Comparisons of Numerical Models on Formation of Sediment Deposition Induced by Tsunami Run-Up

Tsunami sediments provide direct evidence of tsunami arrival histories for tsunami risk assessments. Therefore, it is important to understand the formation process of tsunami sediment for tsunami risk assessment. Numerical simulations can be used to better understand the formation process. However, as the formation of tsunami sediments is affected by various conditions, such as the tsunami hydraulic conditions, topographic conditions, and sediment conditions, many problems remain in such simulations when attempting to accurately reproduce the tsunami sediment formation process. To solve these problems, various numerical models and methods have been proposed, but there have been few comparative studies among such models. In this study, inter-model comparisons of tsunami sediment transport models were performed to improve the reproducibility of tsunami sediment features in models. To verify the reproducibility of the simulations, the simulation results were compared with the results of sediment transport hydraulic experiments using a tsunami run-up to land. Two types of experiments were conducted: a sloping plane with and without coverage by silica sand (fixed and movable beds, respectively). The simulation results confirm that there are conditions and parameters affecting not only the amount of sediment transport, but also the distribution. In particular, the treatment of the sediment coverage ratio in a calculation grid, roughness coefficient, and bedload transport rate formula on the fixed bed within the sediment transport model are considered important.

SIZE MATTERS! (Or the crucial importance of small foraminifera in interpreting tsunami sediments)

<p>Benthic foraminiferal studies were hardly comparable for several decades because of the absence of standardised size criteria. Actually, sample wash and foraminifera investigations in different studies addressed >63µm, >125µm, >150µm or even >250µm fractions. The turning point arrived with Schröder et al. (1987) and Sen Gupta et al. (1987). Both reported significant loss in the foraminifera and species abundances in the >125µm fraction, when compared with the >63µm. Dominant species in oceanic environment became non-significant or disappear, and the larger sieves record became obviously less informative. Schönfeld et al. (2012) consider that >125µm is adequate for ecological monitoring but point that, in some environments, to prevent losing smaller species and juveniles it is required to use the >63µm fraction. Recently, a worrying trend argues that solely the >150μm residue should be investigated to save time, even if it results on assemblages bias. Such trend represents an unacceptable step back. In fact 1) the analysis of coarser fractions reduces representativity of small, but relevant, adult species, effectively biasing both the associations and interpretations, 2) up to 50% (in cases 99%) of foraminiferal fauna may be lost, 3) this constrains comparison with published research and jeopardizes future work and 4) the contribution of juveniles (regardless of their identification) for sedimentary dynamic interpretations is lost. This is clearly the case of foraminiferal studies on tsunami deposits, where small species and juveniles often represent an important proxy to understand tsunami flow dynamics. For instance, in the Algarve 1755AD tsunami deposits juveniles represent up to 22% of the assemblage (e.g. Quintela et al., 2016).</p><p>Furthermore, >150µm fraction does not correspond to any Wentworth’s grain-size classes, precluding correlation between foraminifera and sediment textural features in tsunami deposits analysis (e.g., Hawkes et al., 2007;Mamo et al., 2009; Pilarczyk et al., 2019). Consequently it must be assumed that foraminiferal research is a time consuming task, and that “Yes, size matters!” thus small foraminifera cannot be disregarded and fraction >63µm should be mandatory in multiproxy analyses.</p><p> </p><p>Authors acknowledge the financial support of FCT through projects <strong>OnOff – PTDC/CTAGEO/28941/2017 </strong>and  <strong>UIDB/50019/2020–IDL.</strong></p><p>Hawkes, AD et al. (2007). Sediments deposited by the 2004 Indian Ocean Tsunami along the Malaysia-Thailand Peninsula. Marine Geology 242, 169-190.</p><p>Mamo, B et al (2009). Tsunami sediments and their foraminiferal assemblages. Earth-Science Reviews 96, 263-278.</p><p>Pilarczyk, J et al. (2019).Constraining sediment provenance for tsunami deposits using distributions of grain size and foraminifera from the Kujukuri coastline and shelf, Japan. Sedimentology doi: 10.1111/sed.12591</p><p>Quintela, M et al. (2016). The AD 1755 tsunami deposits onshore and offshore of Algarve (south Portugal): Sediment transport interpretations based on the study of Foraminifera assemblages. Quaternary International, 408: 123-138.</p><p>Schönfeld, J and FOBIMO group (2012). The FOBIMO (FOraminiferal BIo-MOnitoring) initiative—Towards a standardized protocol for soft-bottom benthic foraminiferal monitoring studies. Marine Micropaeontology 94-95, 1-13.</p><p>Schröder, CJ et al. (1987). Can smaller benthic foraminifera be ignored in Paleoenvironmental analysis? Journal of Foraminiferal Research 17, 101-105.</p><p>Sen Gupta, BK et al. (1987). Relevance of specimen size in distribution studies of deep-sea benthic foraminifera. Palaios 2, 332-338.</p>

Tsunami deposits of September 21st 1985 in Barra de Potosí: comparison with other studies and evaluation of some geological proxies for southwestern Mexico

Residents of Barra de Potosí village in southwestern Mexico witnessed inundation by waves up to a distance of ~500 m from the shore after the Mw 7.5 earthquake on September 21st, 1985. Sediments deposited by the tsunami wave were identified near El Potosí estuary and their geological characteristics (sedimentology, mineralogy and chemical composition) were compared with pre-tsunami sediments and deposits from the nearby-unaffected area. Tsunami sediments were characterized by well and moderately well sorted (standard deviation: 0.4-0.7 Φ) fine sand (mean size: 2.13-2.47 Φ) and contain higher amounts of both finer and coarser fractions (negative to positive skewed) and had leptokurtic to extremely leptokurtic distribution. Sedimentological characteristics of tsunami and pre-tsunami deposits were similar. Abundance and association of heavy minerals were also comparable both in tsunami and pre-tsunami deposits. However, lower amounts of Br and Fe2O3 and higher SiO2 and TiO2 differentiate tsunami deposits from the pre-tsunami sediments. Comparison with sediments deposited during the tsunamis of March 14th, 1979, September 21st, 1985, and March 11th, 2011, in the region did not yield any characteristic signature. Except for stratigraphy (i.e., erosive base), no other geological characteristic was useful for identifying paleo-tsunami in the region.

A record of local storms and trans-Pacific tsunamis, eastern Banks Peninsula, New Zealand

This study of five sand units at Lavericks Bay, New Zealand, reports on the sedimentary evidence for three trans-Pacific tsunamis and two local storms. The 1868 Arica, 1877 Iquique and 1960 Valdivia tsunamis from Chile were the largest distantly generated events in New Zealand’s history but have never before been identified at the same location. It is also the first time that the 1877 Iquique tsunami deposit has been found in New Zealand. Two further sand units were identified as local storm deposits laid down in 1869/1870 and 1929. The identification and chronology of these events were established through the use of geochemistry, palynology, diatoms, charcoal abundance and historical documents. Their relative magnitudes were estimated through the use of grain size parameters and lateral extent of the recognisable sand layers. The recognisable sandy tsunami deposits extend about 60% of the inundation distance, while the storm sediments are finer and less extensive. There were two notable geochemical differences between the storm and tsunami deposits. Both storm deposits had lower concentrations of marine proxy elements associated with lower Ca–Ti and Sr–Ba ratios. Other differences were noted between some of the tsunami and storm deposits such as rip-up clasts and sediment characteristics, but these were by no means unequivocal. It is possible that geochemistry may prove to be the only proxy capable of not only differentiating effectively between storm and tsunami sediments but also identifying the maximum inland extent of a deposit and of inundation. It is the ability to better understand the nature and extent of such catastrophic events through these subtle differences in event characteristics that will help improve risk management for coastlines around the world.