hypothesis suggesting that the large-scale tsunami can generateturbidity currents in deep sea. This hypothesis was verified by the numerical experiments of tsunamis and turbidity currents. The
result of this research indicates that understanding of the initiation mechanism and behavior of the tsunami-generated turbidity
currents are important to reconstruct paleo-events and paleo-environment change such as surface disturbance associated withearthquakes and tsunamis events.
The 2011 Tohoku-Oki Earthquake and Tsunami occurred at 5:46 (UTC) on March 11, 2011 off Tohoku region, Japan. At about
3 hours after the main shock of the Tohoku-Oki Earthquake, the sensors on the seafloor recorded that the anomalous event occurred (Arai et al., 2013). Subsurface sediment cores were collected at 16 sites over range of water depth 170-2000 m, and event
deposits (newly emplaced sediment layers) were observed identified obviously at the top of 14 core samples. Sedimentologicalanalysis of these layers implies that the event deposits can be interpreted as turbidites, and it is suggested that this anomalous
event was affected by the turbidity current run from shallower regions.
Because of the absence of related submarine landslides in the shallow marine area, it is reasonable to consider that the turbidity
current was developed from the tsunami itself (Arai et al., 2013). It is hypothesized that the suspension cloud was stirred up by
the tsunami at shallower depths and it grew into the turbidity current via the self-accelerating process. Both the condition (flowvelocity and distribution of event deposits) of turbidity currents and turbidites estimated from the observation and results of numerical simulations of the unsteady turbidity current were quite conformable to this hypothesis. The numerical experiments ofturbidity currents suggested that the tsunami-generated turbidity currents can occur when seafloor sediment in shallow marine iseroded at least 1.4 cm in thickness (in case of the porosity 50%) by the tsunami. The numerical experiments of tsunamis (using
iRIC-ELIMO) indicates that the Tohoku-Oki Tsunami can eroded substrate 1-2 cm in thickness off Miyagi Prefecture, suggestingthat the tsunami erosion may exceed the requirement to develop the tsunami-generated turbidity currents. Thus, it is concluded
that tsunamis that have a similar scale to the Tohoku-Oki Tsunami potentially produce turbidity currents.

Keywords: Tohoku-Oki earthquake, tsunami-generated turbidity current, event deposit

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MIS33-P05

Room:Convention Hall

Time:May 26 18:15-19:30

A 100-year stratum record and a 2011 Tohoku-Oki event record in the Japan Trench
FUJII, Minami2 ; KAWAMURA, Kiichiro1∗ ; OGURI, Kazumasa3 ; TOYOFUKU, Takashi3 ; KANAMATSU, Toshiya3 ;MURAYAMA, Masafumi4 ; ARAI, Kazuno5
1

Yamaguchi University/Present:Aratani CEC., 2 Yamaguchi University/JAMSTEC, 3 JAMSTEC, 4 Kochi University, 5 Saitama
University
Introduction
In this study, we described detailed sedimentary structures in the core samples which was collected from a shelf, shelf break,gentle slope and deep-sea fan off Hachinohe, a bottom of the canyon off Sanriku, and a gentle slope off Sendai after the TohokuOki earthquake. We observed a sandy sediment layer due to the 2011 event at the surface layer. To estimate the depositional age
in the sediment layer, we analyzed 210Pb and 137Cs being radionuclides.
A depositional age can be typically estimated using a profile of 210Pbex calculated from various radionuclide analyses. 137Cs
was dispersed into the atmosphere and settled into sediments after the nuclear weapons testing, so that the presence of 137Cs
means that the sediment was deposited within 60 years.
Based on these radionuclide analyses, we disclosed a disturbance on the seafloor and in the sediments by the 2011 event.
Additionally, we revealed steady and calm sedimentation processes within about 100 years throughout the core sediments. Thus,
we deciphered an abrupt [short-term sedimentation] being an event deposition and a steady and calm [long-term stratum records]
being a background deposition within about 100 years in the Tohoku area.
Samples
The sediments off Hachinohe were collected using a multiple core system during the cruise KT-11-20 by R/V Tansei-Maru(JAMSTEC) in August 2011. The sediments off Sendai and off Sanriku were collected using a MBARI core on the ROV HyperDorphin during the cruise NT12-12 by R/V Natsushima (JAMSTEC) in May 2012. These sampling methods are possible tocollect sediments at the seafloor without any disturbances.
Results
As a result we summarized the [short-term sedimentation] as below.
1. At the shelf and shelf break off Hachinohe, there are coarse sandy sediment layers. Based on the 210Pb profiles and sedimentary structures, we concluded this sandy layer rapidly deposited due to the 2011 tsunami. However, we could not observe
any coarse-grained sediment layers at the gentle slope and deep-sea fan.
2. At the gentle slope off Sendai, we observed a sandy sediment layer of 4 cm thick that was formed by a current from west toeast; from landward to seaward at the top. The base of the layer is unconformity. We concluded that this sediment layer would
be the 2011 event deposit.
3. At the bottom of the canyon off Sanriku, there is a sand layer that was likely formed rapidly at the top based on 210Pb
profiles.
We summarized [long-term stratum records] as below.
1. At the gentle slope and deep-sea fan off Hachinohe, the sediment is deposited and consolidated gradually with burial. We
can not extract any paleocurrent directions because of probably heavy bioturbation.
2. At off Sendai, we detected a paleocurrent from northeast to southwest throughout the core sediments. This might be a
predominant bottom current within ˜100 years in this area.
3. At off Sanriku, we extracted a paleocurrent from southeast to northwest. Based on 210Pb profiles, the depositional rates
were 0.061 cm/yr (0.068˜0.072 g/cm2/yr) at 5˜13 cm below seafloor, 0.109˜0.166 cm/yr (0.045˜0.076 g/cm2/yr).
Concluding remarks
We observed a deep-sea [off Sanriku] 2011 event layer which was also ever reported off Sendai. This layer was only observedat the shelf area off Hachinohe. Thus, our study disclosed in detail depositional area of the 2011 event layer. The distribution,
depositional processes and preservation processes of this layer should be a clue to understanding for the [deep-sea] paleoseismological study. We need more information for this new deep-sea science. The study continues.

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MIS33-P06

Room:Convention Hall

Time:May 26 18:15-19:30

Temporal change of the 2011 Tohoku-oki earthquake- and tsunami-related event beds at
off Sanriku forearc region
IKEHARA, Ken1∗ ; USAMI, Kazuko1
1

Geological Survey of Japan, AIST

Wide distribution of the 2011 Tohoku-oki earthquake- and tsunami-related submarine event deposits has been reported. Some
event beds were formed by the repeated generation of turbidity currents with its interval of more than a few - a few tens days.
These facts indicate the formation of the 2011 event deposits was occurred in wide range both in spatially and temporally. Largefriction velocity of the tsunami waves might contribute to generate sediment resuspension and redeposition at shallow waters,
and strong ground motion of the earthquake might affect the sediment remobilization in deep waters. Radiological measurementsof the event deposits suggest the remobilization of surface sediments. However, we still do not know exact image what happenedby the 2011 Tohoku-oki earthquake and its related tsunami in the entire off Sanriku region. To clarify the recurrence of thegreat earthquakes from marine sediment records, evaluation of preservation potential of the event deposits is essential. Repeatedexamination of sedimentary structures of the event deposits indicates that high sedimentation rate and low benthos activities areimportant factor for the preservation. A terrace at the lower slope and the Japan Trench floor, where has high sedimentation
rates and low benthos activities, and sediments at which contains many fine-grained turbidites, is a potential area for the turbidite
paleoseismology along the northern Japan Trench.
Keywords: event deposit, marine sediment, temporal change, 2011 Tohoku-oki earthquake and tsunami

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MIS33-P07

Room:Convention Hall

Time:May 26 18:15-19:30

Effects of mass sedimentation event after the 2011 Tohoku earthquake on benthic organisms in the upper bathyal sediments
NOMAKI, Hidetaka1∗ ; MOCHIZUKI, Tomohiro2 ; KITAHASHI, Tomo3 ; NUNOURA, Takuro1 ; ARAI, Kazuno4 ;TOYOFUKU, Takashi1 ; SUGA, Hisami1 ; WAKITA, Masahide1 ; TANAKA, Gengo5 ; SHIGENO, Shuichi1 ;TASUMI, Eiji1 ; FUJIKURA, Katsunori1 ; WATANABE, Shuichi1
1

JAMSTEC, 2 Tokyo Institute of Technology, 3 The University of Tokyo, 4 Saitama University, 5 Kumamoto University

We examined the effects of mass sedimentation events caused by the 2011 off the Pacific coast of Tohoku earthquake onabundances and vertical distributions of prokaryotes and metazoan meiofauna in sediments, using sediment cores collected fromeight bathyal stations off Tohoku 1 and 2.5 years after the M9.0 earthquake. Event deposits of 1 to 7 cm thick were observed at
the topmost part of the sediment cores at all sampling stations. At some stations, prokaryotic cell abundances were lower in thesurface event-deposit layers compared to those in deeper sediments. These variations were explained by environmental parameters such as a sorting factor and mean grain size, suggesting that turbidite sedimentation affected prokaryotic cell abundances.
Nematodes had anomalously higher subsurface abundances at the stations where subsurface peak prokaryotic cell numbers wereobserved. Although there are no corresponding data before the earthquake from the same sites, it is likely that the subsurface
peaks in prokaryotic cell numbers and meiofaunal density resulted from the sedimentation events. The effects of sedimentation
events on the organisms were observed 2.5 years after the earthquake, indicating that episodic sedimentation events on scales ofseveral centimeters have a large effect on small organisms inhabiting sediments.
Keywords: Meiobenthos, Sedimentary microbe, the 2011 off the Pacific coast of Tohoku earthquake, Vertical distribution

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MIS33-P08

Room:Convention Hall

Time:May 26 18:15-19:30

A secular variation of sub-bottom environment after the 3.11 Tohoku Earthquake andTsunami disaster around Hirota-Bay
ARAKAWA, Takuya1∗ ; SAKAMOTO, Izumi1 ; YOKOYAMA, Yuka1 ; YAGI, Masatoshi1 ; IIJIMA, Satsuki1 ;INOUE, Tomohito1 ; NEMOTO, Kenji1 ; FUJIMAKI, Mikio2
1

School of marine science and technology, Tokai university, 2 Costal Ocean Resech

On March 11, 2011, Tohoku Earthquake and Tsunami disaster were generated and the Tohoku district Pacific side sufferedserious damage over a wide area. It intends that I clarify a change of sub-bottom environment and I predict it in the future at
Hirota bay.
The sample which was gathered near the front of Kesen River in September, 2013 change from the gravel sediment to the mud
sediment. As for this change, the influence of the typhoon and the heavy rain are thought about. The sediment from the mouthof Kesen River will be changed by the weather condition.
The strong reflector of SSS data which is distributed over the front of Kesen River is a tendency to decrease from 2013 through2014. On the other hand, outer layer sediments which is distributed over the front of Kesen River change from the fine sediment
to the corse sediment. And, a distribution range of the mud sediment spreads out in the east side of the Osabe fishing port. Thus,
it is thought that the mud is carried to the offshore and the bay central part such as river water. Sub-bottom environment of thecenter part from Hirota bay will change to the mud sediment in the future.

Keywords: Tsunami, Sediment, Environmennt

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MIS33-P09

Room:Convention Hall

Time:May 26 18:15-19:30

Characteristic of palaeoenvironment based on the diatom assembles of the core drilledfrom Hirota bay, Iwate, JapanINOUE, Tomohito1∗ ; SAKAMOTO, Izumi1 ; YOKOYAMA, Yuka1 ; YAGI, Masatoshi1 ; IIJIMA, Satsuki1 ;MATSUZAWA, Hiroyuki1 ; SAGAYAMA, Tsumoru2
1

Tokai university, 2 Geol.Survey Hokkaido,HRO

Hirota bay located in Rikuzen takada off the coast is flowed Kesen river from southeastern . The recent 2011Tohoku tsunami
strongly affected the coastal area of the Pacific coast of Tohoku. We will show about characteristic of diatom assembles of thecolumnar core sampled from Hirota bay.
Sakamoto et al(2014) estimated that sandy sediments from 0-71cm is 2011tsunami origin sediments (Unit1) , muddy sedimentsfrom 71-143cm is normal sediments in this bay (Unit2) on the 13HV3 core .Results of diatom analysis of the 13HV3 core arethat brackish˜freshwater species from 0-71cm dominante but brackish-marine species from 75-143cm dominante .
On the 13HV8 core which sampled near the Kesen river , brackish˜freshwater species from 0-1cm dominate but brackishmarine species from 42-160cm dominate(Sagayama et al,2014).
Normal sediments on 13HV3 shows marine environment but normal sediments on 13HV8 shows brackish environment. It
suggests that the difference of distance from river front and seawater environments (seawater/freshwater ratio) on sampling
points.

Keywords: diatom, tsunami, deposit

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