Wakita et al.
Progress in Earth and Planetary Science(2022) 9:56
https://doi.org/10.1186/s40645-022-00515-1

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

Deep‑sea bottom‑water environmentchange caused by sediment resuspension
on the continental slope off Sanriku, Japan,
before and after the 2011 Tohoku Earthquake
Masahide Wakita1* , Shuichi Watanabe1, Jun Yoshino2, Kazumasa Oguri3,4, Hidetaka Nomaki5,
Shinsuke Kawagucci3, Keisuke Ariyoshi6, Akira Nagano7 and Katsunori Fujikura3

Abstract:
The 2011 off the Pacific coast of Tohoku Earthquake occurred off southern Sanriku, Japan, on March 11 and generated strong shaking and huge tsunami along the entire eastern coast of Tohoku. The mainshock and numerous
large aftershocks caused mass-wasting sedimentation events on the continental shelf, slope, and trench bottom.
To investigate the impacts of the 2011 Tohoku Earthquake on the deep-sea bottom-water environment off Sanriku,
we conducted shipboard surveys up to ~ 2000 dbar during 2011–2018 and long-term monitoring of the seafloor on
the continental slope using a deep-sea station (~ 1000 dbar) off Otsuchi Bay during 2012–2018. The high turbidity(maximum ~ 6%) was observed for the bottom water deeper than 500 dbar on the continental slope of the entire area
off Sanriku during 2012–2018. This high turbidity was caused by sporadic sediment resuspension induced by frequent
large aftershocks. Furthermore, dissolved oxygen concentrations in the bottom layer from 1000 to 1500 dbar dropped
significantly by about 10% after the earthquake, while nutrients and dissolved inorganic carbon showed no significant
changes but exhibited wide variations. The high turbidity was associated with the increase in the concentrations ofphosphate, dissolved inorganic carbon, and methane, as well as the decrease in those of dissolved oxygen and nitrate.
This suggests that remineralization of suspended organic matter resulting from the respiration and denitrificationof microbial communities after the earthquake caused the chemical properties of the deep-sea bottom-water. The
deep-sea bottom-water environment change was maintained by sporadic sediment resuspension due to continued
large aftershocks and was likely caused by variations in dissolved inorganic carbon and phosphate. There are two
peaks in the concentration and carbon isotope ratio of methane on the deeper slope from 1000 to 2000 dbar near
the hypocenter, which were advected along isopycnal surfaces of 27.38σθ (1000 dbar) and 27.56σθ (1500 dbar). The
source of the shallower peak of chemical input is considered to be the sediment resuspension from the shallow sediment on the continental slope induced by the mainshock and large aftershocks.
Keywords: The 2011 off the Pacific coast of Tohoku Earthquake, Deep-sea bottom-water environment, Continental
slope, Turbidity, Sediment resuspension, Dissolved oxygen, Dissolved inorganic carbon, Methane*Correspondence: mwakita@jamstec.go.jp
1
Mutsu Institute for Oceanography (MIO), Research Institute for Global
Change (RIGC), Japan Agency for Marine-Earth Science and Technology, 690
Kitasekine, Sekine, Mutsu, Aomori 035‑0022, Japan
Full list of author information is available at the end of the article© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Wakita et al. Progress in Earth and Planetary Science(2022) 9:56

1 Introduction
The 2011 off the Pacific coast of Tohoku Earthquakewith a moment magnitude ­(Mw) of 9.0 (hereafter “the2011 Tohoku Earthquake”) occurred on March 11, 2011,and generated strong shaking and huge tsunami along
the entire eastern coast of the Tohoku area (e.g., Hirose
et al. 2011). On this same date, many large aftershocks
occurred (Fig. 1a), including a ­Mw 7.5 event off Sanriku
and a M­ w 7.7 event off Ibaraki Prefecture (e.g., Hirose
et al. 2011). Aftershocks have been active around thesource region of the mainshock since its occurrence, andthe two largest aftershocks ­(Mw ≥ 7) define the northernand southern limits of the aftershock region (Fig. 1a)(e.g., Uchida and Bürgmann 2021). The annual meanearthquake frequency since the 2011 Tohoku Earthquake­(Mw ≥ 5) has been higher than that during 2001–2010
within the aftershock area of Tohoku (Japan Meteorological Agency (JMA) 2022).
The mainshock together with the numerous large aftershocks triggered mass-wasting sedimentation events
associated with dense turbidity on the continental shelf,
slope, and trench bottom off Sanriku (e.g., Kawagucci
et al. 2012; Noguchi et al. 2012; Arai et al. 2013; Oguri
et al. 2013, 2016; Nomaki et al. 2016). This mass-wasting sedimentation events associated with dense turbidity impacted deep-sea bottom-water environment. For
example, a disturbance in the deep-sea bottom-water
environment below 2000 dbar depth has been detected in
the form of turbulent diffusion of sediments and anomalous increases in the concentrations of manganese, methane, and helium-3, and increases in planktonic microbialcommunities near the hypocenter 36 days after themainshock (Kawagucci et al. 2012; Sano et al. 2014). In
other regions, just after large earthquakes, dense turbidity and high manganese concentration anomaly were also
observed in the bottom water of the Sagami Bay (Kasaya
et al. 2009; Gamo et al. 2007), off Tokachi (Mikada et al.
2006), and off Kii Peninsula (Ashi et al. 2014). Moreover, large aftershocks of the 2011 Tohoku Earthquake
on December 7, 2012, and February 17, 2015, increased
turbidity through sediment resuspension in the bottom water at ~ 1000 m depth, though this did not affectbenthic habitats (e.g., ophiuroids, sea cucumbers, gastropods, deep-sea eels, and rockfish Sebastolobus macrochir), as observed during 2012–2016 at the deep-sea
station (~ 1000 m) off Otsuchi Bay (Oguri et al. 2016,2018). These results indicate deep-sea bottom-water
environments in some areas changed as a result of the2011 Tohoku Earthquake and aftershocks. However,these observed changes have not been fully comparedwith pre-earthquake bottom-water conditions and variability because of the lack of pre-earthquake data, andPage 2 of 16

such a comparison is required to properly assess theimpact of the 2011 earthquake and aftershocks on the
deep-sea bottom-water environment.
Before the 2011 Tohoku Earthquake, the Japan
Meteorological Agency (JMA) and Japan Agency for
Marine-Earth Science and Technology (JAMSTEC)
collected physical and biogeochemical data over theentire continental slope area off Sanriku and the abyssal plain ocean floor of the western North Pacific Ocean(Fig. 1). After the 2011 Tohoku Earthquake, to examine the impacts of the earthquake and aftershocks on
deep-sea bottom-water environment change, we conducted surveys onboard research vessels over the continental slope area off Sanriku during 2012–2018 under
the Tohoku Ecosystems-Associated Marine Sciences(TEAMS) Project. Using these two sets of data andlong-term monitoring data at the deep-sea station off
Otsuchi Bay, we compared the deep-sea bottom water
before and after the 2011 Tohoku Earthquake andaftershocks.

2 Data and method
2.1 Observations off the Pacific coast of Sanriku

After the 2011 Tohoku Earthquake, we performed shipbased hydrographic observations in the entire continental slope area off Sanriku onboard the research vessels
Mirai, Shinsei-Maru, and Hakuho-Maru of JAMSTEC
and the training ships Oshoro-Maru of Hokkaido University and Bosei-Maru of Tokai University (Fig. 1 andTable 1). We conducted vertical hydrocasts using a conductivity–temperature–depth (CTD) profiler with a carousel multiple sampling system (CTD–CMS) from thesea surface to just 10 m above the seafloor (up to 2000
dbar depth). The CTD–CMS system consisted of a CTD
SBE 911plus (Sea-Bird Scientific), a light transmissometer(C-star 25 cm light-path type, WET Lab), and Niskin-X
bottles (12 L type, General Oceanics). Light transmission
anomaly (LTA) values were calculated from the difference
between the in situ light transmission value and the maximum value of the transparent layer in deep water during
each cruise and were interpreted as representing deepsea water turbidity (Kawagucci et al. 2012). We collectedwater samples in Niskin bottles for measurements of dissolved oxygen (DO), nutrients, salinity, dissolved inorganic carbon (DIC), and methane. During 2011–2018,
we performed observations at a total of 166 stations on
14 cruises (Table 1). These data will be publicly available
from the JAMSTEC Web site, Tohoku Ecosystem-Associated Marine Sciences (TEAMS) Data Catalog (https://​www.​jamst​e c.​go.​jp/j/​about/​infor​matio​ns/​notif​icati​on_​
2021_​maint​enance.​html?​lang=​en).

Wakita et al. Progress in Earth and Planetary Science(2022) 9:56

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Fig. 1 Location map and bathymetry (a), and time series as a function of latitude of location (b) of sampling stations in this study. (c) Location mapof our sampling stations (Table 1) encircled by the square in panel (a). The hypocenter of the 2011 Tohoku Earthquake is shown by the red star, andthe hypocenters of large aftershocks ­(Mw ≥ 7) until 2019 in the northern and southern limits of the aftershock region and the earthquakes for the
Ev1 and Ev2 are shown by yellow and purple stars, respectively. Ship-based hydrographic observations were performed onboard research vessels[Mirai (aqua-blue circles), Shinsei-Maru (red asterisks), and Hakuho-Maru (green circles) of JAMSTEC] and Japanese training ships [Oshoro-Maru (pinkcircles) of Hokkaido University and Bosei-Maru (blue circles) of Tokai University], and by the JMA before (yellow squares) and after (pink squares) the2011 Tohoku Earthquake. The open orange triangle denotes the deep-sea station (~ 1000 m depth) used for long-term monitoring of the seafloor
on the continental slope (Oguri et al. 2016, 2018). Bathymetric contours show 200 m depth (solid gray line), 1000 m depth (solid black line), 2000 m
depth (dashed black line), and 3000 m depth (dotted black line). The color bar shows the depth of bathymetry

DO was measured with automatic photometric titrators (DOT-01X, Kimoto Electric, during the Mirai and
Bosei-Maru cruises; and DOT-05, Kimoto Electric, during the Shinsei-Maru cruise). According to results for
replicate samples, the precision of DO measurements wasless than ± 0.2 µmol ­kg−1. Apparent oxygen utilization(AOU) was determined by subtracting the observed DO
concentration from the saturated concentration calculated from temperature and salinity using the formulaby Weiss (1970). Nutrients (nitrate ­(NO3), phosphate­(PO4), and silicate) and salinity were measured with acontinuous-flow analyzer (QuAAtro, BL TEC K.K.) and a

Wakita et al. Progress in Earth and Planetary Science(2022) 9:56

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the Sanriku region (Oguri et al. 2016, 2018) (Fig. 1). CTD
and DO data were obtained from CTD and DO sensors(RDCP 600 with 4050, 4019B, 3830, and 4017D; Xylem)
and calibrated against salinity and DO data measured
onboard for seawater samples obtained in Niskin bottles at 1000 m depth during cruises BO13-20, KS-15-15,
KS-16-2, and KS-17-J08C (Oguri et al. 2016).
To compare the deep-sea bottom-water environment
before and after the 2011 Tohoku Earthquake over theentire area off Sanriku, we used data for CTD, DO, nutrients, and DIC collected during 2005–2018 off the Pacificcoast of Sanriku and along the international repeat
hydrography program P10 section in the abyssal plainof the Pacific plate by the JMA (http://​www.​data.​jma.​go.​jp/​g md/​k aiyou/​db/​vessel_​obs/​d ata-​report/​html/​ship/​
ship_e.​php) (yellow and pink squares in Fig. 1) and during the 2005, 2012, and 2014 Mirai cruises (https://​www.​ncei.​noaa.​gov/​access/​ocean-​carbon-​data-​system/​oceans/​
Repea​tSect​ions/​clivar_​p10.​html).

salinometer (Model 8400B AUTOSAL, Guildline Instruments), respectively. Values of nutrients were calibrated
against the certified reference material (RM) provided
by KANSO TECHNOS, and those of salinity against theInternational Association for the Physical Sciences of the
Ocean (IAPSO) standard seawater provided by Ocean
Scientific International. The precisions for N­ O3, ­PO4,
silicate, and salinity were within ± 0.1 µmol ­kg−1, ± 0.01
µmol ­kg−1, ± 0.2 µmol ­kg−1, and ± 0.001 based on replicate sample determinations, respectively. We measured
DIC with coulometers (CM5012, UIC; MODEL 3000A,
Nippon ANS) (Wakita et al. 2021). DIC values were calibrated against C­ O2 in seawater RM produced by KANSOTECHNOS. This RM value was determined by measuring
DIC relative to the certified RM provided by Prof. A. G.
Dickson (Scripps Institution of Oceanography, Universityof California San Diego). The precision of DIC measurements was within ± 0.8 µmol ­kg−1, based on replicate
sample determinations. Concentrations and carbon isotope ratios of methane were determined simultaneouslywith a combination of purge and trap techniques andcontinuous-flow isotope ratio mass spectrometry (Kawagucci et al. 2018). We present the stable carbon isotope
ratio in delta notation on a per mil scale with respect toVienna Pee Dee Belemnite (VPDB). To perform isopycnal
analysis, we linearly interpolated the values of LTA, DO,nutrients, DIC, and methane onto two selected isopycnal
surfaces (27.38σθ and 27.56σθ).
In addition, to understand the variation in seafloor
environment following aftershocks, we analyzed CTD
and DO data for the period 2012–2018 from the deepsea station deployed at ~ 1000 m depth for long-termmonitoring on the continental slope off Otsuchi Bay in

2.2 Remineralization of organic matter near the seafloor
on the continental slope

To investigate the impacts of disturbances of sediments
caused by the 2011 Tohoku Earthquake and aftershocks
on the deep-sea bottom-water environment, we estimated the remineralization of organic matter by oxygen
respiration and denitrification of microbial communities
near the seafloor on the continental slope. When a watermass on the slope moves along an isopycnal surface, theconcentrations of DIC, nutrients, and AOU increase, andDO decreases owing to the remineralization of organic
matter by respiration, and N­ O3 is consumed by denitrification in the anaerobic sediments. Denitrification

Table 1 Our ship-based hydrographic observations and water sample measurements conducted after the 2011 Tohoku Earthquake
CruiseShip

PeriodMR11-03

Mirai

Apr. 2011

MR11-05

Mirai

Jul. 2011

MR12-E02

Mirai

Mar. 2012

MR12-02

Mirai

Jul. 2012

OS252

Oshoro-MaruMar. 2013

MR13-04

Mirai

Jul. 2013

BO13-20

Bosei-Maru

Nov. 2013

KS-14-3

Shinsei-MaruMar. 2014

KS-15-13

Shinsei-Maru

Oct. 2015

KS-15-15

Shinsei-Maru

Nov. 2015

KS-16-2

Shinsei-MaruMar. 2016

KS-17-J05

Shinsei-MaruMar. 2017

KS-17-J08C

Shinsei-Maru

Jun. 2017

KH-18-J03C

Hakuho-MaruAug. 2018

CTD

LTA

DO

Nutrients

DIC

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CH4

Δ13CH4

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Wakita et al. Progress in Earth and Planetary Science(2022) 9:56

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remineralizes carbon with a very different stoichiometric ratio compared with nitrogen from respiration. The
stoichiometric ratios of the remineralization by respiration and denitrification have been described by Anderson(1995) and Gruber and Sarmiento (1997) as follows:
C106 H175 O42 N16 P + 150 O2 =106 CO2 + 16 HNO3
+ H3 PO4 + 78 H2 O, and(1)

C106 H175 O42 N16 P + 104 HNO3 =106 CO2 + 60 N2+ H3 PO4 + 138 H2 O.

(2)The amount of consumed N­ O3 can be estimated by the
relationship between measured N­ O3 ­(NO3m) and P­O4­(PO4m) (Mintrop et al. 1999). By using ­NO3m and ­PO4m,
apparent ­NO3 utilization (ANU) is defined as(3)

ANU = 16 PO4m − NO3m .

Thus, we estimated remineralization-corrected DIC and­PO4 in the subsurface water near the seafloor on the slope,as follows:Remineralization − corrected DIC = DICm − 117/170 AOU− 106/104 ANU, and(4)Remineralization − corrected PO4 = PO4m − 1/170 AOU− 1/104 ANU,

(5)
where the coefficients 117/170 and 1/170, and 106/104
and 1/104 are the stoichiometric ratios of remineralization of organic matter by respiration (Anderson andSarmiento 1994) and denitrification (Gruber and Sarmiento 1997), and ­DICm is the DIC measured in the oceaninterior.

3 Results and discussion
3.1 Hydrographic features on the continental slope
off Sanriku

Figure 2 shows the sections of LTA-derived turbidity measured during 2012–2017 (i.e., after the 2011 Tohoku Earthquake). Turbidity was higher in the upper water between
depths corresponding to 0 and 500 dbar owing to the highbiomass abundance relative to the deep water. The upper
water (shallower than 26.7σθ) is influenced by the Tsugaru
Warm Current, the Oyashio, and the Kuroshio proceeded
to the region off Sanriku (Fig. 3) and exhibits complicatedwater mass structures (e.g., Hanawa and Mitsudera 1986)
and high primary production and chlorophyll a (e.g., Ishizaka 1998). The intermediate and deep layers below a depthof the 26.7σθ isopycnal are occupied by the low-salinity
North Pacific Intermediate Water (NPIW) (26.6–27.4σθ)(e.g., Talley et al. 1995) and the high-salinity Circumpolar

Deep Water transported from the Antarctic Circumpolar
region (Figs. 2 and 3).
Although potential temperature, salinity, DO, AOU,
­NO3, ­PO4, and DIC became less variable with increasing
depth, as did potential density, particularly below ~ 500
dbar (~ 26.8σθ), LTA became more variable with increasing depth (Fig. 4). Values of LTA were high and variable inthe bottom water deeper than 500 dbar on the continental slope off Ohtsuchi Bay, Onagawa Bay, and Sendai Bay(Fig. 2). These values were higher than those observed
near the bottom off Ohtsuchi Bay in 2017, Onagawa Bay in2017, and Sendai Bay in 2012. The maximum LTA intensity
in the deep-sea bottom water (~ 6%) shown in 2014 (Fig. 4)
was lower than the LTA of ~ 36% measured below 2000
dbar on the MR11-02 cruise 36 days after the 2011 Tohoku
Earthquake (Kawagucci et al. 2012). Moreover, the large
aftershocks of the 2011 Tohoku Earthquake on December 7, 2012, and February 17, 2015, increased turbidity inthe bottom water through sediment resuspension from a
HDTV camera at the deep-sea station (~ 1000 m depth) off
Otsuchi Bay (Fig. 1; Oguri et al. 2016, 2018). The bottom
water returned to the pre-earthquake state, 10 days after
the large aftershocks (Oguri et al. 2016, 2018). This suggeststhat the resuspended sediments resettled near the originalsite or a bottom current flowing southward to southwestward in this area (2.5–4.5 cm/s; Itoh and Sugimoto 2002)
flushed or diluted the turbid bottom water. Turbidity in the
deep-sea bottom water near the seafloor has been observed
to increase as a result of the turbulent resuspension of sediments by the 2011 Tohoku Earthquake and its aftershocks(e.g., Kawagucci et al. 2012; Oguri et al. 2016). Because
dense turbidity and high manganese concentration anomaly was observed in the bottom water of the Sagami Bay
after large earthquake (­ Mw = 5.4) (Kasaya et al. 2009; Gamo
et al. 2007), the annual mean earthquake frequency sincethe 2011 Tohoku Earthquake ­(Mw ≥ 5) has been higher
within the aftershock area of Tohoku (JMA 2022). Accordingly, sporadic enhancement in LTA was widely detectedin the deep-sea bottom water on the continental slope off
Sanriku during 2011 − 2018 (Figs. 2 and 4d). This sporadic
enhancement in LTA would be caused by sediment resuspension induced by the 2011 Tohoku Earthquake and frequent large aftershocks.
3.2 Deep‑sea bottom‑water conditions before and after
the 2011 Tohoku Earthquake

As physical and chemical disturbances in seawater in
conjunction with the disturbance of surface sediments
triggered by earthquakes have been observed previously (e.g., Gamo et al. 2007; Kawagucci et al. 2012),
we compared the deep-sea bottom-water environment between 1000 and 1500 dbar before and after theWakita et al. Progress in Earth and Planetary Science(2022) 9:56

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Fig. 2 Vertical distributions of light transmissometer anomaly (LTA) values measured during cruises MR12-E02 (a, e, and i), BO13-20 (b, f, and j),
KS-15–15 (c, g, and k), and KS-17-J05 (d and h) off Ohtsuchi Bay, Onagawa Bay, and Sendai Bay (l). Contours of LTA are every 1%

Wakita et al. Progress in Earth and Planetary Science(2022) 9:56

Fig. 3 Potential temperature–salinity diagram for our ship-basedhydrographic observations. Contours show isopycnal surfaces.
Colored circles denote the latitude of sampling locations

2011 Tohoku Earthquake on the two isopycnal surfaces(27.38σθ and 27.56σθ) (Fig. 5). The deep water below
1000 dbar (~ 27.38σθ), comprising a lower NPIW, showedno significant natural variation on a decadal timescale
between 1993 and 2005 at the P10 section from 24 to
30°N (Murata et al. 2009). The mean values of potential temperature and salinity off Sanriku are statisticallyconsistent before and after the 2011 Tohoku Earthquake(Fig. 5a and b) and also consistent with those along theP10 section in 2005, 2012, and 2014 (green triangles inFig. 5a and b). These mean values on the 27.38σθ surface after the 2011 Tohoku Earthquake (bold red lines inFig. 5a and b) show no statistically significant difference
to those measured at the deep-sea station (orange lines
in Fig. 5a and b). The potential temperature and salinity on the 27.38σθ surface were more variable than thoseon the 27.56σθ surface. Thus, the deep-sea bottom watermass on the continental slope was similar to that along
the P10 section and showed no significant decadal variation between 2004 and 2018. This deep-sea bottom watermass appears to have been unaffected by the physical disturbance of sediments after the 2011 Tohoku Earthquake.
Our comparison of the deep-sea bottom-water conditions
before and after the 2011 Tohoku Earthquake showed thatthe mean values of DO on the isopycnal surfaces (27.38σθ
and 27.56σθ) between 1000 and 1500 dbar decreased significantly (with an increase in AOU) by ~ 5 μmol/kg (p < 0.001

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based on t test), whereas nutrients exhibited no significant
differences and wide scatter (Fig. 5c–g). The mean value of
DO on the slope was significantly lower (higher for AOU)
than that along the P10 section (green triangles in Fig. 5cand d), whereas nutrients values showed no significant differences from those along the P10 section (green triangles inFig. 5e–g). After the 2011 Tohoku Earthquake, values of DO
and AOU on the 27.38σθ surface varied within the rangeof values measured at the deep-sea station (orange lines inFig. 5c–d). These differences between the shipboard surveys
and the deep-sea station were not significant. Thus, DO in
the deep-sea bottom water on the slope was still observed to
be low (high for AOU) in 2018 along the entire Pacific coast
off Sanriku.
In order to evaluate the decrease in DO due to sediment resuspension caused by earthquakes, we compared the bottom water at the deep-sea station (~ 1000 m
depth) off Otsuchi Bay (Fig. 1) before and after the
events of the large aftershocks: February 17, 2015, off
Sanriku ­(Mw = 6.7, Ev1), September 17, 2017, off Sanriku ­(Mw = 5.9, Ev2), and July 7, 2018, off Boso Peninsula­(Mw = 5.9, Ev3) (Fig. 1a and 6). Because the disturbanceof surface sediments and subsequent high turbid bottom
water at the deep-sea station returned to the pre-earthquake state within 10 days after the large aftershocks
on December 7, 2012 (­Mw = 7.3), and February 17, 2015
(Ev1) (Oguri et al. 2016, 2018), we compared DO values
averaged within 9 days before and after the earthquake.
Along with the increase in turbidity for the Ev1 (Oguri
et al. 2018), a significant decrease in DO (~ 4 μmol/kg)
was observed after the earthquake (Fig. 6a). Ev2 occurred
104 km away from the deep-sea station to the hypocenterand also induced a significant decrease in DO (~ 4 μmol/kg) (Fig. 6b). However, Ev3 494 km apart from the stationinduced no DO change at this station (Fig. 6c).
For the possibility of seafloor turbidity, the intensity of ground motion has been evaluated as the peakground acceleration (PGA) (e.g., Gomberg 2018).
Because of limited seafloor observation, we try toapply PGA to the intensity of ground motion. Sediment resuspension can be linked to the PGA. The valueof the PGA for Ev2 (7−200 cm/s/s) was larger than the
Ev3 (1−4 cm/s/s); these values were estimated from thenegative correlation between distance from hypocenterand PGA for the aftershock observed by the Seafloor
observation network for earthquakes and tsunamis
along the Japan Trench (S-net) (Dhakal et al. 2021;
Dhakal and Kunugi 2021). The PGA value for Ev1 cannot be estimated, as the S-net was constructed in 2016.
Nevertheless, note that the hypocentral distance to the
deep-sea station for Ev1 (88 km) was almost same asthat for Ev2 (106 km) and was much shorter than thatof Ev3 off the Boso Peninsula (483 km). Because of theWakita et al. Progress in Earth and Planetary Science(2022) 9:56

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Fig. 4 Vertical profiles of pressure (a), potential temperature (b), salinity (c), LTA (d), DO (e), AOU (f), ­NO3 (g), ­PO4 (h), and DIC (i) as functions ofpotential density (σθ) and the location map (j) by using the cruises (Table 1). Colored circles denote the latitude of sampling locations. Lines show
the studied isopycnal surfaces (27.38σθ: red; 27.56σθ: blue)

negative correlation with the hypocentral distance (e.g.,
Dhakal et al. 2021; Dhakal and Kunugi 2021), the PGA
values for Ev1 are considered to be similar to that forEv2 and to be much higher than that for Ev3. The difference in the DO changes attributes not to the earthquake magnitudes but to the PGA at the deep-sea
station. Thus, high PGA induced by large aftershocks
would cause an increase in turbidity/LTA besides thedecrease in DO in the bottom water through sedimentresuspension.
However, note that the decrease in the bottom-water
DO due to sediment resuspension by the large aftershocks is affected by the underlying sediment properties
such as sediment grain size and composition. Surface
sediments on the upper slope (500−2000 dbar) over theentire area off Sanriku are dominated by silt and muddy
sediments with high organic carbon contents (Fontanier
et al. 2014; Ikehara et al. 2021; Nomaki et al. 2016). The
oxygen penetration depth in the sediments on the upper

slope was shallow than 0.5−1 cm, that is typical organic
matter-rich sediments (Fontanier et al. 2014; Nomaki
et al. 2016). The C/N ratios and stable carbon isotope
ratios of the surface sediments within a year after the2011 Tohoku Earthquake showed no significant difference between the event deposits, including turbidites,
and the underlying sediment (Nomaki et al. 2016; Ikehara
et al. 2021). These results indicate that high turbidity and
LTA on the upper slope are characterized by sedimentresuspension, short-distance transport, and resettlingnear the original site. Thus, the increase in turbidity and
LTA by organic matter-rich surface sediment resuspension due to aftershock leads to DO decrease in the bottom water at the upper slope off Sanriku. This process
is considered to consist of mixing of bottom water andoxygen-depleted sediment pore water and oxygen consumption due to aerobic decomposition of resuspended
organic matter. The latter process is supported by the following report; in the few days after a hurricane passage

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Fig. 5 Time series of potential temperature (a), salinity (b), DO (c), AOU (d), ­PO4 (e), silicate (f), ­NO3 (g), and DIC (h) in the Sanriku area (circles) andalong the P10 section (green triangles) on the studied isopycnal surfaces (27.38σθ: red; 27.56σθ: blue). Open and closed symbols show values before
and after the 2011 Tohoku Earthquake (3.11), respectively. In panels (a–d), thin orange lines denote values measured at the deep-sea station off
Otsuchi Bay (Fig. 1a). Dashed and bold lines refer to mean values before and after the 2011 Tohoku Earthquake, respectively. Error bars are standard
deviations for 2005, 2012, and 2014 along the P10 section, respectively. Right red (left blue) axes of DO (c), AOU (d), ­PO4 (e), and ­NO3 (g) denote thevalue on 27.38σθ (27.54σθ), respectively

Wakita et al. Progress in Earth and Planetary Science(2022) 9:56

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x[m] = h ( B − A ) / A

Fig. 6 Time series of DO in the bottom water at the deep-sea station(~ 1000 m depth) off Otsuchi Bay (Fig. 1a) before and after the eventsof the large earthquakes (purple double lines): February 17, 2015 (Ev1,
Fig. 1a), September 17, 2017 (Ev2, Fig. 1a), and July 7, 2018, off thenBoso Peninsula (Ev3). Dashed and bold lines refer to the 9-day meanvalues before and after the large earthquakes, respectively

in the northern Gulf of Mexico, organic matter of resuspended shelf sediments consumes the DO in the bottom
water through aerobic decomposition (Bianucci et al.
2018).
To estimate the decrease in bottom-water DO on theupper Sanriku slope after the 2011 Tohoku Earthquakeby the pore water/bottom water mixing and oxygen consumption, we calculate the thickness of surface sediments
resuspended by the 2011 Tohoku Earthquake and large
aftershocks (x [m]), based on simple box model (Fig. 7),
as follows:

A = B h a [mmol] / a ( h + x) m3

(6)

(7)

where B and A are the DO concentration before and after
the 2011 Tohoku Earthquake [mmol/m3], respectively, ais the entire area at the upper Sanriku slope ­[m2], and his the thickness of bottom water affected by surface sediment resuspension due to aftershock [m]. We use 1 [m]as the h, because a HDTV camera at the deep-sea station off Otsuchi Bay was installed at a height of 0.5−1 mabove the seafloor and captured the turbidity throughaftershock (Oguri et al. 2016, 2018). We assume thatthe DO concentration in the porewater is 0 [mmol/m3],
as the depth of oxygen penetration in the upper slope
sediments was very shallow (0.5−1 cm) (Fontanier et al.
2014; Nomaki et al. 2016). The DO concentration after
the 2011 Tohoku Earthquake (A) and the differenceof DO before and after the 2011 Tohoku Earthquake(B−A) on the 27.38σθ and 27.56σθ surfaces are 37.6−44.9[mmol/m3] and 4.9−5.1 [mmol/m3], respectively. Using
these values, x is estimated to be 0.11−0.13 [m]. These
values are almost the same as previously reported earthquake-induced surface sediment remobilization thickness (0.02−0.12 [m]) (Molenaar et al. 2019). Thus, weverified the rationality of the decrease in bottom-water
DO on the upper Sanriku slope after the 2011 Tohoku
Earthquake by the water mixing and oxygen consumption mechanisms.
The relationships between LTA intensity and contentsof chemical components in the deep-sea bottom water
were examined to estimate the source of these components (e.g., Mottl et al. 1995; Kawagucci et al. 2012). The
disturbance of sediments by the 2011 Tohoku Earthquake
is presumed to have been accompanied by the input of
sediment porewater into the overlying deep-sea bottom
water. Because DO and ­NO3 ­(PO4) concentrations in
the bottom water are higher (lower) than those in porewater in the sediments as a result of remineralization, as
shown for the area off Sanriku in March 2012 (Nomaki
et al. 2016), we examined the relationship between bottom-water LTA and DO, ­NO3, ­PO4, and DIC during
2011–2018 after the 2011 Tohoku Earthquake on thecontinental slope over Sanriku (Fig. 8). LTA was foundto be significantly positively (negatively) correlated with
AOU, ­PO4, and DIC (DO) on the two isopycnal surfaces(27.38σθ and 27.56σθ) (Fig. 8a–8c, e). No positive correlation between LTA and N­ O3 concentration was found, but
LTA was positively correlated with ANU (Fig. 8d and f ).
These results indicate that increased turbidity is associated with higher concentrations of ­PO4, and DIC in the
deep-sea water on the continental slope and with lower
DO and ­NO3 concentrations.

Wakita et al. Progress in Earth and Planetary Science(2022) 9:56

a [m2]h [m]Bottom waterx [m]Surface sedimentFig. 7 Schematic simple box model in the bottom water and thesurface sediment at the upper Sanriku slope. “x” is the thickness of
surface sediments resuspended by the 2011 Tohoku Earthquake
and aftershocks. “a” is the area at the upper Sanriku slope ­[m2], and“h” is the thickness of bottom water affected by surface sedimentresuspension due to the 2011 Tohoku Earthquake and aftershock

The chemical influxes associated with sediment resuspension by the 2011 Tohoku Earthquake and large aftershocks are considered to have modified the deep-sea
bottom water on the continental slope by remineralization caused by respiration and denitrification by the
microbial community. To examine this effect, we estimated remineralization-corrected DIC and ­PO4 (Fig. 8g
and h) and found no significant correlation between
LTA and remineralization-corrected DIC and P­ O4 oneither of the studied isopycnal surfaces. The values ofremineralization-corrected DIC and ­PO4 on the slope
after the 2011 Tohoku Earthquake (2154 ± 4 μmol/kg and1.28 ± 0.03 μmol/kg for 27.38σθ, and 2172 ± 3 μmol/kg
and 1.29 ± 0.03 μmol/kg for 27.56σθ) (Fig. 8g and h) areconsistent with those at the P10 section (2156 ± 4 μmol/
kg and 1.30 ± 0.01 μmol/kg for 27.38σθ, and2174 ± 3 μmol/kg and 1.32 ± 0.01 μmol/kg for 27.56σθ).
The values of remineralization-corrected ­PO4 on theslope are statistically consistent before (1.31 ± 0.04 μmol/
kg for 27.38σθ and 1.31 ± 0.03 μmol/kg for 27.56σθ) andafter the 2011 Tohoku Earthquake. The increases in DIC
and ­PO4 were therefore due to remineralization from respiration and denitrification by the microbial community
as a result of the 2011 Tohoku Earthquake and the large
aftershocks, and the chemical influxes caused by earthquake-related sediment resuspension affected the deepsea bottom water on the continental slope.
Moreover, several large earthquakes before the 2011
Tohoku Earthquake during 2001–2010 occurred off
Sanriku (e.g., August 16, 2005, off Miyagi Prefecture,­Mw = 7.2; November 15, 2005, off Sanriku, ­Mw = 7.2; May
8, 2008, off Ibaraki Prefecture, ­Mw = 7.0) (JMA 2022)
and also might have affected the deep-sea bottom wateron the continental slope. We compared the increase in

Page 11 of 16­PO4 due to remineralization by respiration and denitrification (1/170 AOU + 1/104 ANU) before and after the2011 Tohoku Earthquake. The remineralized ­PO4 value
after the 2011 Tohoku Earthquake (1.76 ± 0.03 μmol/
kg for 27.38σθ and 1.75 ± 0.02 μmol/kg for 27.56σθ) was
significantly higher than that before the 2011 Tohoku
Earthquake (1.74 ± 0.03 μmol/kg for 27.38σθ and1.72 ± 0.02 μmol/kg for 27.56σθ) (p < 0.01 based on ttest). Although several large earthquakes before the 2011
Tohoku Earthquake during 2001–2010 (JMA 2022) also
would affect the deep-sea bottom water on the slope, the2011 Tohoku Earthquake and large aftershocks would
have a greater impact on the deep-sea bottom water. In
summary, the post-earthquake chemical changes in the
deep-sea bottom-water environment caused by remineralization from respiration and denitrification after the2011 Tohoku Earthquake are considered to be maintained by sporadic sediment resuspension induced by thepersistent occurrence of large aftershocks.
The regression slopes between LTA and DO, AOU,
­PO4, ­NO3, and DIC on the 27.38σθ surface were steeper
than those on the 27.56σθ surface (Fig. 8a–f ). Thisimplies that the sources of chemical input may be different between isopycnal surfaces. To clarify the difference in these chemical input sources, we examined theconcentrations and carbon isotope ratios of methane off
Ohtsuchi bay and Onagawa bay near the hypocenter ofthe earthquake on October 23, 2015 (­Mw = 4.9), in earlyNovember during cruise KS-15-15 (Fig. 9), as methane isan effective chemical tracer for the input of sub-seafloor
fluid into deep-sea water (e.g., Tsunogai et al. 2000; German et al. 2010).
The methane contents in the deep-sea bottom water(up to ~ 4 nM) measured in this study are an order ofmagnitude smaller than the ~ 60 nM measured 36 days
after the earthquake during cruise MR11-03 (Kawagucci
et al. 2012). This methane in the deep-sea bottom water
has two peaks of LTA intensity near the isopycnal surfaces of 27.38σθ and 27.56σθ (Fig. 9a and b) and is unlikely
to have originated from in situ microbial methanogenesis
because of the aerobic condition of the seawater (Fig. 4e).
The stable carbon isotope ratio of methane decreased
with increasing depth and σθ (Fig. 9c). A Keeling plot
analysis (Keeling 1958) for each isopycnal surface(Fig. 9e) reveals two sources of methane. The methane
source inferred for the 27.56σθ surface (~ 1500 dbar) was
relatively enriched in 13C (− 55‰). This value is similar
to that of methane measured at a depth of > 800 m below
the seafloor near the hypocenter during Ocean Drilling Project Sites 1150 and 1151 (Ijiri et al. 2009) and to
those detected in the water column close to the hypocenter region 36 days after the earthquake (Kawagucci et al. 2012). Moreover, this δ13C value is also similar

Wakita et al. Progress in Earth and Planetary Science(2022) 9:56

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Fig. 8 Relationships between DO (a), AOU (b), ­PO4 (c), ­NO3 (d), DIC (e), ANU (f), remineralization-corrected DIC (g), and remineralization-corrected­PO4 (h) and LTA value on the studied isopycnal surfaces (27.38σθ: red circles; 27.56σθ: blue circles). Solid (dotted) lines indicate statistically significant(insignificant) regressions (p < 0.05)

Wakita et al. Progress in Earth and Planetary Science(2022) 9:56

Page 13 of 16

Fig. 9 a–c Vertical profiles of LTA (a), methane ­(CH4), b and stable carbon isotope ratio of methane (δ13CH4) as functions of potential density (σθ)
at five sampling stations (D8, O2, O3, O4, and O5) during cruise KS-15–15. Lines show the studied isopycnal surfaces (27.38σθ: red; 27.56σθ: blue). d
Location map and bathymetry of sampling stations during cruise KS-15-15, with the hypocenter of the 2011 Tohoku Earthquake and the earthquakeon October 23, 2015 (Mw = 4.9), shown as a red star and a pink star. Bathymetric contours show 200 m depth (solid gray line), 1000 m depth (solid
black line), 2000 m depth (dashed black line), and 3000 m depth (dotted black line). The color bar shows the depth of bathymetry. e Keeling plotof concentrations of ­CH4 and δ13CH4 at five sampling stations (D8, O2, O3, O4, and O5) during cruise KS-15-15. Open circles denote data for thestudied isopycnal surfaces (27.38σθ: red; 27.56σθ: blue), and dashed red and dashed blue lines show the respective linear regression lines

to that in the hadal water below 8000 m at 34°N in the
Izu–Ogasawara Trench (Kawagucci et al. 2018). The 2011
Tohoku Earthquake and aftershocks drive drastic marineorganic carbon supply to the hadal Japan trench (Ikehara et al. 2021; Kioka et al. 2021; Schwestermann et al.
2021), which is possibly associated with the relatively

13

C-enriched methane in the hadal water at the trench.
The sources of methane for the 27.56σθ surface by sediment resuspension after the 2011 Tohoku Earthquake
might be derived from deep sub-seafloor reservoirs or
sedimentary compound releases of marine organic carbon within the trench.

Wakita et al. Progress in Earth and Planetary Science(2022) 9:56

The methane source inferred from the 27.38σθ surface (~ 1000 dbar) showed an enriched 13C signature(δ13C =  − 33‰) (Fig. 9e). After the earthquake on October 23, 2015, DO concentration at the deep-sea station decreased during November (Oguri et al. 2018).
Because the hypocentral distance to the deep-sea stationand station O3 during KS-15-15 was close (~ 34 km),the high LTA on the 27.38σθ surface (Fig. 9a) might beaffected by sediment resuspension by the aftershocks.
The 13C-enriched methane on the 27.38σθ surface could
have been derived from shallow sediment, in which the
deep sub-seafloor-derived methane had been consumed
through microbial methane oxidation, resulting in 13C
enrichment in the remnant methane (e.g., Yanagawa et al.
2016) and the subsequent release of this methane into
the water column by sediment resuspension. Thus, thesources of chemical input on the upper slope by sedimentresuspension after the 2011 Tohoku Earthquake would bederived from shallow sediment on the continental slope.
The influx of methane into the deep bottom water causedby the 2011 Tohoku Earthquake might have remained
until 2015.

4 Conclusions
We conducted shipboard hydrographic observations during 2011–2018 to investigate changes in the deep-sea
bottom-water environment on the continental slope off
Sanriku, Japan, before and after the 2011 Tohoku Earthquake. Focusing on the deep-sea bottom-water environment between 1000 and 1500 dbar, particularly on the
isopycnal surfaces of 27.38σθ and 27.56σθ, we found that
DO underwent a significant drop of about 10% after the2011 Tohoku Earthquake, whereas nutrients and DIC
showed no significant difference and wide variation.
After the 2011 Tohoku Earthquake, turbidity as calculated from LTA was detected in the bottom layer on thecontinental slope of the entire area off Sanriku. This highturbidity was caused by sporadic sediment resuspension
resulting from frequent large aftershocks. LTA was positively correlated with ­PO4, DIC, and methane and negatively with DO on the isopycnal surfaces of 27.38σθ and27.56σθ. Using these relationships, we ascertained thatthe deep-sea bottom-water environment was chemicallychanged by remineralization due to respiration and denitrification by the microbial community after the 2011 Tohoku
Earthquake. Change in the deep-sea bottom-water chemical environment was maintained by sporadic sedimentresuspension induced by continuing large aftershocks andmight have been caused by the variations in DIC and ­PO4
on the slope after the mainshock and large aftershocks.
Deep-sea station monitoring at 1000 dbar has shownthat benthic biota remained unchanged by the increased

Page 14 of 16

turbidity caused by sediment resuspension from large
aftershocks (Oguri et al. 2016, 2018) and that the changed
deep-sea microbial community recorded 36 days after
the earthquake had returned to its original state 98 days
after the earthquake (Kawagucci et al. 2012). The present study revealed that the impact of the 2011 Tohoku
Earthquake on the deep-sea bottom-water environment
expanded in extent across the continental slope area withtime through chemical influxes associated with sedimentresuspension caused by large aftershocks. This continuous and widespread impact likely affected the marine
ecosystem and microbial community on the continental slope. It is therefore essential to continue to monitor the marine ecosystem and microbial community off
Sanriku through ship-based hydrographic and chemical
observations.
Abbreviations
ANU: Apparent nitrogen utilization; AOU: Apparent oxygen utilization; CTD–
CMS: Conductivity–temperature–depth profiler with carousel multiple sampling; DIC: Dissolved inorganic carbon; DO: Dissolved oxygen; JAMSTEC: Japan
Agency for Marine-Earth Science and Technology; JMA: Japan Meteorological
Agency; LTA: Light transmissometer anomaly; NO3: Nitrate; NPIW: North PacificIntermediate Water; Mw: Moment magnitude; PO4: Phosphate; TEAMS: Tohoku
Ecosystems-Associated Marine Sciences; RM: Reference material.
Acknowledgements
We acknowledge the help of the staff of the Mutsu Institute for Oceanography
and the Research Institute for Global Change of JAMSTEC, and the captains
and crews of the R/V Mirai, T/V Oshoro-Maru, T/V Bosei-Maru, R/V Shinsei-Maru,
and R/V Hakuho-Maru for their kind cooperation in sample collection andhydrographic measurements during the 2012–2018 cruises. We are gratefulto K. Sasaki, H. Kawakami, T. Kasaya, Y. Furushima, T. Totyofuku, S. Tsuchida,
Y. Fujiwara, H. Suga, and H. Uchida (JAMSTEC); I. Sakamoto and Y. Yokoyama(Tokai University); S. Otosaka and H. Ogawa (University of Tokyo); M. Takada, K.
Tsubata, and T. Ohmura (Tohoku Environmental Science Services Corporation);and Y. Tanaka (Hachinohe Institute of Technology) for valuable discussion andcomments. Methane analysis was supported by K. Tanaka and the Deep301
laboratory. We also thank the marine technicians of Marine Works Japan
onboard the R/V Mirai, T/V Bosei-Maru, R/V Shinsei-Maru, and R/V HakuhoMaru. Finally, we also express our deep thanks to editor and two anonymous
reviewers who provided us many useful comments.
Author contributions
MW proposed the topic and conceived and designed the study. JY, KO, HN,
and SK analyzed the data and assisted in their interpretation. SW, KA, AN, and
KF collaborated with the corresponding author in the construction of themanuscript. All authors read and approved the final manuscript.
Funding
This study was supported by Tohoku Ecosystems-Associated Marine Sciences
Project Grant Number JPMXD1111105260 from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
Availability of data and materials
The datasets supporting the conclusions of this article are available in thephysical and biogeochemical database for 2005–2018 collected off the Pacificcoast of Sanriku and along the international repeat hydrography program P10
section (https://​www.​jamst​ec.​go.​jp/j/​about/​infor​matio​ns/​notif​i cati​on_​2021_​
maint​enance.​html?​lang=​en; http://​www.​data.​jma.​go.​jp/​gmd/​kaiyou/​db/​
vessel_​obs/​data-​report/​html/​ship/​ship_e.​php; https://​www.​ncei.​noaa.​gov/​
access/​ocean-​carbon-​data-​system/​oceans/​Repea​tSect​ions/​clivar_​p10.​html).

Wakita et al. Progress in Earth and Planetary Science(2022) 9:56

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Mutsu Institute for Oceanography (MIO), Research Institute for Global
Change (RIGC), Japan Agency for Marine-Earth Science and Technology, 690
Kitasekine, Sekine, Mutsu, Aomori 035‑0022, Japan. 2 Tohoku Environmental
Science Services Corporation (TESSCO), 330‑2, Nozuki, Obuchi, Rokkasho,
Aomori 039‑3212, Japan. 3 Marine Biodiversity and Environmental Assessment
Research Center (BioEnv), Research Institute for Global Change (RIGC), Japan
Agency for Marine-Earth Science and Technology, 2‑15, Natsushima, Yokosuka,
Kanagawa 237‑0061, Japan. 4 HADAL and Nordcee, Department of Biology, University of Southern Denmark (SDU), Campusvej 55, 5230 Odense
M, Denmark. 5 Super‑Cutting‑Edge Grand and Advanced Research (SUGAR),
Program Institute for Extra‑Cutting‑Edge Science and Technology Avant‑Garde
Research (X‑star), Japan Agency for Marine-Earth Science and Technology,
2‑15, Natsushima, Yokosuka, Kanagawa 237‑0061, Japan. 6 Research and Development Center for Earthquake and Tsunami Forecasting (FEAT), Research Institute for Marine Geodynamics (IMG), Japan Agency for Marine-Earth Scienceand Technology, 3173‑25, Showa‑machi, Kanazawa, Yokohama, Kanagawa
236‑0001, Japan. 7 Global Ocean Observation Research Center (GOORC),
Research Institute for Global Change (RIGC), Japan Agency for Marine-Earth
Science and Technology, 2‑15, Natsushima, Yokosuka, Kanagawa 237‑0061,
Japan.
Received: 28 March 2022 Accepted: 4 October 2022

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