Offshore Tsunami Deposits Research Articles

Luminescence dating of tsunami deposits from the algarve shelf, Portugal

Offshore tsunami deposits have the potential to substantially improve our understanding of long-term tsunami hazard. This may be particularly true for the Portuguese Algarve coast, where onshore sediment records are restricted to the last 3000 years due to scarcity of sediment archives suitable for identifying tsunami deposits. This study investigates the potential of optically stimulated luminescence (OSL) for dating offshore tsunami layers from the Portuguese shelf and its background sedimentation cored during METEOR cruise M-152, by comparing luminescence ages with radiocarbon chronologies. Accurate luminescence ages for both offshore tsunami deposits and constraining shelf sediments can be achieved by using single grains of quartz in combination with the minimum age model. However, even single grain ages significantly over-estimate by several thousand years for some very poorly bleached tsunami deposits and underlying shelf sediments affected by erosion during the event. This demonstrates the necessity of using chronologies that are based on a large number of ages, as well as a combination of direct tsunami ages and limiting ages for background sedimentation. The combination of luminescence ages from this study with radiocarbon ages using a Bayesian approach yields an age of 3600–3800 years for a predecessor event of the 1755 CE Lisbon tsunami, formerly radiocarbon dated in the same offshore sediment cores to ca. 3600 cal years BP.

New coring study in Augusta Bay expands understanding of offshore tsunami deposits (Eastern Sicily, Italy)

AbstractTsunami deposits present an important archive for understanding tsunami histories and dynamics. Most research in this field has focused on onshore preserved remains, while the offshore deposits have received less attention. In 2009, during a coring campaign with the Italian Navy Magnaghi, four 1 m long gravity cores (MG cores) were sampled from the northern part of Augusta Bay, along a transect in 60 to 110 m water depth. These cores were taken in the same area where a core (MS06) was collected in 2007 about 2·3 km offshore Augusta at a water depth of 72 m below sea level. Core MS06 consisted of a 6·7 m long sequence that included 12 anomalous intervals interpreted as the primary effect of tsunami backwash waves in the last 4500 years. In this study, tsunami deposits were identified, based on sedimentology and displaced benthic foraminifera (as for core MS06) reinforced by X‐ray fluorescence data. Two erosional surfaces (L1 and L2) were recognized coupled with grain‐size increase, abundant Posidonia oceanica seagrass remains and a significant amount of Nubecularia lucifuga, an epiphytic sessile benthic foraminifera considered to be transported from the inner shelf. The occurrence of Ti/Ca and Ti/Sr increments, coinciding with peaks in organic matter (Mo incoherent/coherent) suggests terrestrial run‐off coupled with an input of organic matter. The L1 and L2 horizons were attributed to two distinct historical tsunamis (ad 1542 and ad 1693) by indirect age‐estimation methods using 210Pb profiles and the comparison of Volume Magnetic Susceptibility data between MG cores and MS06 cores. One most recent bioturbated horizon (Bh), despite not matching the above listed interpretative features, recorded an important palaeoenvironmental change that may correspond to the ad 1908 tsunami. These findings reinforce the value of offshore sediment records as an underutilized resource for the identification of past tsunamis.

Internal structure of event layers preserved on the Andaman Sea continental shelf, Thailand: tsunami vs. storm and flash-flood deposits

Abstract. 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.

Shallow-marine deposits associated with the 2011 Tohoku-oki tsunami in Sendai Bay, Japan

Little is known about offshore tsunami deposits, although they have the potential to improve palaeotsunami reconstructions, especially in areas with limited preservation of onshore tsunami deposits. We analysed 44 sediment cores collected in 2012 at water depths of 14–30 m in Sendai Bay, north-eastern Japan, to identify the deposits of the 2011 Tohoku-oki tsunami with the aid of 134Cs released from the Fukushima-Daiichi Nuclear Plant. The preserved tsunami deposits in the inner shelf are interbedded with very fine sand and mud, showing similar features to storm deposits. In the shoreface, a distinct layer of coarse sand overlies fine sand, appearing to have originated as beach sand transported offshore by the tsunami backwash. The coarse sand fades offshore around the boundary between the shoreface and inner shelf. Many of the core sites show evidence that surface sediment was reworked, probably by post-tsunami storm waves, suggesting that further reworking would make the ultimate preservation of the tsunami deposits unlikely.

The 2011 Tohoku-oki tsunami — Three years on

Abstract 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. Offshore tsunami deposits related to the 2011 Tohoku-oki tsunami have also been well described. This recent research indicates that sedimentation and erosion in inner bay and open ocean (~ 20 m water depth) locations can be in the order of several meters, suggesting that the tsunami shear force was strong in the nearshore zone. On the other hand, sandy to muddy deposits a few centimeters thick were observed at about 100 to 6000 m water depth. It is likely that the tsunami resulted in resuspension of sea bottom sediments and that suspended material flowed downslope as a turbidity current or suspended flow, although many authors recognize the possibility that strong earthquake groundshaking might have also generated turbidity currents. Studies of the 2011 Tohoku-oki event have led researchers back to two of the fundamental issues of tsunami geology: understanding the linkage between onshore and offshore sedimentation and erosion, and establishing identification criteria for tsunami deposits. Moreover though, beyond the issue of simple tsunami geology, it is important for all researchers to communicate with governments and the general public in order to reduce future casualties by using risk assessments based on our understanding of infrequent large tsunamis.

Sedimentary deposits left by the 2004 Indian Ocean tsunami on the inner continental shelf offshore of Khao Lak, Andaman Sea (Thailand)

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.

Probable late Messinian tsunamiites near Monte Dei Corvi, Italy, and the Nijar Basin, Spain: expected architecture of offshore tsunami deposits

Three distinct, 30- to 80-cm-thick, graded, multilayered, coarse-grained sandstone layers, intercalated in the late Messinian mudstones of the Colombacci formation in Lago Mare facies of the Trave section are interpreted as tsunamiites (Ts1–Ts3). Each of these layers is sheet-like and could be followed along strike over several tens of meters. The lower two layers (Ts1–Ts2) occur in the lower part of the Colombacci formation and the third (Ts3) just below a conspicuous white “colombacci” limestone near the top of the formation. The three sandstone layers represent unique sedimentary events within the 120-m-thick San Donato-Colombacci mudstones, which contain many thin, fine-grained, possibly storm-related turbidites. Each of the three clastic layers is overall graded and strongly cross-bedded. A single layer consists of a stack of several graded sublayers that are eroded into the underlying mudstones and into each other. Absence of hummocky cross-stratification (HCS) indicates that the layers are not produced during a large, catastrophic storm event. Current ripples such as dm-sized trough cross-beds suggest strong, prolonged, unidirectional currents, capable of carrying coarse conglomeratic sands. Climbing ripples in middle-fine sand units indicate a high suspension load settling under waning current strength. Each of the Ts1–Ts3 beds satisfies a combination of criteria, described in this paper, that allow interpretation as a tsunamiite in an offshore environment. Tsunamiite Ts2 is underlain by a 15-cm-thick meshwork of synsedimentary fissures, filled with coarse sand. Ground movements induced by strong earthquakes probably caused these crevasses. The uniqueness of each layer, the erosion of the base of each of the sublayers into underlying mudstones and previously deposited sublayer and the consistent stacking of graded sandstone beds within each of the three layers, underlain by earthquake-produced fissures, strongly point to deposition by traction currents produced by the surges of a large tsunami event, triggered by a large vertical fault movements. Vertical fault displacements most likely occurred along the thrust faults like the Sibilline thrust at the SW of the Laga foreland basin, which were active at late Messinian times. A series of cyclic graded turbidites, underlain by seismically induced sand-filled fissures in the Late Messinian Feos formation in SE Spain, are interpreted as tsunamiites produced by a tsunami or seiche.

A Note on the Preservation of Offshore Tsunami Deposits

Abstract In this contribution we explore the preservation potential of offshore tsunami deposits. The application of linear wave theory and flat-bottom conditions allows a simplified representation of the physical environment. In such an environment it is possible to compute the boundary water depth, below which the influence of storm waves on tsunami deposits is negligible. The majority of the tsunami deposits described in the literature were deposited onshore where the tsunami wave is transformed into bores and rollers. These deposits differ from tsunami sediments of the deeper shelf, where the water-sediment interaction corresponds more to wave influences. We define the boundary depth db as the depth below which tsunami deposits cannot be reworked due to storm waves. This is based on the assumption that the wave base coincides with the seabed. If the wave base is below the bed, a reworking of bed sediments occurs. If it is above, no energy is transferred to the bottom. As an example, we use tide-gauge records at Pointe de La Rue, Seychelles, and ca. 50 km offshore from Brisbane, Australia, at a water depth of 70 m, to derive the major wave parameters of the Sumatra tsunami and a Category 1 Cyclone that occurred from 4–6 March 2004 in Brisbane. The Brisbane storm wave reached the maximum wave height of 14.3 m. Wave periods varied between 6 s and 14 s and the computed maximum wavelength was 237 m. The largest height of the Sumatra tsunami was 3.01 m. Periods of between 24 min and 61 min were computed for the time between two subsequent peaks. The combination of our hydrodynamic considerations with information from a simplified Hjulstrom-Sundborg diagram implies that the most powerful storm and tsunami waves produce conditions near and at the sea bed that allow the transport of sediment grains from decimeters to meters in diameter. That means a sandy tsunami deposit in the area of the Brisbane tide gauge would be reworked by storm waves. Assuming the highest wave amplitude during the 2004 March storm at Brisbane as such a boundary wave, the water depth below which preservation of tsunami deposits is most likely is greater than 65 m.