Study Of Tsunami Deposits Research Articles

Examination of the largest-possible tsunamis (Level 2) generated along the Nankai and Suruga troughs during the past 4000 years based on studies of tsunami deposits from the 2011 Tohoku-oki tsunami

Japanese historical documents reveal that Mw 8 class earthquakes have occurred every 100–150 years along the Suruga and Nankai troughs since the 684 Hakuho earthquake. These earthquakes have commonly caused large tsunamis with wave heights of up to 10 m in the Japanese coastal area along the Suruga and Nankai troughs. From the perspective of tsunami disaster management, these tsunamis are designated as Level 1 tsunamis and are the basis for the design of coastal protection facilities. A Mw 9.0 earthquake (the 2011 Tohoku-oki earthquake) and a mega-tsunami with wave heights of 10–40 m struck the Pacific coast of the northeastern Japanese mainland on 11 March 2011, and far exceeded pre-disaster predictions of wave height. Based on the lessons learned from the 2011 Tohoku-oki earthquake, the Japanese Government predicted the tsunami heights of the largest-possible tsunami (termed a Level 2 tsunami) that could be generated in the Suruga and Nankai troughs. The difference in wave heights between Level 1 and Level 2 tsunamis exceeds 20 m in some areas, including the southern Izu Peninsula. This study reviews the distribution of prehistorical tsunami deposits and tsunami boulders during the past 4000 years, based on previous studies in the coastal area of Shizuoka Prefecture, Japan. The results show that a tsunami deposit dated at 3400–3300 cal BP can be traced between the Shimizu, Shizuoka and Rokken-gawa lowlands, whereas no geologic evidence related to the corresponding tsunami (the Rokken-gawa–Oya tsunami) was found on the southern Izu Peninsula. Thus, the Rokken-gawa–Oya tsunami is not classified as a Level 2 tsunami.

Coupling eruption and tsunami records: the Krakatau 1883 case study, Indonesia

The well-documented 1883 eruption of Krakatau volcano (Indonesia) offers an opportunity to couple the eruption’s history with the tsunami record. The aim of this paper is not to re-analyse the scenario for the 1883 eruption but to demonstrate that the study of tsunami deposits provides information for reconstructing past eruptions. Indeed, though the characteristics of volcanogenic tsunami deposits are similar to those of other tsunami deposits, they may include juvenile material (e.g. fresh pumice) or be interbedded with distal pyroclastic deposits (ash fall, surges), due to their simultaneity with the eruption. Five kinds of sedimentary and volcanic facies related to the 1883 events were identified along the coasts of Java and Sumatra: (1) bioclastic tsunami sands and (2) pumiceous tsunami sands, deposited respectively before and during the Plinian phase (26–27 August); (3) rounded pumice lapilli reworked by tsunami; (4) pumiceous ash fall deposits and (5) pyroclastic surge deposits (only in Sumatra). The stratigraphic record on the coasts of Java and Sumatra, which agrees particularly well with observations of the 1883 events, is tentatively linked to the proximal stratigraphy of the eruption.

Sedimentary Features of Tsunami Deposits in Carbonate-Dominated Beach Environments: A Case Study from the 25 October 2010 Mentawai Tsunami

This study provides a case history of tsunami deposition in a carbonate-dominated environmental setting. We present the results of a sedimentological investigation conducted on deposits formed by the 25 October 2010 Mentawai Island Tsunami and report on deposits analyzed at three sites on North Pagai Island: Sabeu Gunggung, Macaroni Resort, and Tumalei. The deposits are mainly composed of medium to coarse sand-sized fragments of corals, shells and foraminifera, with thickness ranging from 3 to 26 cm. The deposits consist of two to five layers, with fining-upward trends dominating. Local topography noticeably affects the thickness, number of layers, and distribution of tsunami deposits. The foraminiferal assemblage and diversity vary at each sample point, along transect and between different transects. Based on the foraminiferal content, most of the deposit material likely came from shallower depths. In addition, density distribution of the tsunami deposit material exhibits some degree of variability in terms of the range of densities in each sample and the trend of the overall density along each transect. In contrast to heavy mineral-dominated deposits, the density of carbonate grains as a function of size may be an important consideration when carbonate-dominated deposits are used to interpret hydraulic conditions that created a particular tsunami deposit. Since there are relatively few studies of tsunami deposits in carbonate-dominated environmental settings such as the Mentawai Islands, our study provides a useful case history of tsunami deposition in such an environmental setting.

Heavy minerals in the 2011 Tohoku-oki tsunami deposits—insights into sediment sources and hydrodynamics

The 2011 Tohoku-oki tsunami left sand and mud deposits more than 4km 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 0.125–0.25mm 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 1.5km of the shoreline were mostly eroded from the beach, dune and local soils, while deposits farther inland (>1.5km) 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”.

Use of anisotropy of magnetic susceptibility (AMS) in the study of tsunami deposits: Application to the 2004 deposits on the eastern coast of Banda Aceh, North Sumatra, Indonesia

We studied emplacement dynamics of the December 26, 2004 tsunami deposits in artificial shallow ponds and in shallow lagoons, along the northern coast of Sumatra in Banda Aceh area. Successive tsunami waves inundated topographic lows, which acted as sediment traps and favored preservation of deposits. Individual waves led to the deposition of fining upwards sedimentary sequences. This multiple normal grading deposit with a thickness ranging from 2 to 80 cm constitutes a site of major interest for the study of the deposits emplaced by a tsunami wave train. The main interest lies in the characteristics of the deposits that can be correlated to the behavior of the waves attested by numerous testimonies. In this setting, we aimed at testing the applicability of the AMS (Anisotropy of Magnetic Susceptibility) technique as a proxy of unconsolidated sediment fabric. The combination of this proxy and traditional sedimentary analysis helped reconstructing the hydrodynamic conditions — particularly the flow direction, prevailing during sedimentation. We applied the AMS technique on eight sections stretching inland from 1.5 km to 2.2 km from the pre-tsunami coastline. Grain-size and AMS data from these deposits provide similar and partially complementary information. Traction dominates in the basal parts of the deposits and occurs mainly for the basal layers of the sequences whereas settling dominates for the upper layers of these sequences. The vertical evolution of the AMS parameters within the deposits is related to the arrival of the successive waves with variations in the traction and decantation processes during deposition. Variations of current strength during sedimentation are related to the wave's momentum and to the progressive infill of topographic depressions. The vertical change of the fifth coarsest percentile vs. the median grain sizes of the deposits suggests strong variations of the energy of the currents, and confirms the evolution of the AMS parameters. In the present study, the fabric of the deposits, which developed exclusively during the uprush phase of the cycle, also allows the reconstruction of flood directions that are both normal and parallel to the shoreline, with variations related to flow dynamics, mean grain size and local topographic control. Due to the absence of backwash in between the waves and low velocity of the final backwash, information about the sedimentation conditions of the tsunami deposits appears to be better preserved in topographic depressions in areas located remote from the seashore. The AMS proxy appears to be a promising tool for further studies of recent- and paleo-tsunami deposits.