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Provenance of grains in submarine event deposits inferred from benthic foraminiferal assemblages: Examples of deposits formed by the 2011 Tohoku earthquake and tsunami
... On the other hand, vertical changes of several centimeters in sandy to muddy deposits were observed between 100 and 6000 m water depth. Although many authors do not exclude the possibility that strong earthquake groundshaking might have generated turbidity currents (e.g., Usami et al., 2014), it is more likely that the tsunami not only resulted in resuspension of sea bottom sediments but also transported material in suspension from the nearshore zone to deeper water in some form of turbidity current or suspended flow (Arai et al., 2013; in this issue). ...
The 2011 Tohoku-oki tsunami that devastated the Pacific coast of Tohoku, Japan was a turning point for modern research. As a result of this event it was recognized that paleotsunami research is vital to help understand the size and recurrence interval of low-frequency large tsunamis. This paper reviews the progress of geological research on the 2011 Tohoku-oki tsunami and summarizes new questions that are arising out of this work. For example, recent work suggests that the landward extent and thickness of the sandy deposit, as well as the presence or absence of marine microfossils in the sediment are most likely to be mainly controlled by the initial wave properties, sediment source, offshore bathymetry and onshore topography. This in turn implies that there are certain relationships between the characteristics of a tsunami deposit and the wave properties and it may be possible to reconstruct the latter from the deposits.
We show the first real-time record of a turbidity current associated with a great earthquake, the Mw 9.0, 2011 Tohoku-Oki event offshore Japan. Turbidity current deposits (turbidites) have been used to estimate earthquake recurrence intervals from geologic records. Until now, however, there has been no direct evidence for large-scale earthquakes in subduction plate margins. After the 2011 Tohoku-Oki earthquake and tsunami, an anomalous event on the seafloor consistent with a turbidity current was recorded by ocean-bottom pressure recorders and seismometers deployed off Sendai, Japan. Freshly emplaced turbidites were collected from a wide area of seafloor off the Tohoku coastal region. We analyzed these measurements and sedimentary records to determine conditions of the modern tsunamigenic turbidity current. We anticipate our discovery to be a starting point for more detailed characterization of modern tsunamigenic turbidites, and for the identification of tsunamigenic turbidites in geologic records.
Eastern Hokkaido forearc, Japan, is located on the Pacific side of the
country along the Kuril Trench where large earthquakes and tsunamis
have frequently occurred. The earthquake history in this region during
the last few centuries is known, based on the historical literature. In
order to examine prehistoric earthquakes, geological approach, such as
analysis of seismo-turbidites or tusnami deposits, are effective. Two
gravity cores (GH03-1033 and GH03-1034) were collected from the bottom
of the Kushiro Submarine Canyon, offshore of Kushiro, eastern Hokkaido
forearc. The cores contain evidence of several turbidites.
Sedimentological, geochemical, and micropaleontological data and
high-resolution seismic data have been used to identify character,
provenance, and recurrence intervals of the turbidites. Upstream (<
1000 m of the water depth), the Kushiro Submarine Canyon is presumed not
to be a pathway for turbidity currents during Holocene, because an
accumulation of thick mud lies in the channel. Tephras of Ta-a (AD1739),
Ko-c2 (AD1694), Ta-b (AD1667), and Us-b (AD1663) were identified, and
used to define the recurrence ages of the turbidite depositions. The
provenance of the turbidites in core GH03-1034 collected from the middle
part of the canyon was inferred to be the northern slope of the canyon
deeper than the shelf edge, on the basis of sand composition and benthic
foraminiferal analysis. Strong seismic shocks would have caused repeated
turbidity currents into the Kushiro Submarine Canyon. The determined
turbidite recurrences (rate of 68 years) are similar to the historical
(rate of 79.7 years) and estimated (rate of 77.4 years) recurrence
intervals of the earthquakes along the Kuril Trench.
Tsunami waves leave sedimentary signatures both onshore and offshore, although the latter are hardly known. The objective of the present study is to provide new evidence for the 2004 Indian Ocean tsunami deposits left on the inner continental shelf of the Andaman Sea (Thailand) and to identify diagnostic sedimentological and geochemical properties of these deposits. Based on extensive seafloor mapping, three sediment cores were selected for study and were analysed for their sedimentary structures, grain size composition, chemical elemental composition, physical properties and 210Pb activity. Sediment cores retrieved from shallow water (9-15 m) within 7.5 km off the shore revealed distinct event layers, which were interpreted as being tsunami deposits. These 20-25 cm thick deposits were already covered with post-tsunami marine sediments. They were composed of several units, marine sand layers alternating with poorly sorted mud with terrigenous and anthropogenic components, representing different hydrodynamic conditions (probably during run-up and backwash phase). These sedimentological observations were supported by geochemical and physical data and were confirmed using 210Pb dating. A sediment core taken from a depth of 57 m at a distance of 25 km offshore did not reveal clear event deposits. Comparisons with available data from offshore tsunami deposits showed that there is no single set of signatures that could be applied to identify this kind of deposits.
This study documents seafloor morphology and sediments based on multibeam, side-scan sonar and
boomer surveys, as well as sediment samples taken on the inner to mid shelf of the Andaman Sea after the
2004 Indian Ocean tsunami. Preservation of submarine relief in former underwater mining areas points to limited
impact of the tsunami, while channel structures parallel to the observed tsunami backwash indicate a possible
higher impact. Therefore, the tsunami impact seems to be focused on some areas. The impact was probably
most effective during the backwash, when stiff mud deposits containing grass, wood fragments and shells
were transported by high density backwash flows. Moreover, several boulders, which might have been
deposited during the tsunami backwash flow, were found in the channels in front of Pakarang Cape.
The 11 March 2011 off the Pacific coast of Tohoku (M
w 9.0) Earthquake ruptured a 200 km wide megathrust fault, with average displacements of ~15–20 m. Early estimates of the co-seismic slip distribution using seismic, geodetic and tsunami observations vary significantly in the placement of slip, particularly in the vicinity of the trench. All methods have difficulty resolving the up-dip extent of rupture; onshore geodetic inversions have limited sensitivity to slip far offshore, seismic inversions have instabilities in seismic moment estimation as subfault segments get very shallow, and tsunami inversions average over the total region of ocean bottom uplift. Seismic wave estimates depend strongly on the velocity structure used in the model, which affects both seismic moment estimation and inferred mapping to slip. We explore these ideas using a least-squares inversion of teleseismic P-waves that yields surprisingly large fault displacements (up to ~60 m) at shallow depth under a protrusion of the upper plate into the trench. This model provides good prediction of GPS static displacements on Honshu. We emphasize the importance of poorly-constrained rigidity variations with depth for estimating fault displacement near the trench. The possibility of large slip at very shallow depth holds implications for up-dip strain accumulation and tsunamigenic earthquake potential of megathrusts elsewhere.
Sedimentary successions in small coastal lakes situated from 0 to 11 m above the 7000 year BP shoreline along the western coast of Norway, contain a distinctive deposit, very different from the sediments above and below. The deposit is interpreted to be the result of a tsunami inundating the coastal lakes. An erosional unconformity underlies the tsunami facies and is traced throughout the basins, with most erosion found at the seaward portion of the lakes. The lowermost tsunami facies is a graded or massive sand that locally contains marine fossils. The sand thins and decreases in grain size in a landward direction. Above follows coarse organic detritus with rip-up clasts, here termed ‘organic conglomerate’, and finer organic detritus. The tsunami unit generally fines and thins upwards. The higher basins (6–11 m above the 7000 year shoreline) show one sand bed, whereas basins closer to the sea level 7000 years ago, may show several sand beds separated by organic detritus. These alternations in the lower basins may reflect repeated waves of sea water entering the lakes. In basins that were some few metres below sea level at 7000 years BP, the tsunami deposit is more minerogenic and commonly present as graded sand beds, but also in some of these shallow marine basins organic-rich facies occur between the sand beds. The total thickness of the tsunami deposit is 20–100 cm in most studied sites. An erosional and depositional model of the tsunami facies is developed.
It was demonstrated that submarine mega mass wasting due to quite recent submarine eruptions (possibly including the Teisi Knoll eruption in 1989) should have occurred along the eastern continental slope of the Izu Peninsula, central Japan. Large area of the slope failed, and turbidite and debris flow deposits partially covered the sea bottom near Hatsushima Island, where deep-sea clam communities are associated with fluid venting and high heat flow. The catastrophe was evidenced from turbidite sequences having pumice fragments and shallow water benthic foraminifers, heat flow values, and side-looking sonar image including large scale of flow mountains. Significant increase of bottom depth since 1982 was observed as well as chaotic multi-ecograms of PDR both northward and southward from the deep-sea clam community site along the foot of the escarpment with a length of about 10km.
Three piston cores were extracted from three different turbidite fans developed near a coral reef region in the southern Ryukyu Island Arc, northwestern Pacific Ocean. The ages of these cores were determined by AMS (accelerator mass spectrometer) 14C dating and two planktic foraminiferal δ18O events. Many turbidity currents, recognized in the studied cores, were triggered mainly by earthquakes. We hypothesize that the age discrepancies of the turbidite layers, from core to core, were due to differences in the source areas and passages (submarine canyons) of the respective turbidity currents. A core from the youngest fan suggests that turbidity currents frequently occurred after ~10 Ka with ~1000 yr interval on average, possibly in relation to periodical earthquakes. This possibility is demonstrated in a multi-channel reflection survey over ~113km, near the core-sites, where a recent accretionary prism was recognized.
Fault rupture during the 2011 Tohoku-oki earthquake, which generated a huge tsunami, is thought to have propagated to a shallow part of the subduction zone. This observation calls into question conceptual models that assume that the shallow part of the plate boundary interface in a seismogenic subduction zone slips aseismically. However, the available observations of the earthquake and tsunami do not have sufficient resolution near to the subduction trench to determine whether coseismic fault slip extended all the way to the trench axis. Here we use seismic reflection data to image the subduction trench axis seawards of the Tohoku-oki earthquake epicentre. We compare an image of a profile taken in 1999 with one acquired along the same profile 11 days after the earthquake. Before the earthquake, we observe a triangular wedge of sediments at the trench axis. After the earthquake, we observe a deformed upheaval structure in the sedimentary layer that is 3km long and 350m thick. We suggest that this remarkable deformation structure formed as a result of compression during coseismic slip on the shallow plate interface, implying that fault rupture during the Tohoku-oki earthquake did reach the sea floor at the trench axis. We conclude that the shallow plate interface at the subduction trench axis can slip seismically.
We describe in detail possible large submarine landslides, several tens
of kilometers in length and width, on the trench landward slope of the
Japan Trench on the basis of high-resolution topographic surveys and
detailed seafloor observations. These slides stopped at the toe of the
trench slope. After initial movement of the toe along a basal
decollement or thrust of the trench landward slope wedge during an
earthquake, the basal frictional condition(s) might change drastically
from static to dynamic, thus reducing the frictional strength. As a
result, rapid submarine landsliding push downward on the toe, generating
large horizontal displacements for tsunamis. This hypothesis should
explain suitably the relation between large displacement of the thrust
fault and tsunami generation by the 2011 Tohoku earthquake as well as
tsunami generation by the 1896 Tohoku earthquake.
This paper examines the results of a numerical modeling of the 2011 Tohoku-oki earthquake tsunami to investigate its offshore propagation and inundation along a shore-normal transect located in the center of the Sendai Plain. The inundation distance, flow depth, and flow speed were compared with the available data measured during a post-tsunami field survey and estimated from the video records. The calculated inundation distance reached 4.5–5.5 km from the coastline, which is comparable to the actual inundation distance. The variation of tsunami heights from 2.4–6 m and flow speeds from 3.4–6.2 m/s along the transect is generally consistent with the measured heights and estimated speeds. The waveform on the beach showed that the wave train was composed of a high (10–11 m) first wave followed by low (~ 4 m) waves. Considering the waveform and the topographic change, the erosion of the beach and the sedimentation inland are explained mainly by the first wave. In addition, the calculated flow speed and friction velocities in the offshore may account for the formation of the possible tsunami deposits, which were reported recently by a sea-bottom survey. The modeling results are generally consistent with the available data and are considered to be useful for understanding the inundation pattern and sedimentation process of the Tohoku-oki tsunami on the Sendai Plain.
Turbidity currents may be generated in the oceans as part of the sequence from landsliding through debris flow to turbidity current flow. Three aspects of this sequence examined here are 1) the transition from landsliding to debris flow, 2) the mechanics of subaqueous debris flow, and 3) the transition from subaqueous debris flow to turbidity-current flow. The transition from landsliding to debris flow, as observed in the subaerial environment, occurs readily if water is incorporated into the landslide debris as it is jostled and remoulded during downslope movement. Remoulding and incorporation of water reduce the strength and increase the fluid behavior of the debris, thereby causing it to flow rather than slide. Incorporation of only a few percent water typically decreases the strength of landslide debris by a factor of two or more; therefore, landslide debris commonly becomes very fluid with incorporation of a small amount of water. The ready availability of water in the marine environment suggests that conditions are favorable for the development of subaqueous debris flows from subaqueous landslides. Debris flow has been modeled as flow of a plastico-viscous substance, which has a yield strength and deforms viscously at stresses greater than the yield strength. The conditions required for movement of a subaqueous debris flow are described in terms of a critical thickness of debris, which varies directly with strength and inversely with submerged trait weight and slope angle. Within a debris flow, viscous shear occurs where shear stress exceeds the shear strength of the debris, but where shear stress is less than shear strength the material is rafted along as a nondeforming plug. Distinct zones of viscous shear and nondeformation exist in a subaqueous debris flow. Transition from subaqueous debris flow to turbidity-current flow involves extensive dilution of debris-flow material, reducing the density from about 2.0 gm/cm3 to about 1.1 gm/cm3. In experiments, subaqueous debris-flow material was mixed with the surrounding water by erosion of material from the front of the flow and ejection of the material into the overlying water to form a dilute turbulent cloud (turbidity current). The amount of mixing, and hence the size of the turbidity current, varied inversely with the strength of the debris. Conditions that cause mixing at the front of a subaqueous debris flow are illustrated by analyzing flow around a half-body, with boundary-layer separation. Turbidity, currents also may be generated from subaqueous debris flows by mixing water directly into the body of the flow, behind the front, although this type of mixing was not observed in experiments. Mixing into the body of the flow can result from flow instability, either by breaking interface waves or by momentum transfer associated with turbulence, but available information suggests that mixing due to instability is inhibited by the presence of clay and coarse granular solids in debris. Mixing by erosion from the front of a debris flow is favored as being a more typical process of generating turbidity currents because this mixing is a natural consequence of debris flowing through water; it requires no special conditions to operate.
Transformation of turbidity currents into debris flows across a channel-to-lobe transition zone was recognized on the basis of lateral mapping of a falling-stage-systems-tract deposit in the lower Pleistocene Otadai Formation submarine-fan sequence on the Boso Peninsula, Japan. The most proximal outcrops comprise 1 to 4 in thick turbidites, which are laterally equivalent to debrites encased in turbidites; in outcrops farther down flow. Such turbidites and encased debrites; were deposited from single flow events. The most distal outcrops comprise only thinner (2 to 40 cm thick) turbidites. Debrites contain many siltstone clasts and finer-grained sediments, which are characterized by deep-water faunas and clay-mineral composition similar to those of interbedded hemipelagites. Therefore, the transformation of turbidity currents into debris flows is interpreted to have occurred in response to the incorporation of many siltstone clasts and liner-grained sediment particles into the precursor turbidity currents through erosion of muddy substrates. Subsequently, turbulence in the precursor turbidity currents was suppressed and near-bed flow with higher sediment concentration developed. Intense erosion of muddy substrates is interpreted to have occurred in response to the increase in intensity of turbulence in the precursor turbidity currents at the mouths of middle-fan channels. The precursor turbidity currents (or turbidity currents generated by dilution of debris flows) produced a turbidite deposit, which was subsequently overrun by the debris flow in a proximal lobe environment. The debris flows are interpreted to have subsequently been transformed into turbidity currents, which are recorded by turbidite deposition in the more distal-lobe environment. Debrites can characterize the proximal portion of an attached-lobe deposit in a channel-to-lobe transition zone. Such debrites; can be significant heterogeneities for fluid flows in sand-prone successions from middle-fan channel to attached-lobe deposits.
We present preliminary evidence for a 10,000-year earthquake record from
two major fault systems based on sediment cores collected along the
continental margins of western North America. New stratigraphic evidence
from Cascadia demonstrates that 13 earthquakes ruptured the entire
margin from Vancouver Island to at least the California border since the
eruption of the Mazama ash 7700 years ago. The 13 events above this
prominent stratigraphic marker have an average repeat time of 600 years,
and the youngest event 300 years ago coincides with the coastal record.
We also extend the record of past earthquakes to the base of the
Holocene (at least 9800 years ago), during which 18 events correlate
along the same region. The sequence of Holocene events in Cascadia
appears to contain a repeating pattern of events, a tantalizing first
look at what may be the long-term behavior of a major fault system.
On February 5, 1663, perhaps the largest earthquake ever witnessed in eastern North America struck the Saguenay Fjord basin with such force that landslides and submarine slides were triggered over a wide region. Over 3 km3 of clayey Holocene sediments collapsed from the margins of the fjord, resulting in amalgamated debris flow and slide deposits that covered much of the basin floor. These deposits reached thicknesses of up to 100 m, and extended over an area of 100 km2. The ambient seafloor was compressed under the impact and load of these debris masses, and/or liquefied under the cyclic stress of the seismic wave. A 0.2 km3 landslide also occurred at this time, blocking the Saguenay River and possibly facilitating an extra large spring freshet (discharge).Biotracers (planktonic and benthic foraminifera, arcellaceans, pollen and wood), in conjuction with mass physical properties of sediment cores and high-resolution seismic and sidescan profiler data, have been used to identify local and distal sediment sources, including their original water depth of deposition, and their subsequent mode of mass gravitational transport. The landslide material was eroded by the river, and super-elevated sediment concentrations are considered responsible for a long-lasting (28 days) turbidity current that entered the Saguenay Fjord depositing a single 0.3 km3 turbidite. The 2 to 10 m thick turbidite is considered to have been self-igniting having eroded into the surface of the collapsed basin debris. Paleo-hydraulic calculations suggest that this turbid flow averaged 0.45 m/s and reached a flow thickness of 30 m.
Dead specimens of a minute fusiform rotaliid foraminifer are common in the 28–63 μm fraction of multiple corer samples from a 4850 m-deep site on the Porcupine Abyssal Plain (PAP). Their test morphology is remarkably similar to small specimens of Stainforthia fusiformis (Williamson, 1858), a species which is well known from coastal settings (intertidal to outer shelf) around NW Europe and North America. A detailed comparison of the PAP form with typical individuals of S. fusiformis from Norwegian waters (55–203 m depth), however, reveals slight but consistent morphological differences. The PAP specimens are smaller (test length 40–140 μm) than those from Norway (test length 80–380 μm), the chambers tend to be rather less elongate, the density of pores in the test wall is much lower, and there are differences in apertural features. We therefore conclude that the diminutive abyssal form is a distinct species, here referred to as Stainforthia sp. This interpretation is consistent with increasing evidence for genetic differentiation in deep-sea organisms, particularly along bathymetric gradients. Stainforthia sp. was previously illustrated by Pawlowski as Fursenkoina sp. and appears to be widespread and abundant in the abyssal North Atlantic (>4000 m depth). Stainforthia fusiformis, on the other hand, is most abundant in continental shelf and coastal settings. It extends onto the continental slope in the North Atlantic but has not been reported reliably from depths greater than about 2500 m.
The Tsubutegaura conglomeratic tsunamiites occur in the middle section of the Miocene storm-related sand-silt alternation system in the Chita Peninsula, central Japan. Deposition of the system in the upper bathyal environment in a bay and synsedimentary seismic activity has been elucidated by palaeontological, palaeogeographical and sedimentological studies. Two coupled units, each built up by a conglomerate layer with an overlying sandstone layer and the alignment of lenticular sedimentary bodies are exceptional in this sequence. Some typical lenticular bodies, confined within sedimentary troughs, consist of a boulder bearing conglomerate layer and calcareous sandstone layer of the lower couple, and another conglomerate layer of the upper couple. The tuffaceous sandstone layer of the upper couple is distributed more widely than the other lithologies.The framework gravels in the coupled conglomerates form a clast-supported fabric and are quite notably monomictic. The clasts are angular and imbricated partly, forming peculiar gravel clusters due to traction current transportation.Conspicuous laminations develop in the coupled sandstone layers. Antidunes with chute and pool structure in the upper couple sandstone layer indicate deposition from an upper flow regime current. On the other hand, the calcareous composition of the lower couple sandstone layer reveals shallow-water provenance. Thixotropic deformations and diastasis cracks in the siltstone bed immediately beneath the layers of the couples show that a severe earthquake tremor and a rapid change of water pressure occurred just before the deposition of the tsunamiites.Submarine debris flows due to collapse of a fault scarp in a shallow bank and the ensuing wash by two pulses of the tsunami-induced ebb current comprise the scenario for the formation of the Tsubutegaura conglomeratic tsunamiites.
The tsunami wave induced by the collapse of the Santorini caldera after the Bronze age (Minoan) eruption (3500 BP) produced turbidites and large volume mega-turbidites in the abyssal plains of the Ionian Sea as well as on the floor of small basins of the Mediterranean and Calabrian Ridges, characterized by the so-called ‘Cobblestone Topography’. Since the first discovery in 1978, a Holocene mud layer which has been termed ‘homogenite’ and which typically shows a graded interval at its base, has been identified and correlated in over 50 deep-sea cores recovered in the eastern Mediterranean.
Mapping on high-resolution satellite images and in the field shows differences in landforms and characteristics of tsunami flow on two contrasting coastal plains following the giant earthquake on December 26, 2004: the plain of Banda Aceh on the northern tip of the Sumatra island, Indonesia, and the Nam Khem plain in the Andaman Sea coast of Thailand. The landforms of the Banda Aceh coastal plain are characterized as deltaic lowland with tidal plains in the western and central parts, and strand plain with beach ridges in the eastern part. The run-up tsunami flow invaded areas about 3–4 km from the coast. Strong tsunami flow severely damaged the tidal plain and the landforms along the coast except coastal dunes in the east. Most of the landforms except sand dunes along the coast had almost no effect on the protection against the tsunami, but the higher micro-landforms such as beach ridges and natural levees prevented the flow of the tsunami from its invasion into the inland near the end of tsunami inundation.The tsunami inundation spread out over the entire Nam Khem coastal plain with an average depth of 4–5 m. The direction of run-up flow was almost perpendicular to the coastline, whereas backwash flow directions were controlled by topography. Backwash flow was concentrated in the lower portions of the plain, for example in small stream channels. Wedge-shaped channels in the lower parts of the streams were formed due to the concentration of backwash flow. The existence of the swales between parallel beach ridges corresponds well with the distribution of thick tsunami deposits. Coastal erosion of the plain was caused by the direct attack of tsunami waves, and the lower reaches of small rivers were eroded by strong backwash flow.
Debris flows and turbidites have been described in piston cores, bottom photographs and side-looking sonar records from basin floors amid the rugged, irregular topography of the Calabrian Ridge. A sediment transport process is inferred which began with a rotational slump on the steep basin wall, subsequently transformed into a debris flow and turbid cloud. Such events occurred approximately every 1500 years. Because the debris flow/turbidite deposits are correlative between basins, some relatively widespread mechanism such as an earthquake is required to initiate the slope failure. The estimated intensity of ground shaking as a result of historical earthquakes on the nearby Calabrian and Hellenic Arcs has been used to develop a relationship between frequency of occurrence and intensity of ground shaking. Extrapolation of this relation to infrequent, large magnitude events shows that the 1500-yr event is small enough to be geologically plausible but large enough to cause slope failure on basin walls. It is possible that the historical record fortuitously contains one of these large magnitude, long repeat time events, which may have caused the most recent of the observed debris flows in 1903.
We detected and measured coseismic displacement caused by the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (M(W)) 9.0] by using multibeam bathymetric surveys. The difference between bathymetric data acquired before and after the earthquake revealed that the displacement extended out to the axis of the Japan Trench, suggesting that the fault rupture reached the trench axis. The sea floor on the outermost landward area moved about 50 meters horizontally east-southeast and ~10 meters upward. The large horizontal displacement lifted the sea floor by up to 16 meters on the landward slope in addition to the vertical displacement.
Micropaleontological analysis of nearshore to offshore sediments recovered from the southwestern coast of Thailand was performed to clarify the submarine processes of sediment transport and deposition during the 2004 Indian Ocean tsunami. The distribution pattern of benthic foraminifers showed seaward migration after the tsunami event. Agglutinated foraminifers, which are characteristic of an intertidal brackish environment, were identified in the post-tsunami samples from foreshore to offshore zones. These suggest that sediments originally distributed in foreshore to nearshore zones were transported offshore due to the tsunami backwash. On the other hand, the distribution pattern of planktonic and benthic species living in offshore zones showed slight evidence of landward migration by the tsunami. This suggests that landward redistribution of sediments by the tsunami run-up did not occur in the offshore seafloor of the study area. Our results and a review of previous studies provide an interpretation of submarine sedimentation by tsunamis. It is possible that tsunami backwashes induce sediment flows that transport a large amount of coastal materials seaward. Thus, traces of paleotsunami backwashes can be identified in offshore sedimentary environments as the accumulation of allochthonous materials. This can be recognized as changes in benthic foraminiferal assemblages. © 2009 The Authors Journal compilation © 2009 Blackwell Publishing Asia Pty Ltd.
Tsunami sedimentation:an example from Indian Ocean Tsunami in southwestern Thailand
- S Fujino
- H Naruse
- K Fujita
- A Suphawajruksakul
- T Jarupongsakul
Numerical modeling of the
- D Sugawara
- K Goto
Sugawara, D. and Goto, K., 2012, Numerical modeling of the 2011