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Potential tsunamigenic faults of the 2011 off the Pacific coast of Tohoku Earthquake

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Takeshi Tsuji
Yoshihiro Ito
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Motoyuki Kido at Tohoku University
Motoyuki Kido
Yukihito Osada
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Abstract and Figures

Faults related to the tsunamigenic 2011 Tohoku-Oki Earthquake (Mw 9.0) were investigated by using multi-channel seismic reflection data acquired in 1999 and submersible seafloor observations from 2008. The location of the fault system interpreted in the seismic reflection profile is distributed around the area with largest slip and tsunami induction of the 2011 event. Cold-seep communities along the trace of the branch reverse fault and a high scarp associated with the trace of a normal fault suggest current activity on these faults. We interpret the fault system in the seismic profile as a shallow extension of the seismogenic fault that may have contributed to the resulting huge tsunami.
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LETTER Earth Planets Space,63, 831–834, 2011
Potential tsunamigenic faults of the 2011 off the Pacific coast
of Tohoku Earthquake
Takeshi Tsuji1, Yoshihiro Ito2, Motoyuki Kido2, Yukihito Osada2, Hiromi Fujimoto2,
Juichiro Ashi3, Masataka Kinoshita4, and Toshifumi Matsuoka1
1Graduate School of Engineering, Kyoto University, C1-1-110 Kyotodaigaku-Katsura, Nishikyoku, Kyoto 615-8540, Japan
2Graduate School of Science, Tohoku University, 6-6 Aramaki-aza-aoba, Aoba-ku, Sendai, Miyagi 981-8578, Japan
3Atmosphere and Ocean Research Institute, the University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan
4Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC),
2-5 Natsushima-cho, Yokosuka-shi, Kanagawa 237-0061, Japan
(Received April 13, 2011; Revised May 19, 2011; Accepted May 20, 2011; Online published September 27, 2011)
Faults related to the tsunamigenic 2011 Tohoku-Oki Earthquake (Mw9.0) were investigated by using multi-
channel seismic reflection data acquired in 1999 and submersible seafloor observations from 2008. The location
of the fault system interpreted in the seismic reflection profile is distributed around the area with largest slip and
tsunami induction of the 2011 event. Cold-seep communities along the trace of the branch reverse fault and a
high scarp associated with the trace of a normal fault suggest current activity on these faults. We interpret the
fault system in the seismic profile as a shallow extension of the seismogenic fault that may have contributed to
the resulting huge tsunami.
Key words: 2011 Tohoku-Oki Earthquake, tsunamigenic faults, seismic reflection data, seafloor observation,
cold-seep communities, high scarp.
1. Introduction
The 11 March 2011 earthquake (Mw9.0) ruptured a wide
area along the plate interface off the Pacific coast of To-
hoku, Japan (Japan Meteorological Agency JMA, 2011;
Yagi, 2011; Fig. 1(a)). The northwestern margin of the Pa-
cific plate is subducting beneath the northeastern Japan Arc
at a convergence rate of 8.6 cm/yr (DeMets et al., 1990) and
frequently generates interplate earthquakes and tsunamis
(e.g., Yamanaka and Kikuchi, 2004). However, the tsunami
caused by this earthquake was extremely huge, and ob-
servations with an ocean bottom pressure gauge revealed
short-period spike-shaped sea surface uplift (e.g., Fujii et
al., 2011). In order to reveal mechanisms of the impulsive
tsunami generation, shallow fault distributions and geome-
tries are important.
Here, we identify a series of faults from a seismic re-
flection profile obtained in 1999 near the hypocenter (JMA,
2011) and seafloor observations of the fault trace made in
2008 by the manned submersible Shinkai 6500. Because
the surveyed area includes the region where the largest ver-
tical displacement is predicted to have occurred (Ueno and
Satake, 2011; Shao et al., 2011; Fig. 1(a)), the shallow
faults here are likely to be directly related to the tsunami
characteristics.
Copyright c
The Society of Geomagnetism and Earth, Planetary and Space Sci-
ences (SGEPSS); The Seismological Society of Japan; The Volcanological Society
of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sci-
ences; TERRAPUB.
doi:10.5047/eps.2011.05.028
2. Seismic Reflection Data
Multi-channel seismic reflection data acquired by R/V
Kairei (JAMSTEC) in 1999 (Line MY102 of KR99-08
cruise; Tsuru et al., 2002) were analyzed for an investiga-
tion of fault geometry and to select dive points for seafloor
observations (Fig. 1(b)). In the seismic survey, the sound
source was an array of ∼200-L (12,000 cubic inch) airguns
fired every 50 m. The receiver array was a 156-channel,
4-km streamer, and the record length was 13.5 s.
We applied conventional seismic processing, including
trace editing, multiple suppression, deconvolution, velocity
analysis, stacking, and post-stack migration (Yilmaz, 2001).
We then obtained the depth-domain profile (Fig. 2) by us-
ing stacking velocity. Due to the limitation of the streamer
length, it was difficult to determine seismic velocities accu-
rately in the deeper lithology.
3. Geological Interpretation
On the reflection profile (Fig. 2), we identified three pre-
dominant faults branching from the plate boundary fault:
(A) a backstop reverse fault acting as a boundary between
a seaward accreted sequence and a landward less-deformed
Cretaceous sequence (von Huene et al., 1994; Tsuru et al.,
2002), (B) a branch reverse fault constructing the signifi-
cant seafloor slope break (Fig. 2(d)), and (C) a steeply dip-
ping normal fault branching from a plate boundary fault and
extending towards a seafloor ridge (Fig. 2(c)). However,
potential underplating structures are observed landward of
the backstop reverse fault defined by Tsuru et al. (2002)
(Fig. 2(b)).
Displacement along the steeply-dipping normal fault has
831
832 T. TSUJI et al.: POTENTIAL TSUNAMIGENIC FAULTS
Fig. 1. (a) Seismic survey line (yellow line), energy and slip distribution of the 2011 earthquake, and area of tsunami induction. Yellow rectangle
indicates the area of panel (b). (b) Bathymetric map around the seismic survey line (red line) (Sasaki, 2004). Reverse triangles indicate the location
of a landward margin of the trench. We identify the ridge associated with steeply dipping normal fault and the seafloor slope break associated with
branch reverse faults. Red rectangles indicate the areas of panels (c) and (d). (c) Dive track at the seafloor ridge (Dive #1071). Red triangle indicates
the location where a dead clam was observed. (d) Dive tracks at the seafloor trace of the branch reverse fault (Dive #1069, #1072, #1073, and #1074).
Red triangles indicate the locations of clam colonies.
offset a Cretaceous sequence surface by ∼800 m (Fig. 2(c)).
The plate boundary fault and steep normal fault appear to
bound a pop-up structure. The seafloor ridge associated
with the normal fault displacement is well identified on the
seafloor topography (Fig. 1(b)) and is as long as several
tens of kilometres parallel to the trench axis. Therefore,
the normal fault should play an important role in the plate
convergent margin off Miyagi.
4. Submersible Seafloor Observations
In May 2008, we used the manned submersible Shinkai
6500 (YK08-06) to visit two points along this seismic line:
the seafloor trace of the (B) branch reverse fault (Fig. 2(d))
and a ridge associated with displacement along the (C)
T. TSUJI et al.: POTENTIAL TSUNAMIGENIC FAULTS 833
Fig. 2. (a) Original seismic reflection profile with amplitude gain control (AGC). (b) Composite seismic reflection profile with geological interpretations.
(c) Detailed profile around the normal fault and ridge structure. Displacement of steeply dipping fault (red dots) offsets the sediment basement surface
(yellow dots). (d) Detailed profile around the seafloor trace of branch reverse fault (red dots). The fault can be identified as a clear reflection.
branch normal fault (Fig. 2(c)). Because of an insuffi-
cient depth capability of Shinkai 6500, we could not dive
to the seafloor trace of the (A) backstop reverse fault lo-
cated at a depth of ∼7000 m as well as plate boundary fault
(∼7500 m).
Chemosynthetic communities observed along the branch
reverse fault trace (Figs. 1(d), 2(d), 3(a)) indicate that fluid
passes through open fractures along the fault plane. Similar
cold seeps on the seafloor traces of active faults in the north-
ern Japan Trench (Ogawa et al., 1996) and other convergent
margins (Toki et al., 2004) suggest that the interpreted faults
on the off-Tohoku seismic profile are also active.
A scarp ∼150 m high marks the trace of the normal fault
(Figs. 1(c), 2(c), 3(b)), and continuously exists along the
ridge (∼2 km dive track of the Shinkai). The slope angle
of the scarp is nearly vertical and overhanging in places
(Fig. 3(b)). We sampled rocks from the steep scarp and
estimated the depositional age as 0.51–0.85 Ma from both
calcareous nannofossils (e.g., Okada and Bukry, 1980) and
fossil diatoms (e.g., Yanagisawa and Akiba, 1998). Because
there is a fresh scarp surface without any manganese coating
and because we observed a dead clam there, this scarp may
have been generated by recent earthquake activity.
5. Summary and Discussions
Faults related to the tsunamigenic 2011 Tohoku Earth-
quake were investigated by using seismic reflection data and
submersible seafloor observations. The fault system identi-
834 T. TSUJI et al.: POTENTIAL TSUNAMIGENIC FAULTS
Fig. 3. (a) Chemosynthetic biological communities observed along the
seafloor trace of the branch reverse fault (Fig. 2(d)). (b) Still video
image (view from north) of the scarp attributed to the displacement
of a steeply dipping normal fault (Fig. 2(c)). A fresh surface without
manganese coating suggests recent activity of the normal fault.
fied in this study is located at the seaward edge of the rup-
ture area (Yagi, 2011) or consistent with the maximum rup-
ture area (JMA, 2011; Shao et al., 2011) (Fig. 1(a)). The
largest tsunami was inferred to have been induced near the
interpreted fault (Ueno and Satake, 2011) where the largest
vertical static displacement is expected (Shao et al., 2011).
Therefore, the interpreted fault system may be shallow ex-
tensions of the seismogenic fault that also slipped during
the earthquake.
Seafloor displacement during the earthquake estimated
by the Japan Agency for Marine-Earth Science and Tech-
nology (JAMSTEC, 2011) demonstrated that the seafloor
between the trench and the normal fault is significantly de-
formed in a seaward direction (50 m horizontal displace-
ment) and uplifted (7 m vertical displacement). Therefore,
the normal fault seems to act as a landward boundary of a
significant displacement region. This observation suggests
that the geological unit between the normal fault and the
plate boundary fault should be uplifted (moves to seaward
direction), as inferred from the fault interpretations on seis-
mic profile (Fig. 2(b)).
Because displacement along the plate boundary fault near
the trench is much larger than the deeper fault landward of
our survey area (e.g., Fujii et al., 2011), the geological unit
above the plate boundary fault is in a tensile state of stress.
Due to the tensile stress state, the normal faults should be
ruptured during this earthquake event.
If the steeply-dipping normal faults slipped during the
earthquake, in addition to the plate boundary fault, they
can induce the huge tsunami even as a result of a smaller
displacement. The potential underplating unit landward of
the backstop reverse fault may also have caused uplift that
contributed to the tsunami.
Acknowledgments. We thank T. Sasaki (University of Tokyo) for
the bathymetric map. We are grateful to two reviewers for their
useful comments. The bathymetric data in Fig. 1(b) were acquired
by R/V Kairei,Yokosuka, and Mirai (JAMSTEC). The seismic
data were acquired by R/V Kairei (JAMSTEC). This study is
supported by Grant-in-Aid for Scientific Research on Innovative
Areas (21107003).
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... On the other hand, Bletery et al. (2014) highlighted the crucial role of normal faulting in the overriding plate and reported that the largest slip occurred just in the up-dip direction of an important normal fault revealed in seismic lines (Tsuji et al., 2011(Tsuji et al., , 2013 co-seismically reactivated. Bletery et al. (2014) also explained, based on Cubas et al. (2013), that this normal fault (yellow line in Figure 7) could have activated coseismically (under very low dynamic friction conditions) allowing a large amount of slip to propagate up to the free surface. ...
... These authors proposed that low friction authorized large slip with a low-stress drop in the shallowest part of the megathrust defined as block A (consistent with the large Tsunami), which is a coherent unit separated by the normal fault from another block B (Figure 7) located to the west (see Bletery et al., 2014 for more details). Finally, they proposed that the rupture of a long-term locked asperity on the megathrust (located below the normal fault) caused block A to move as a coherent unit with respect to block B. Our results show that the eastern edge (i.e., trenchwards) of the low Tzz lobe N°2 coincides with the location of this normal fault (from Tsuji et al., 2011) separating very low Tzz values (<−20 Eötvös) on the ceiling ...
... The last (i.e., Baba and Yoshida, 2020), suggests that the structural blocks (in that work referred to as Blocks B, C, and D, named in Figure 7 as BB, BC and BD respectively), bounded by the offshore Hidaka tectonic line and the Honjo-Sendai tectonic line, slipped rapidly trenchward when the 2011 Tohoku-Oki earthquake occurred (Figure 7). Thus, the low Tzz lobe (identified as N°2) could be mapping a main crustal block along the marine forearc, limited along-strike by strike-slip faults (Baba and Yoshida, 2020) and up-dip by a trench parallel normal fault (Tsuji et al., 2011;Tsuji et al., 2013). This main block is probably limited towards the down-dip direction by another fault just over the slab/Moho intersection (mapped by Bassett et al., 2016). ...
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Despite a lack of modern large earthquakes on shallowly dipping normal faults, Holocene Mw > 7 low-angle normal fault (LANF; dip<30°) ruptures are preserved paleoseismically and inferred from historical earthquake and tsunami accounts. Even in well-recorded megathrust earthquakes, the effects of non-linear off-fault plasticity and dynamically reactivated splay faults on shallow deformation and surface displacements, and thus hazard, remain elusive. We develop data-constrained 3D dynamic rupture models of the active Mai’iu LANF that highlight how multiple dynamic shallow deformation mechanisms compete during large LANF earthquakes. We show that shallowly-dipping synthetic splays host more coseismic slip and limit shallow LANF rupture more than steeper antithetic splays. Inelastic hanging-wall yielding localizes into subplanar shear bands indicative of newly initiated splay faults, most prominently above LANFs with thick sedimentary basins. Dynamic splay faulting and sediment failure limit shallow LANF rupture, modulating coseismic subsidence patterns, near-shore slip velocities, and the seismic and tsunami hazards posed by LANF earthquakes.
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Coseismic Folding During Ramp Failure at the Front of the Sulaiman Fold‐and‐Thrust Belt
Article
Full-text available
The Sulaiman Fold Thrust (SFT) in Central Pakistan formed during the India‐Eurasia collision in the late Cenozoic. However, the mechanics of shortening of the brittle crust at time scales of seismic cycles is still poorly understood. Here, we use radar interferometry to analyze the deformation associated with the 2015 magnitude (Mw) 5.7 Dajal blind earthquake at the eastern boundary of the SFT. We use kinematic inversions to determine the distribution of slip on the frontal ramp and of flexural slip along active axial surfaces for the forward‐ and backward‐verging two end‐member models: a double fault‐bend‐fold system and a fault‐propagation‐fold. In both models, a décollement branches into a shallow ramp at approximately 7.5 km depth with coseismic folding in the hanging wall. The Dajal earthquake ruptured the base of the Boundary Thrust buried under the sediment from the Indus‐River floodplain, representing fault‐bend or fault‐propagation folding some 30 km off its nearest surface exposure.
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Forearc Topography: Mirror of Megathrust Rupture Properties
Chapter
  • Sep 2022
This chapter explores how large subduction earthquakes impact the forearc topography. It explains the critical taper theory, which describes the deformation of brittle wedges. The chapter replaces this theory in a forearc context with megathrusts composed of seismic and aseismic patches. It explores how the study of the deformation and topography of forearcs can help in estimating the seismic and tsunamigenic hazards. In a compressive setting, sand, sediments or rocks are deformed by a sequence of thrusts, above a basal decollement to form a prism, or wedge. Aseismic zones are described with a rate‐strengthening friction, and the seismogenic zone with a rate‐weakening behavior. The difference in friction also implies different displacement quantities between aseismic and seismic segments. The type of splay fault, normal or reverse, depends on the difference in effective friction and the position of the prism with respect to the critical envelope.
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Simple topographic parameter reveals the along-trench distribution of frictional properties on shallow plate boundary fault
Article
Full-text available
  • Dec 2022
  • EARTH PLANETS SPACE
The critical taper model best describes the first-order mechanics of subduction zone wedges. The wedge geometry, which is conventionally defined by two parameters, slope angle and basal dip angle, accounts for the strength of megathrust. By applying this theoretical model, fault frictional properties and earthquake occurrences can be compared among subduction zones, and within a single subduction zone, and the spatial distribution or temporal change of fault strength can be investigated. Slope angle can be accurately estimated from bathymetry data, but basal dip angle must be inferred from subsurface structure, which requires highly accurate depth-migrated seismic reflection profiles. Thus, application of the critical taper model is often limited by an insufficient number of highly accurate profiles, and the spatial distribution of frictional coefficients must be inferred from relatively few data. To improve this situation, we revisited the theoretical formula of the critical taper model. We found that the effect of basal dip angle on the critical taper model is small, and slope angle can be a proxy for the effective friction when the pore fluid pressure ratio is high, internal friction is small, or both. These conditions are met in many subduction zones. The validity of the approximation can be checked with a parameter newly introduced in this study. Therefore, this finding allows use of variations in slope angle, which could be obtained accurately from only the bathymetry as an approximation for relative variations in the effective coefficient of basal friction, if the targeted subduction meets the validity. We applied this approximation to the Japan Trench and estimated the variations in the friction coefficient distribution on the shallow plate boundary fault from 71 data points. We found that the area where the friction coefficient was smaller than the mean corresponded to a segment, where a large coseismic shallow rupture occurred during the 2011 Tohoku-Oki earthquake (Mw 9.0). Thus, by approximating tapered wedge geometry with a simple topographic parameter that can be obtained from existing global bathymetry, we can quickly estimate the distribution of frictional properties on a plate boundary fault along a trench and related seismic activity. Graphical Abstract
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Effects of coseismic megasplay fault activity on earthquake hazards: Insights from discrete element simulations
Article
  • Jan 2022
  • J STRUCT GEOL
Deep-water megasplay faults may promote or limit earthquake rupture and tsunami genesis. To better understand how megasplay faults affect earthquake rupture and associated tsunami potential, we use the Discrete Element Method (DEM) to model the upper plate as a wedge that is partitioned into a seismic (velocity-weakening, VW) inner wedge and an aseismic outer (velocity-strengthening, VS) wedge, combined with a splay fault rooting at the decollement. We examine the effects of the width of the outer (VS) wedge, as well as the dip and friction along the splay fault during earthquake rupture. Our results suggest that along-strike variations in the width of the VS outer wedge along the Chile Margin may play a key role in splay fault activity in the ruptured segment of the 2010 Maule earthquake. In addition, our model fit to the published slip distribution for the 2010 Maule earthquake suggests that megasplay fault activation did not significantly impact earthquake size along the south-central Chile Margin. In contrast, our model fit to the slip distribution for the 2011 Tohoku earthquake shows that megasplay fault reactivation may have moderately affected earthquake coseismic rupture. Splay faults can slip coseismically, contributing to associated tsunamis. However, the presence of a VS outer wedge is the predominant constraint on rupture size and tsunami generation.
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Tsunami source of the 2011 off the Pacific coast of Tohoku Earthquake
Article
Full-text available
  • Jul 2011
  • EARTH PLANETS SPACE
Tsunami waveform inversion for the 11 March, 2011, off the Pacific coast of Tohoku Earthquake (M 9.0) indicates that the source of the largest tsunami was located near the axis of the Japan trench. Ocean-bottom pressure, and GPS wave, gauges recorded two-step tsunami waveforms: a gradual increase of sea level (˜2 m) followed by an impulsive tsunami wave (3 to 5 m). The slip distribution estimated from 33 coastal tide gauges, offshore GPS wave gauges and bottom-pressure gauges show that the large slip, more than 40 m, was located along the trench axis. This offshore slip, similar but much larger than the 1896 Sanriku "tsunami earthquake," is responsible for the recorded large impulsive peak. Large slip on the plate interface at southern Sanriku-oki (˜30 m) and Miyagi-oki (˜17 m) around the epicenter, a similar location with larger slip than the previously proposed fault model of the 869 Jogan earthquake, is responsible for the initial water-level rise and, presumably, the large tsunami inundation in Sendai plain. The interplate slip is ˜10 m in Fukushima-oki, and less than 3 m in the Ibaraki-oki region. The total seismic moment is estimated as $3.8 × 1022 N m (Mw = 9.0).
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Current Plate Motions
Article
Full-text available
  • May 1990
  • GEOPHYS J INT
Best-fitting Euler vectors, closure-fitting Euler vectors, and a new global model (NUVEL-1) describing the geologically current motion between 12 assumed-rigid plates, were determined. The 1122 data from 22 plate boundaries inverted to obtain NUVEL-1 consist of 277 spreading rates, 121 transform fault azimuths, and 724 earthquake slip vectors. The model fits the data well. The strikes of transform faults mapped with GLORIA and Seabeam along the Mid-Atlantic Ridge greatly improve the accuracy of estimates of the direction of plate motion. Data shows that motion about the Azores triple junction is consistent with plate circuit closure, and better resolves motion between North America and South America. Motion of the Caribbean plate relative to North or South America is about 7 mm yr-1 slower than in prior global models. The direction of slip in trench earthquakes tends to be between the direction of plate motion and the normal to the trench strike. -from Authors
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Asperity Map Along the Subduction Zone in the Northeastern Japan Inferred From Historical Seismograms
Article
  • Dec 2002
In the off-Tohoku region, northern Japan, M7 class earthquakes have repeatedly occurred in an interval of about 30years. The seismic observation by low-gain seismometers has been carried out by Japan Meteorological Agency and universities in Japan since the beginning of 1900's. Collecting these historical seismograms, we examined the fault asperity (large slip area) for individual large earthquakes greater than M7.0. They include the earthquakes of 1931(Mw7.3), 1936(7.4), 1937(7.1), 1960(7.2), 1968(8.3), 1968(7.0), 1978(7.5), 1980(7.1), 1989(7.0), 1994(7.7). We obtained that the events of 1931, 1968, and 1994 shared a common asperity and the events of 1960, 1968, and 1989 also shared another common asperity. Some characteristic features of the asperities are as follows: (1) The individual asperity has its own location and extent. (2) The asperity locates away from the hypocenter. (3) The asperity is surrounded by aftershocks. The patterns of asperity distribution in northern Japan subduction zone are divided into three different categories. In northern part (40N-41.3N) the seismic coupling in asperity is almost 100% and the size of asperity is large. In central part (39N-40N) little seismic moment is released by large earthquakes and asperity size is small. In southern part (37.8N-39N) the seismic coupling coefficient is about 50%. The variation in seismic coupling along the Japan Trench seems to correlate with bathymetry of the subducting plate such that the graven-rich structure at the ocean bottom corresponds to the aseismic moment release. It is also interesting to note that a large back-slip motion in the northern part, which is derived from GPS data, may be consistent with the existence of two large asperities.
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Along-arc structural variation of the plate boundary at the Japan Trench margin: Implication of interplate coupling
Article
  • Dec 2002
At the Japan Trench convergent margin, many large interplate earthquakes of greater than M7.5 frequently occur. Their epicenters have uneven distribution, mostly located in the northern area. To investigate the relationship between this distribution and tectonic structures, we have conducted multichannel seismic surveys since 1996. Our data show two kinds of interplate sedimentary units: a wedge-shaped unit and a channel-like unit. Both units have a lower P wave velocity than the basal part of the overriding island arc crust. The wedge-shaped unit having a velocity of 2-3 km/s is widely distributed over the forearc region in the northern area. Its thickness decreases with depth, becoming several hundred meters at a depth of ~12 km. The channel-like unit having a velocity of 3-4 km/s is observed in the southern area, extending in the downdip direction. Its thickness reaches ~2 km at a depth of ~12 km. If the low velocity of these units results from the existence of fluid, as many authors assume, the units being thick implies higher fluid content assuming constant porosity. Considering that fluid reduces basal friction and with an assumption that fluid available at a specific interface is proportional to the total fluid content in the sediment, the thickness variation of the units would cause different degrees of coupling at the plate boundary along the arc. This may provide one explanation for the regional disparity in the interplate earthquake occurrence in the margin. Furthermore, we attempt to call attention to the possibility that the channel-like sediment works as a shear stress releaser.
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Tectonic structure across the accretionary and erosional parts of the Japan Trench margin
Article
  • Nov 1994
Structure sections across the Japan Trench from prestack depth migrated seismic data define the accretionary prism, a wedge-shaped seaward end of the continental framework, and a tectonized middle slope between them. The 10-km-wide accretionary prism is underthrust by sediment remaining with the subducted oceanic crust. Structure beneath the steep middle slope more nearly resembles the adjacent continental margin than the accreted mass. This deformed middle slope domain forms a buffer between the compliant accretionary wedge and a more rigid older continental framework. Along the plate boundary, subducted strata thicken beneath the middle and upper slopes and, where last imaged, are four times the input thickness beneath the accretionary prism. This interplate stratified layer appears to control interplate friction because the layer is imaged 45 km landward from the trench axis to 12-km depth and in that expanse, few upper plate earthquakes are recorded. Erosion of the base of the upper plate beneath the middle continental slope is indicated by margin subsidence and material flux. This erosion is concurrent with accretion at the front of the margin.
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Asperity map along the subduction zone in northeastern Japan inferred from regional seismic data
Article
  • Jul 2004
In an attempt to examine the characteristic behavior of asperities, we studied the source processes of large interplate earthquakes offshore of the Tohoku district, northeastern Japan, over the past 70 years. In this area, earthquakes of M7 class have a recurrence interval of about 30 years. Seismic observation using a strong-motion seismometer has been carried out by the Japan Meteorological Agency since the beginning of the 1900s. We collected these seismograms in order to make a waveform inversion. On the basis of the derived heterogeneous fault slip, we identified large slip areas (asperities) for eight earthquakes which occurred after 1930, and we constructed an asperity map. The typical size of individual asperities in northeastern Japan is M7 class, and an M8 class earthquake can be caused when several asperities are synchronized. We propose that the patterns of asperity distribution beneath offshore Tohoku fall into three different categories. In the northern part (40°-41.3°N) the seismic coupling in the asperity is almost 100%, and the size is large. In the central part (39°-40°N), little seismic moment has been released by large earthquakes, and the asperity size is small. In the southern part (37.8°-39°N) the seismic coupling is medium. The weak seismic coupling may be related to submarine topographical features and to the sediment and water along the subducting plate. Our results also suggest a general tendency for the asperities to be located away from the hypocenters (initial break), with aftershocks occurring in the area surrounding the asperity.
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Detection of low-chloride fluids beneath a cold seep field on the Nankai Accretionary Wedge off Kumano, south of Japan
Article
  • Nov 2004
  • EARTH PLANET SC LETT
Chemical and isotopic characteristics were determined for interstitial waters extracted from surface sediments in and around dense biological communities on the seafloor of the Nankai accretionary prism off Kumano, south of Japan. We found the following unique features when compared with usual interstitial water samples of normal seafloor in those of samples from bacterial mats on the Oomine Ridge, one of the outer ridge in the Nankai accretionary prism: (1) significant depletion of chloride concentration (maximum 10% depletion from bottom seawater), (2) high concentrations of CH4 and ΣCO2 (more than 660 μmol/kg and 60 mmol/kg, respectively), (3) sulfate depletion (more than 90% depletion compared to bottom seawater), and (4) δDH2O and δ18OH2O depletion [more than 4‰ and 0.7‰ depletion, respectively, compared to standard mean ocean water (SMOW)]. The highest CH4 value among these samples was comparable to the highest value so far reported at one of the most active seep areas in the Nankai Trough, suggesting that these sites should also be regarded as one of the most active seep sites in the Nankai Trough. The chemical compositions of the samples taken from the Oomine Ridge strongly suggest that the fluid originates not from normal sediment–seawater interaction at the sediment surface of hemipelagic environments, but from active seepage of fluids that are rich in CH4 and ΣCO2, depleted in Cl− and SO42−, and low in δDH2O and δ18OH2O compared to normal seawater. Values for the carbon isotopic composition (δ13CCH4) of the dissolved methane in the interstitial fluid [less than −70‰ PeeDee Belemnite (PDB)] and for the C2H6/CH4 ratio (less than 10−3) suggest that the methane originates from microbial production in a relatively shallow layer of sediment, not from the deep sedimentary layer of higher temperature than 60 °C at the depth of more than 300 m below the seafloor. The Cl−=0 mmol/kg extrapolated end-member δDH2O and δ18OH2O values of low-chloride fluids were −46±7‰ and −6.3±0.7‰ SMOW, respectively, suggesting that land-derived groundwater could be one of the possible sources for the low-Cl− fluids. Depth profiles of chloride concentrations of interstitial fluids show the heterogeneity of end members and upward fluid flow velocities suggest that active fluid seepage on the Oomine Ridge seems to be a localized phenomenon. Assuming steady-state emission of fluid from the cold seep vent, upward fluid flow velocities from the seeping vent are estimated to be 40–200 cm year−1, comparable to the previously reported values within the bacterial mats in the Nankai Trough accretionary wedge. Development of bacterial mat might favor slower advection, which might allow longer time for diagenetic reactions in the vent conduits, and consequently, carry more reductive compounds in the fluids.
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Seismic data analysis
Chapter
  • Jan 2001
Oz Yilmaz has expanded his original volume on processing to include inversion and interpretation of seismic data. In addition to the developments in all aspects of conventional processing, this two-volume set represents a comprehensive and complete coverage of the modern trends in the seismic industry-from time to depth, from 3-D to 4-D, from 4-D to 4-C, and from isotropy to anisotropy.
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