Conference Paper

The Indian Ocean tsunami 2004: Identification of Tsunami Deposits in the Andaman Sea by Different Proxis

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Abstract

It was doubted for a long time that in wave dominated coastal areas the impact of tsunami waves on shoreface deposits can be preserved. Following high resolution mapping with different hydroacoustic methods, positions for grab sampling and coring were identified, where tsunami deposits were supposed to occur. The sampled material was analyzed using a wide range of sedimentological, geochemical, micropalaeontological, chemical, and physical methods. Storm and tsunami event layers could be identified and distinguished. Individual layers, ranging from 12 - 39 cm in thickness, were interpreted as tsunami deposits. Run-up and backwash deposits could be distinguished. Based on foraminiferal transfer functions and textural analyses re-suspension of sediment during run-up seemed to be restricted to about 20 m water depth. On the other hand it could be shown by using PAHs as a chemical proxy that the loaded backwash extends up to 25 km offshore.

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Tsunamiite beds can provide excellent keys to stratigraphic correlations over great distances, as they can have a wide extent. On the other hand, they occur only sporadically because of the gigantic tsunami energy and due to a great variety in sedimentary settings. The cooperation of a few elements of water movement, that is, (1) shuttle movement of water currents and (2) deceleration (of the run-up tsunami) and acceleration (of the backwash tsunami) caused by gravitation, fundamentally results in a variety of tsunamiite features that depend on the variable conditions of the surrounding sedimentary settings and environments. Successive layers showing opposite palaeocurrent directions and intercalated mud drapes are characteristics and common features of tsunamiites in some environments such as coastal lakes and shallow-sea basins. Single sand sheets also occur inland, depending on their distance from the sea. An erosion surface may be the only record of a tsunami over an extensive coastal area where step-form run-up tsunami waves go forward and break and where backwash tsunami flows (currents) are markedly accelerated by gravitation. The eroded sediments, including organic material, are transported both offshore and landward, forming a surge with some density. Because of the nature of the tsunami-induced water movement, many kinds of current structures, including those that indicate high-energy regimes, are common structural components in tsunamiites. Hummocky and swaley structures occur in submarine tsunamiites of various environments. Rip-up mud clasts are also common in submarine tsunamiites, showing the special nature of tractive tsunami currents that lack internal shear stress except in the thin bottom boundary layer. Tsunamiites are lacking almost completely in modern deep-sea areas. The Mediterranean homogenites induced by the collapse of the Santorini caldera provide a rare example of tsunami-induced sediments in a deep-sea area. Deposits from extremely gigantic tsunami, such as some Cretaceous/Paleogene (K/Pq)-boundary tsunamiites, can be found in deep-sea records of the geological history. There is not one single convenient key to distinguish tsunami-generated sediments from those generated by other events. Integrated studies including facies analysis of the background environmental sediments are indispensable for identifying and clarifying tsunamiites and their implications.
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Along the Andaman (west) coast of Thailand, the 2004 tsunami depositional features associated with the 2004 tsunami were used to describe the characteristics of tsunamis in a place far away from the effect of both recent and ancient storms. The current challenge is that a lack of precise sedimentological characteristics have been described that will differentiate tsunami deposits from storm deposits. Here, in sedimentological senses, we reviewed the imprints of the sedimentological characteristics of the 2004 tsunami and older deposits and then compared them with storm deposits, as analyzed from the deposits found along the eastern (Gulf of Thailand; GOT) coast of Thailand. We discuss the hydraulic conditions of the 2004 tsunami and its predecessors, on the Andaman coast, and compare them to storm flows found on the coast of the GOT. Similar to an extensive tsunami inflow deposit, a storm flow overwash has very similar sedimentary structures. Well-preserved sedimentary structures recognized in sand sheets from both tsunami and storms include single and multiple normal gradings, reverse grading, parallel, incline and foreset lamina, rip-up clasts, and mud drapes. All these sedimentary structures verify the similarity of tsunami and storm inflow behavior as both types of high-energy flow start to scour the beach zone. Antidunes are likely to be the only unique internal sedimentary structures observed in the 2004 tsunami deposit. Rip-up clasts are rare within storm deposits compared to tsunami deposits. We found that the deposition during the outflow from both tsunami and storms was rarely preserved, suggesting that it does not persist for very long in the geological record. Keywords2004 Indian Ocean tsunami–Storm surge–Washover deposits–Flow regime–Andaman coast
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The Indian Ocean tsunami of 26 December 2004 reached maximum wave heights of 35 m in Aceh, the northernmost province of Sumatra. Both the tsunami and the associated Sumatra-Andaman earthquake were unprecedented in Acehnese history. Here we use sand sheets to extend tsunami history 1,000 years into Aceh's past. The 2004 tsunami deposited a sand sheet up to 1.8 km inland on a marshy beach ridge plain. Sediment cores from these coastal marshes revealed two older extensive sand sheets with similar sediment characteristics. These sheets, deposited soon after ad 1290-1400 and ad 780-990, probably resulted from earlier tsunamis. An additional sand sheet of limited extent might correlate with a documented smaller tsunami of ad 1907. These findings, a first step towards a palaeotsunami record for northern Sumatra, suggest that damage-causing tsunamis in Aceh recur infrequently enough for entire human lifetimes to typically elapse between them. Such recurrence adds to the challenge of preparing communities along the northern Indian Ocean shorelines for future tsunamis.
Article
Tsunami, storm and flash-flood event layers, which have been deposited over the last century on the shelf offshore Khao Lak (Thailand, Andaman Sea), are identified in sediment cores based on sedimentary structures, grain size compositions, Ti = Ca ratios and 210Pb activity. Individual offshore tsunami deposits are 12 to 30 cm in thickness and originate from the 2004 Indian Ocean Tsunami. They are characterized by (1) the appearance of sand layers enriched in shells and shell debris and (2) the appearance of mud and sand clasts. Storm deposits found in core depths between 5 and 82 cm could be attributed to recent storm events by using 210Pb profiles in conjunction with historical data of typhoons and tropical storms. Massive sand layers enriched in shells and shell debris characterize storm deposits. The last classified type of event layer represents reworked flash-flood deposits, which are characterized by a fining-upward sequence of muddy sediment. The most distinct difference between storm and tsunami deposits is the lack of mud and sand clasts, mud content and terrigenous material within storm deposits. Terrigenous material transported offshore during the tsunami backwash is therefore an important indicator to distinguish between storm and tsunami deposits in offshore environments.
Article
In regions with a short historical tsunami record, the assessment of long-term tsunami risk strongly depends on geological evidence of prehistoric events. Whereas dating tsunami deposits is already well established, magnitude assessment based on remaining sedimentary structures is still a major challenge. In this study, two approaches were applied to deduce transport processes and hydrodynamic parameters of tsunami events from onshore deposits found in the coastal plain of Ban Bang Sak, SW Thailand: (1) The maximum offshore sediment source was determined using granulometry, geochemistry, mineralogy and foraminifera of the tsunamites, and reference samples from various marine and terrestrial environments, and (2) the onshore flow velocities and flow depths of associated tsunami waves were estimated by means of sedimentation modelling. In the case of the Indian Ocean tsunami (IOT) of 2004, modelled flow velocities of 3.7 to 4.9 m/s, modelled onshore flow depths of up to 5.5 m, and a sediment source from offshore areas shallower than a 45-m water depth—including littoral sediments transported as bedload and suspended load from the shallow subtidal zone—are in agreement with quotations based on survivor videos and posttsunami surveys. For a 500-to 700-year-old predecessor, comparable flow velocities and flow depths of 4.1 to 5.9 m/s and 4.0 to 7.5 m, respectively, were modelled, indicating a similar magnitude as the IOT 2004. Comparable values of maximum transport distance and depth of wave erosion were also found. In the case of three older tsunami candidates, dated to 1180 to 2000 cal BP, the deposits indicate partly similar source areas with water depths of less than 45 m and partly shallower source areas restricted solely to the beach. Whereas the former tsunamis are interpreted as events similar to 2004, the latter are more likely storms or tsunamis of a lower magnitude.
Article
Although it is accepted that large tsunami waves impact the sea floor, the response of surface sediments to tsunami is not yet fully understood. Tsunami by the 2011 off the Pacific coast of Tohoku earthquake caused considerable damage to Northeast Japan. Surface sediments on the outer shelf of Sendai Bay also show evidence of strong ground motion and agitation by the subsequent tsunami. Sediments in the hemipelagic mud below the erosional surface contain vein-like structures, providing evidence of strong ground motions induced by the earthquake. Two turbidite sequences are present above the erosional surface, in which 134Cs and 137Cs only occur in the upper turbidite. Unconsolidated mud on the inner–mid Sendai Shelf was agitated and resuspended by the high frictional velocities associated with the tsunami waves. The high concentration of suspended sediment in the water column above the shelf is likely to have generated turbidity currents and have made the lower turbidite. After the release of 134Cs by the accident at the Fukushima No.1 Nuclear Power Plant, the second turbidity current generated and created the upper turbidite.
Article
In this paper we summarize the regional setting, our previous understanding of historical and pre-historical tsunamis on the Pacific coast of Tohoku, Japan, prior to the 2011 Tohoku-oki tsunami, and our current understanding of the sedimentological, geochemical and paleontological features of the onshore and offshore deposits of the event. Post-tsunami surveys revealed many new insights, such as; (1) the maximum extent of the sand deposit is sometimes only 60% of the inundation distance, (2) the inundation limit can be estimated by geochemical analysis even a few months after the event, (3) a minor amount of marine sediment was transported inland by the tsunami on the Sendai and adjacent plains with the major sediment sources being from beach and dune erosion or vented sediments from liquefaction, although nearshore and offshore surveys revealed that there was a significant amount of sediment transport on the seafloor, (4) coarse gravel deposits (~ 1 m in thickness) were usually thicker than the sand ones (~ 30 cm in thickness), and (5) beach erosion was minimal in some places while severe in others. Another important aspect of this event is that it was a large, infrequent, tsunami that took place where possible predecessors (e.g., AD869 Jōgan) were already known to have occurred based on historical and geological evidence. The AD869 Jōgan tsunami deposits are noticeably similar to the 2011 Tohoku-oki sands, therefore suggesting that the Jōgan and its source mechanism may have been larger than previously thought. While we have learned many lessons from the 2011 Tohoku-oki event, more research is needed to provide reliable tsunami risk assessments around the world.
Article
The 11 March 2011 MW 9.0 Tohoku megathrust earthquake off the Pacific coast of Japan was a salient event in the history of Japan. The resultant huge tsunami (the 2011 Tohoku-oki tsunami) inundated a vast coastal area of northeastern Japan, causing widespread devastation. Twenty days after the tsunami, we analyzed the impact of the tsunami on the sea bottom of the Kesennuma inner bay using side-scan sonar to explore the damage and bathymetric change in the harbor. Herein we present the first direct evidence that the sea bottom sediments of around 10–15 m were largely reworked by the tsunami to thickness of a few meters, and that large dunes were formed by the tsunami. Considering that the sea wave influence is as weak as it is inside the inner bay, the potential exists that even meter-thick paleo-tsunami deposits are preserved in shallow sea bottoms with large bedforms. This finding will be a stepping-stone to future geological studies of tsunami effects in shallow sea regions.
Article
The total concentrations of twelve, likely carcinogenic, polycyclic aromatic hydrocarbons (PAHs) (i.e., phenanthrene (Phe), anthracene (An), fluoranthene (Fluo), pyrene (Pyr), benz[a]anthracene (B[a]A), chrysene (Chry), benzo[b]fluoranthene (B[b]F), benzo[k]fluoranthene (B[k]F), benzo[a]pyrene (B[a]P), indeno[1,2,3-cd]pyrene (Ind), dibenz[a,h]anthracene (D[a,h]A), and benzo[g,h,i]perylene (B[g,h]P) in backwash deposits of the 2004 Khao Lak tsunami were carefully investigated and compared with the concentrations of world marine sediments (WMS). In general, ∑12PAHs in this study (i.e., 69.43±70.67ngg(-1)) were considerably lower than those values observed in marine sediments from Boston (54,253ngg(-1)), coastal sediments from Barcelona Harbour (15,069ngg(-1)), and riverine sediment from Guangzhou Channel (12,525ngg(-1)), but were greater than values from coastal sediments in Rosas Bay (12ngg(-1)), Santa Ponsa Bay (26ngg(-1)) and Le Planier (34ngg(-1)). The total toxic benzo[a]pyrene equivalent (TEQ(Carc)) values calculated for Khao Lak coastal sediments (KCS), Khao Lak terrestrial soils (KTS), and Songkhla Lake sediments (SLS) were 10.3±12.2ngg(-1), 16.0±47.7ngg(-1), and 5.67±5.39ngg(-1), respectively. Concentrations of PAHs at all study sites resulted in risk levels that fell into the "acceptable" range of the US EPA model and were much lower than those of other WMS. The cancer risk levels of PAH content in KCS ranged from 7.44×10(-8) to 2.90×10(-7), with an average of 1.64×10(-7)±8.01×10(-8); this value is 119 times lower than that of WMS. In addition, soil cleanup target levels (SCTLs) for both non-carcinogens (i.e., Phe, An, Fluo and Pyr) and carcinogens (i.e., B[a]A, Chry, B[b]F, B[k]F, B[a]P, Ind, D[a,h]A and B[g,h,i]P) in the KTS samples were estimated for all target groups, with an average value of 115,902±197,229ngg(-1).
Article
The Sumatra-Andaman earthquake of December 26, 2004 is the first giant earthquake to occur since the advent of modern space-based geodesy and broadband seismology and therefore provides an unprecedented opportunity to investigate the characteristics of one of these most dreadful and rare events. We determine co-seismic and post-seismic deformation over the first month following the main shock using a variety of geodetic data. These include ground displacements from near-field Global Positioning System (GPS) surveys in northwestern Sumatra and in-situ paleogeodetic and remotely sensed observations of the vertical motion of coral reefs, campaign data and continuous GPS measurements from Thailand and Malaysia. The co-seismic model is mainly constrained from co-seismic displacement derived from daily solutions at 34 cGPS stations. It shows that earthquake ruptured the Sunda subduction megathrust over a distance of about 1300 km and a width of less than 150 km releasing a total moment of 6.7-7.0 1022 Nm, (equivalent to Mw=9.15. This moment is slightly in excess of the 6.2 1022 Nm moment released over the first 500s, as estimated from the inversion of seismic records. The latitudinal distribution of released moment derived from the two models compare remarkably well. This pattern is also found consistent with the 500s long source time function and rupture velocity derived from T waves recorded in the Indian Ocean. Finally, this co-seismic model is found consistent with the observed tsunami as measured from altimetric satellite measurements of the tsunami by JASON and TOPEX, as well as with the arrival times of the tsunami recorded by tide gage records at a number of sites bordering the Indian Ocean and Andaman Sea. We find no need for slow slip or delayed slip as proposed in some early studies. However, the geodetic data postdating the main shock by up to 40 days, require that slip must have continued on the plate interface after the 500s long seismic rupture. The corresponding additional geodetic moment is about 1.5 1022 Nm, representing about 20 per cent of the co-seismic moment release. Comparison with the moment released by aftershocks, which amounts less than 1 per cent over the same period, shows that this deformation was mostly aseismic. Constraints on the depth distribution of afterslip are loose, but it seems that it must have occurred at depths less than about 50km, both updip and downdip of the seismically ruptured area. Time evolution of afterslip is consistent with rate-strengthening frictional afterslip. The proportion of aseismic slip is larger to the north, possibly due to the effect of the thick sediment cover entering the trench. These data shed some light on the physical parameters controlling the mode of slip along the plate interface. The ruptured area seems to coincide with the portion of the plate interface shallower than about 40km that was locked before the earthquake, as indicated from the previous background seismicity. From modelling the thermal structure of the plate interface we found that the position of the downdip end of the Locked fault Zone (LFZ) might be governed by temperature, with a transition to aseismic creep occurring around 300 to 350C, possibly controlled by the rheology of quartzo-feldspathic rocks dragged along the plate interface.
Article
The post-tsunami runups on the damaged Andaman Sea coastline of Thailand from the tsunami of 26 December 2004 were surveyed by Thai and Korean teams for 99 transects from 23 January to 7 February 2005. The highest runup in Thailand was 15.68 m at Cape Coral, in Phang-nga province, and the longest inundation distance was 3 km at Bang Nieng, in Phang-nga province. The causes of the variation in runup were analyzed by using the method of splitting tsunami (MOST) model, tide gauges, satellite imagery, and field data with topographic charts. The distribution of runups reflects the effects of bathymetry, coastal topography, coastline configuration and slope, the pattern and density of land use, and the biological and geomorphological characteristics of offshore and near-shore areas.
Article
In the past, marine sedimentologists have been unable to quantitatively investigate modern sediment accumulation on continental shelves, but recent development of Pb-210 geochronology provides a tool to overcome this limitation. It is used in this study to examine the accumulation of modern Columbia River sediments on the Washington continental shelf.Pb-210 profiles in Washington shelf sediments reveal three characteristic regions: a homogeneous surface layer (about 10 cm thick) where sediments are actively mixed by physical and biological processes, a region where Pb-210 activities decrease logarithmically with depth in the sediment, and a lower region of background activities. The surface mixed layer generally reflects erosion by wave and current activity in inner shelf sediments (shallower than about 60 m), and biological mixing in mid and outer shelf sediments. The region of logarithmic activities (i.e., radioactive decay) in the Pb-210 profiles provides the rate of sediment accumulation, which is on the order of mm/yr. The predominant depositional feature is a mid-shelf silt deposit in which accumulation rates progressively decrease north-northwestward away from the Columbia River.These results are consistent with other sedimentological observations, and indicate that Pb-210 geochronology not only allows determination of modern sediment accumulation rates, but also provides additional insight into processes affecting accumulation. Pb-210 geochronology can be an important sedimentological tool for future studies of continental shelf sedimentation in areas undergoing modern sediment accumulation.
Article
Distinctive diatom assemblages may be associated with tsunami sediments and may often contrast with the assemblages found within sediments underlying the tsunami deposit as well as those associated with the modern coastal environment. Sediments associated with the 1998 tsunami that destroyed much of the Sissano lagoon area in northern Papua New Guinea have been investigated. Surface sediments from three transects across the sediment spit near Warapu have been examined for diatom content and preservation. The preservation is variable, and the data show an, often chaotic, assemblage that can be attributed to the tsunami waves incorporating and depositing diatoms from distinctive habitat zones during their runup and subsequent backwash. The diatoms identified within the Warapu sediments indicate an origin from within the inter-tidal and offshore area rather than from the beach–sand spit complex. The sand deposits disclose a high percentage, in excess of 75%, of broken diatom valves, and a predominance of centric (circular) species due to preferential preservation. The study demonstrates that the application of diatom biostratigraphy to modern tsunami deposits can be used in conjunction with other stratigraphical lines of evidence to interpret the source and provenance of historical and palaeo-tsunami deposits.
Article
The 11th March 2011 Tohoku-oki tsunami inundated the low-lying Sendai Plain (Japan) more than 5 km inland leaving sand and mud deposits over most of the area. In order to establish the sources of the tsunami deposits and interpret processes of their sedimentation, samples were collected from the deposits, underlying soils and the beach along a shore-perpendicular transect and analysed for grain size, diatom assemblages and nannoliths. The fining-inland tsunami deposits consisted of poorly to moderately sorted medium to coarse sand within 2 km of the coastline and very poorly to poorly sorted mud farther inland. More specifically, there was a slight fining of the coarse to medium sand mode within the sandy deposits and an increased contribution of the coarse and very coarse silt fraction in the mud deposits. The tsunami deposits also exhibited vertical changes including fining upward and coupled coarsening-fining upward trends. Few diatoms were present in beach sediments, soils and tsunami deposits within 1 km of the coastline, while diatoms were more abundant farther inland. Diatom assemblages in the soil and tsunami deposits were similar and dominated by species typical of freshwater-brackish habitats, while no typically marine species were encountered. Nannoliths were generally absent in the studied sediments, except for few specimens. Our data indicate that there was probably no or only a very minor component of marine sediments transported onland by the tsunami. The sandy tsunami deposits within ~ 1 km of the coastline were mostly derived from beach and dune erosion. From 1 to 2 km landward the contribution of these sources decreased, while sources comprising local soil erosion and the entrainment of sediments from the Teizan-bori canal increased. Farther inland, local soil erosion was the major sediment source for the mud deposits. The tsunami deposits were most likely deposited during at least two inundations, mostly out from suspension resulting in an upward grain size fining trend. However, bed load deposition was also important in the sandy deposits as suggested by a common grain size upward coarsening, position on C–M diagrams and frequent horizontal lamination. The present study reveals that even very large tsunamis may not transport marine sediments onland and thus many commonly applied indicators of tsunami deposits based on the assumption of their offshore origin (marine diatoms, foraminifera, nannoliths, marine sediments) may be of limited use.
Article
The 2011 Tohoku-oki tsunami left sand and mud deposits more than 4 km inland on the coastal plain of Sendai, Japan. The tsunami deposits, pre-tsunami soils and beach sediments were analysed for grain size, and heavy mineral content and assemblages to test the applicability of heavy mineral analyses in the identification of tsunami deposits and interpretation of associated sedimentation processes. Heavy minerals comprised on average 35% of the tsunami deposit in the 0.125–0.25 mm grain size fraction. The most common were orthopyroxenes, clinopyroxenes, amphiboles, limonites and opaque minerals. Heavy mineral concentrations and assemblages were similar in the tsunami deposits, beach and pre-tsunami soils and sediments and thus tsunami deposits could not simply be identified based on their heavy minerals. Sediment provenance analysis revealed that tsunami deposits left within 1.5 km of the shoreline were mostly eroded from the beach, dune and local soils, while deposits farther inland (> 1.5 km) were mostly derived from local soil erosion. No evidence was found for a significant contribution of offshore sediments. Detailed analyses revealed that the lowermost portion of tsunami deposits was mostly of local origin, while the sediment source of the upper portion was variable. A comparison with a previous study of heavy minerals in 2004 IOT deposits confirms that heavy minerals in tsunami deposits are mostly source-dependent and may represent a useful supplementary tool in studies of tsunami deposits. However, the interpretation must always be placed in the local geological context and corroborated with other “tsunami proxies”.
Article
Foraminiferal tests are commonly found in tsunami deposits and provide evidence of transport of sea floor sediments, sometimes from source areas more than 100 m deep and several kilometers away. These data contribute to estimates of the physical properties of tsunami waves, such as their amplitude and period. The tractive force of tsunami waves is inversely proportional to the water depth at sediment source areas, whereas the horizontal sediment transport distance by tsunami waves is proportional to the wave period and amplitude. We derived formulas for the amplitudes and periods of tsunami waves as functions of water depth at the sediment source area and sediment transport distance based on foraminiferal assemblages in tsunami deposits. We applied these formulas to derive wave amplitudes and periods from data on tsunami deposits in previous studies. For some examples, estimated wave parameters were reasonable matches for the actual tsunamis, although other cases had improbably large values. Such inconsistencies probably reflect: (i) local amplification of tsunami waves by submarine topography, such as submarine canyons; and (ii) errors in estimated water depth at the sediment source area and sediment transport distance, which mainly derive from insufficient identification of foraminiferal tests.
Article
A combined approach of field geology and numerical simulation was conducted for evaluating the tsunami impacts on the shelf sediments. The 2003 Tokachi-oki earthquake, M 8.0, that occurred on 25 September 2003 off southeastern Hokkaido, northern Japan, generated a locally destructive tsunami. Maximum run-up height of the tsunami waves reached 4 m above sea level. In order to estimate the tsunami impacts on shallow marine sediments, we compared pre-and post-tsunami marine sediments in water depths of 38–112 m in terms of grain size, sedimentary structure, and microfossil content. Decreases of fine fractions, especially finer than very fine sand, which led to coarsen the mean grain size, were detected in the inner shelf of the northern part of the study area. On the other hand, the other shelf sediments largely unchanged or slightly fined. Foraminiferal analysis also indicated a change in the surficial sediments. We also simulated the tsunami wave velocity and direction, and grain size entrained by the tsunami. The numerical simulation resulted in that the 2003 tsunami could transport very fine sand in water depths of shallower than 45–95 m at the northern part of the study area. This is comparable with the actual grain-size changes after the tsunami had passed. However, some storms and tidal currents might also be possible to stir the surface sediments after pre-tsunami survey, so we could not conclude the grain-size changes had been caused by the tsunami. Nevertheless, a combined approach of sampling and modeling was powerful for estimating the tsunami impacts under the sea.
Article
The 2004 Indian Ocean tsunami devastated the coastal areas along the Andaman western coast of Thailand and left unique physical evidence of its impact, including the erosional landforms of the pre-tsunami topography. Here we show the results from monitoring the natural recovery of beach areas at Khuk Khak and Bang Niang tidal channels of Khao Lak area, Phang-nga, Thailand. A series of satellite images before and after the tsunami event was employed for calculating the beach area and locating the position of the changed shoreline. Field surveys to follow-up the development of the post-tsunami beach area were conducted from 2005 to 2007 and the yearly beach profile was measured in 2006. As a result, the scoured beach areas where the tidal channel inlets were located underwent continuous recovery. The return of post-tsunami sediments within the beach zone was either achieved by normal wind and wave processes or during the storm surges in the rainy season. Post-2004 beach sediments were derived mainly from near offshore sources. The present situation of the beach zone has almost completed reversion back to the equilibrium stage and this has occurred within 2 years after the tsunami event. We suggest these results provide a better understanding of the geomorphological process involved in beach recovery after severe erosion such as by tsunami events.
Article
Tsunami hazard assessment begins with a compilation of past events that have affected a specific location. Given the inherent limitations of historical archives, the geological record has the potential to provide an independent dataset useful for establishing a richer, chronologically deeper time series of past events. Recent geological studies of tsunami are helping to improve our understanding of the nature and character of tsunami sediments. Wherever possible, geologists should be working to improve the research ‘tool kit’ available to identify past tsunami events. Marine foraminifera (single celled heterotrophic protists) have often been reported as present within tsunami-deposited sediments but in reality, little information about environmental conditions, and by analogy, the tsunami that deposited them, has been reported even though foraminifera have an enormous capacity to provide meaningful palaeo-environmental data. Here, we review what foraminifera are, describe their basic form and significance, summarise where they have been reported in tsunami sediments and identify what can be learnt from them. We review the gaps in our understanding and make recommendations to assist researchers who examine foraminiferal assemblages in order to enhance their use within tsunami geology.
Article
The degree of post-depositional alteration and hence the preservation potential of sedimentary event beds and transient signals is determined by the outcome of a ‘race’ between biological (and to a lesser extent physical) processes that conspire to destroy a signal and sediment accumulation which advects the layer or signal out of the surface mixing zone. Preservation potential specifically depends on the relative magnitudes of the (1) biological mixing intensity (sometimes parameterized as a biodiffusivity, Db), (2) mixing-layer thickness, (3) layer or signal thickness, and (4) sediment accumulation rate. These terms control the dissipation time (i.e. time required to destroy a signal) and the transit time (i.e. time necessary to advect a signal through the surface mixing layer). On fine-grained, upper continental margins in general, and the Eel River shelf (northern California) in particular, biological mixing intensity is high (i.e. Db ranges from 10 to 100 cm2/yr), mixing-layer thickness is large (>10 cm), and sediment accumulation rates are rapid (0.1–1 cm/yr). Despite the high sediment accumulation rates, transit times through the surface mixing-layer range from decades to a century. Signal dissipation times are considerably shorter: (1) <3 yr for beds up to 6 cm thick imaged in X-radiographs, and (2) <15 yr for the grain-size signature of beds up to 8 cm thick. Therefore event layers and their corresponding grain-size signature have a low probability of preservation. However, short-lived episodic sedimentation events (e.g. oceanic floods) can instantaneously advect material through the surface mixing layer, thereby preserving event beds and transient signals. On the Eel River shelf the sequential timing of episodic sedimentation events has exerted a first-order control on the resultant stratigraphic record (presence/absence of layers and grain-size fluctuations). Episodic sedimentation – a hallmark of continental shelf settings – is key to understanding strata formation and preservation on margins.
Article
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.
Article
Numerical model simulations, combined with tide-gauge and satellite altimetry data, reveal that wave amplitudes, directionality, and global propagation patterns of the 26 December 2004 Sumatra tsunami were primarily determined by the orientation and intensity of the offshore seismic line source and subsequently by the trapping effect of mid-ocean ridge topographic waveguides.
Article
The Taunton River is a partially mixed tidal estuary in southeastern Massachusetts (USA) which has received significant contaminant inputs, yet little information exists on the history of discharge and the subsequent fate of these contaminants. Three sediment cores taken along a transect were analyzed, reconstructing the spatial and temporal trends of pollution in the estuary. A combination of radiometric dating, contaminant markers, and storm layers from major hurricanes were used to establish age models and sedimentation rates. Age estimates obtained from the different dating methods compared well, establishing an accurate history of contaminant release to the estuary. Polycyclic aromatic hydrocarbons (PAHs) were present in one core at depths corresponding to the early 1860s, earlier than previously established dates of introduction. Temporal and spatial trends of Cr, Cu, Hg and Pb indicated multiple sources of varying input to the river. Polychlorinated biphenyls (PCBs) were present in each of the cores from the 1930s onward, with elevated levels still present in surficial sediments at several sites. A unique organic compound, Topanol, which was produced locally was used as a tracer to track contaminant transport in the river. Tracer data indicates that contaminants are still being transported and deposited to surficial sediments at high concentrations well after their discharge. This reconstruction demonstrates the utility of using multiple dating proxies where often the sole use of radiometric dating techniques is not an option and provides insights into the fate of contaminants discharged decades ago but continue to represent environmental risks.
Erosion and sedimentation from the 17
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Chlieh, M., Avouac, J.-P., Hjorleifdottir, V., Song, T.-R. A., Ji, C., Sieh, K., Sladen, A., Hebert, H., Prawiorodirdjo, L., Bock, Y., Galetzka, J. (2007). "Coseismic Slip and Afterslip of the Great Mw 9.15 Sumatra-Earthquake of 2004", Bull. Seismol. Soc. Am., 97, 152 -173.