Sediment Transport and Hydrodynamic Parameters of Tsunami Waves Recorded in Onshore Geoarchives (original) (raw)
Pure and Applied Geophysics, 2007
The Andaman-Sumatra Tsunami of Dec. 26, 2004, was by far the largest tsunami catastrophe in human history. An earthquake of 9 to 9.3 on the Richter scale, the extension of waves over more than 5000 km of ocean and run-ups up to 35 m are its key features. These characteristics suggest significant changes in coastal morphology and high sediment transport rates. A field survey along the west coast of Thailand (Phuket Island, Khao Lak region including some Similan Islands, Nang Pha mangrove areas and Phi Phi Don Islands) seven to nine weeks after the tsunami, however, discovered only small changes in coastal morphology and a limited amount of dislocated sediments, restricted to the lower meters of the tsunami waves. This is in striking contrast to many paleo-tsunami's events of the Atlantic region. Explanations for this discrepancy are sought in: a. Mechanics of the earthquake. A rather slow shock impulse on the water masses over the very long earthquake zone, b. Shallow water in the earthquake zone, and c. Bathymetry of the foreshore zone at the impacted sites. Shallow water west of Thailand has diminished wave energy significantly.The differences in geomorphological and sedimentological signatures of this tsunami compared with many paleo-tsunami worldwide makes it unsuitable to be used as a model for old and future tsunami imprints by an event of this extreme energy and extension.
Quaternary International, 2012
Where historical records are short and/or fragmentary, geological evidence is an important tool to reconstruct the recurrence rate of extreme wave events (tsunamis and/or storms). This is particularly true for those coastal zones around the Indian Ocean, where predecessors of similar magnitude as the 2004 Indian Ocean Tsunami (IOT) have not been reported by written sources. In this context, the sedimentary record of the Holocene coastal plain of Ban Bang Sak (Phang-nga province, Thailand) provides evidence of multiple prehistoric coastal flooding events in the form of allochthonous sand beds, which were radiocarbon dated to 700-500, 1350-1180, and younger than 2000 cal BP. The layers were assigned to high-energy events of marine origin, which could be either tsunamis or tropical storms, by means of granulometry, geochemistry, vertical structure, and macrofossil content. Although no landfall of a strong storm has occurred in the last 150 years of meteorological data recording, cyclones cannot be ruled out for the last centuries and millennia. However, discrimination between tsunami and storm origin was mainly based on the comparison of the palaeoevent beds with the local deposit of the IOT, which revealed similar characteristics in regard to spatial extend and sediment properties. Furthermore, the youngest palaeoevent correlates with contemporaneous deposits from Thailand and more distant coasts. Hence, we relate it to a basin wide tsunami which took place 700-500 years ago. For the sediments of older extreme events, deposited between 2000 and 1180 cal BP, we found no unambiguous counterparts at other sites; nevertheless, at least for now, they are treated as tsunami candidates.
Marine Geology, 2007
We investigated horizontal and vertical variations in modern tsunami deposits along two transects at Nam Khem and Khao Lak, western coast of Thailand, deposited by the large tsunami associated with the earthquake (magnitude 9.0) of 26 December 2004 off Sumatra, Indonesia. Tsunami waves 6-10 m high struck the area approximately 2 h after the earthquake. Tsunami deposits cover the low-lying coastal plains and extend more than 1 km inland from the shoreline. No landward decrease in sediment thicknesses was found clearly at either transect. Terrace scarps and a steep slope behind the coastal plain probably stopped tsunami deposition further inland, causing substantial sediments to be deposited in front of these features. Very clear vertical variations in grain size and multiple layers are found in the deposits to about 600 m inland at Nam Khem. Fine-grained sediments overlie the coarsegrained sediments of the basal layer of the tsunami deposits. At some sites, the fine-grained sediments are overlain by another layer of coarse-grained sediments, suggesting deposits laid down in succession by multiple waves. The basal coarse-grained sediments at Nam Khem fine landward. The up-flow waned inland based on the assumption that the grain size of the basal deposits relates to the strength of the up-flow. These results are potentially useful in disaster prevention and coastal environmental change management as well as for interpreting paleotsunami deposits in geological records.
Tsunamis versus storm deposits from Thailand
Natural Hazards, 2012
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.
Characteristics of 2004 tsunami deposits of the northern Tamil Nadu coast, southeastern India
The 2004 Indian Ocean tsunami left significant sand deposits along the coastal tract of southeast India (Tamil Nadu state). These deposits serve as a benchmark to understand the effects of present day tsunami on the coastline. Additionally, the geological signatures of tsunami in the coastal stratigraphy can assist in providing modern analogs for identification and interpretation of ancient tsunami. This article presents the field observations of tsunami deposits, their internal stratigraphy and foraminiferal distribution, all of which varied from north to south depending upon coastal geomorphology, near shore bathymetry and sediment sources. In a few places, the tsunami deposits have been reworked due to subsequent events that caused modification in the internal stratigraphy. The tsunami deposits of the northern Tamil Nadu coast comprise at least 50% or more reworked foraminiferal specimens, indicating that the tsunami sediments may have been derived from a paleostrandline from a water depth of at least 45 m Key words: 2004 tsunami, geomorphology, foraminifera, sand deposits, southeast coast of India.
Natural Hazards, 2012
The Indian Ocean tsunami flooded the coastal zone of the Andaman Sea and left tsunami deposits with a thickness of a few millimetres to tens of centimetres over a roughly one-kilometre-wide tsunami inundation zone. The preservation potential and the post-depositional changes of the onshore tsunami deposits in the coastal plain setting, under conditions of a tropical climate with high seasonal rainfall, were assessed by reinvestigating trenches located along 13 shore-perpendicular transects; the trenches were documented shortly after the tsunami and after 1, 2, 3 and 4 years. The tsunami deposits were found preserved after 4 years at only half of the studied sites. In about 30% of the sites, the tsunami deposits were not preserved due to human activity; in a further 20% of the sites, the thin tsunami deposits were eroded or not recognised due to new soil formation. The most significant changes took place during the first rainy season when the relief of the tsunami deposits was levelled; moderate sediment redeposition took place, and fine surface sediments were washed away, which frequently left a residual layer of coarse sand and gravel. The fast recovery of new plant cover stabilised the tsunami deposits and protected them against further remobilisation during the subsequent years. After five rainy seasons, tsunami deposits with a thickness of at least a few centimetres were relatively well preserved; however, their internal structures were often significantly blurred by roots and animal bioturbation. Moreover, soil formation within the deposits caused alterations, and in the case of thin layers, it was not possible to recognise them anymore. Tsunami boulders were only slightly weathered but not moved. Among the various factors influencing the preservation potential, the thickness of the original tsunami deposits is the most important. A comparison between the first post-tsunami survey and the preserved record suggests that tsunamis with a run-up smaller than three metres are not likely to be preserved; for larger tsunamis, only about 50% of their inundation area is likely to be presented by the preserved extent of the tsunami deposits. Any modelling of paleotsunamis from their deposits must take into account post-depositional changes.
Island Arc, 2010
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.
Stratigraphic evidence for pre-2004 tsunamis in southwestern Thailand
Marine Geology, 2009
The 2004 Sumatra-Andaman earthquake and the associated Indian Ocean tsunami vastly exceeded the size of their predecessors from the previous two centuries or more. We sought clues on how often large tsunamis occur by taking 9 shallow cores 0.5-2.0 km inland from the modern shore on a Holocene beach-ridge plain of Phra Thong Island, 125 km north of Phuket. We tentatively correlated one sand sheet among these cores and found a second sand bed beneath in one of the cores. We infer that these deposits represent two pre-2004 tsunamis. The island's setting precludes river floods and tends to rule out storms as causes of the sand deposition. Establishment of the beach-ridge plain requires that both the inferred tsunamis occurred after Holocene sea levels approached their present position. Radiocarbon ages of bulk soil samples are consistent with this conclusion. Though root penetration into soil samples probably makes the ages appear younger, one inferred tsunami occurred before AD 1300 and the other occurred before AD 1900, based on the age estimates. The tsunamis probably originated along the Sunda megathrust, from where tsunamis can travel directly to the Thailand coast.
Shallow water sediment structures in a tsunami-affected area (Pakarang Cape, Thailand)
The influence of tsunami on the floor is poorly understood. Detailed hydroacoustic surveys and sediment sampling campaigns were carried out in 2007 and 2008 offshore Pakarang Cape (Thailand) to catalogue the geomarine effects of the 2004 Indian Ocean tsunami. A major problem in determining tsunami influence in offshore deposits is the lack of pre-tsunami mappings. Starting in 15 m water depth, a system of sand ridges composed of coarse sand exists offshore Pakarang Cape. Elongated sediment transport structures on the NW-flanks of the sand ridges, slowly fading during the annual cycle, indicate the presence of a current oblique to the coastline. This current might coincide with the 2004 Indian Ocean Tsunami. A several cm-thick event layer found at the base of a sand ridge is composed of silty sediment, which could be related to the tsunami backwash or strong floods during the monsoon. These event deposits are covered by coarse sand. They might enter the geological record.