NH5.3

Ludao Island in south eastern Taiwan regularly experiences strong Pacific typhoons.  Fieldwork was undertaken to investigate the characteristics of a boulder field comprising massive limestone and volcanic clasts (10³–10⁴ kg) on the exposed SE coast.  Old large clasts on the Holocene emerged platform provide evidence for multiple high-energy palaeowave events.  Of particular interest were clasts stacked and imbricated together to form distinct boulder trains.  Inferred minimum flow velocities of 4.3–13.8 m/s were needed for their deposition.  What can imbricated boulder trains tell us about the wave processes and geomorphic influences responsible?  One hypothesis here is that localized funnelling of water flow through narrow relict channels is able to concentrate onshore flow energy into powerful jets.  These channels represent inherited (fossil) spur-and-groove morphology, oriented perpendicular to the modern reef edge, now overdeepened by subaerial karstic solution.  Support for this idea is the location and train-of-direction of the main imbricated boulder cluster at the landward head of one such feature.  Geomorphic controls amplifying wave-breaking flow velocities across Ludao's coastal platform mean that a palaeotyphoon origin is sufficient to account for large rock clast stacking and imbrication, without recourse to a tsunami hypothesis.

How to cite: Terry, J., Lau, A., Nguyen, K. A., Liou, Y.-A., and Switzer, A.: Imbricated trains of massive coastal rock clasts (103–104 kg) on Ludao Island, Taiwan: what they can and cannot tell us about palaeotyphoons, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1600, https://doi.org/10.5194/egusphere-egu22-1600, 2022.

This presentation gives an overview of the results of a five-year research project on tsunami-induced boulder transport. It stresses the importance of the exact determination of boulder shapes in contrast to simplified bodies (such as cuboids), especially with regard to the transport distance. It also provides insights about a newly developed numerical boulder-transport model based on Pudasaini (2012). Additionally, some ideas how experimental research on tsunami-induced boulder transport may be improved and coordinated in the future will be presented.

The investigations by physical experiments are based on three boulder shapes of which one depicts the replica of an original boulder from the island of Bonaire (Caribbean Sea, Lesser Antilles). The experiments clearly reveal that the available impact area of the boulder has a great significance; however, this is so far insufficiently considered in analytical equations. In the given case, the comparison between the more streamline-shaped replica of the Bonaire boulder and an idealised cuboid boulder resulted in reduced transport distances of 30 %, in average. Additionally, statistical evaluations revealed that the entire process is highly sensitive with partly stochastic behaviour. Thus, we support the statement of Bressan et al. (2018) in this regard. We show, how important it is to calculate and communicate wave thresholds for mobilisation in terms of probability ranges instead of fixed values.

Based on the results of our own physical experiments and the evaluation of published physical experiments, we developed a tool, which supports researchers in assessing the accuracy of analytical equations for specific in-situ settings (Oetjen et al., 2021). This tool encompasses the crucial parameters (e.g., bottom roughness, boulder shape), combines their influence on the transport process and finally gives an indication of whether the present conditions tend to amplify or hamper the boulder transport. The benefit and the usage of the above-mentioned tool will be demonstrated exemplarily.

Furthermore, within the framework of the project a numerical Boulder-Transport-Model was developed which is based on the Immersed Boundary Method and the Two-Phase Flow Model of Pudasaini (2012). Insights into the functionality of the model and the importance of the increased flow density will be highlighted, while the further development steps will be indicated.

As part of the project, we also dealt with the future development of research on tsunami-induced boulder transport (cf. Oetjen et al., 2021). One important suggestion is to establish a standardised reference setup for experimental investigations within the research community. It would enable researchers to compare the results of their own experiments and the effect of the investigated parameters with well-documented reference values and assist them to evaluate and classify their experimental results accordingly.

Bressan, L., Guerrero, M., Antonini, A., Petruzzelli, V., Archetti, R., Lamberti, A., Tinti, S. (2018): A laboratory experiment on the incipient motion of boulders by high-energy coastal flows. Earth Surface Processes and Landforms 43 (14), 2935–2947. DOI: 10.1002/esp.4461.

Oetjen, J., Engel., M., Schüttrumpf, H. (2021): Experiments on tsunami induced boulder transport – a review. Earth-Science Reviews 220. DOI: 10.1016/j.earscirev.2021.103714.

Pudasaini, S.P. (2012): A general two-phase debris flow model. Journal of Geophysical Research: Earth Surface 117, F03010. DOI: 10.1029/2011JF002186.

How to cite: Oetjen, J., Engel, M., Schüttrumpf, H., and Brückner, H.: Tsunamis in the Caribbean Sea – Implications from coarse-clast deposits and the importance of their shape, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6914, https://doi.org/10.5194/egusphere-egu22-6914, 2022.

Intertidal and supratidal coastal boulder deposits (CBD) result from extreme marine inundation on rocky shores. They are important for understanding long-term coastal wave patterns, have predictive value for future events and can support coastal hazard assessment. But they are poorly studied, and their interpretation remains contentious, with debate on whether they record storms, tsunami, or both. In the case of older deposits, uncertainties about paleo-sea level contribute additional uncertainty. These ambiguities impact risk analysis: should CBD data be part of tsunami risk catalogues, or storminess indices? The hydrodynamics and climatology leading to CBD generation are also still uncertain. Two main obstacles to deeper understanding have been identified: a lack of data on CBD worldwide; and discrepant approaches that lead to difficulties in comparing data from different sites. Building community and interaction among CBD researchers, and awareness of CBD as research targets, can help grow our knowledge and tackle these obstacles.

ISROC (www.isroc.network)—Inundation Signatures on Rocky Coastlines—is an NSF-funded Research Coordination Network to define the CBD problem chain and identify research gaps by developing a broad and diverse network of researchers. The authors of this paper are the PIs and steering group. We plan to extend the community of researchers, in particular to include underrepresented groups; to facilitate development of standards and best practices for gathering and archiving CBD data; to develop cyberinfrastructure for uploading, visualizing, and analyzing data; and train the next generation of CBD researchers. To do this, we will create opportunities for cross-disciplinary collaboration and exchange. Using CBD to reconstruct coastal inundation history and extreme climatological states is a prime example of convergence research that cannot be solved by one discipline. The network includes geologists, geographers, oceanographers, engineers, hydrodynamicists, geophysicists, climatologists and paleoclimatologists. Activities include meetings, student training and exchanges; sessions in future years at major conferences in geoscience and coastal engineering; consolidation of survey/mapping approaches; building a global database; and user-friendly, fully accessible online data archiving. Understanding past inundation and how CBD form and evolve will both help to quantify present-day risk and will provide guidance for what to expect from future climate and sea level.

How to cite: Engel, M., Cox, R., Kennedy, A. B., Berke, M. A., Guannel, G., Lau, A. Y. A., and Mori, N.: ISROC—Inundation Signatures on ROcky Coastlines—A new Research Coordination Network targeting coastal boulder deposits, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8120, https://doi.org/10.5194/egusphere-egu22-8120, 2022.

The Cabo Verde Archipelago holds a remarkable sedimentary record of tsunami inundations, as highlighted by recent finds on Santiago and Maio Islands. Santiago, in particular, constitutes an exceptional site to study in detail the proximal impacts of the megatsunami(s) triggered by the well-known catastrophic flank collapse of Fogo volcano (~60 km to the west of this island) and one of the most active ocean island volcanoes in the Atlantic. Previous studies identified and documented deposits – fields of megaclasts and chaotic conglomerates on northern Santiago – which were attributed to the impact of this megatsunami(s); moreover, the pioneer use of cosmogenic ³He geochronology on basaltic megaclasts quarried/displaced by the event bracketed its occurrence within the 65-84 ka time interval. Here we present the results of a recent study conducted within the remit of the project UNTIeD, which combined detailed field surveys and U-Th disequilibrium geochronology to review and further document the tsunamigenic conglomerates of Santiago and gain additional insights into their formative event(s). We can confirm the presence of tsunami conglomerates on several sectors of the island, chiefly in the north and southeast of the island. Furthermore, on the northern sector, our study suggests the presence of two distinct sets of deposits, of differing ages, as corroborated by U-Th geochronology on corals entrained in the conglomerates. The older set of deposits is restricted to 60–100 m in elevation and yielded coral ages at the upper limit of U-Th techniques (200-450 ka); its origin is still uncertain. The younger and most widespread chaotic deposit can be found from 0 to 100 m in elevation, is poorly consolidated, and mostly mantles a topography partially carved on the older deposit. Coral ages are very widespread from ~58 to >400 ka (as commonly observed in tsunami deposits) but with the higher-confidence younger ages clustering around 58-65 ka, in agreement with the youngest cosmogenic ages. This suggests a more constricted timing for Fogo's (main) megatsunami at around 58-65 ka, in close agreement with recent studies at Fogo. A distinct and younger deposit, of Holocene age, can only be found at low elevations in Nossa Senhora da Luz Bay and likely represents a local event possibly triggered by a small submarine landslide. Taken together, these finds not only provide a better time constraint and insights on the impact of Fogo's megatsunami but reinforce the notion that the Cabo Verde Islands have been impacted by multiple tsunamis in the last 500 ka.

This work was supported by project PTDC/CTA-GEO/28588/2017 - LISBOA-01-0145-FEDER-028588 UNTIeD, co-funded by the ERDF through POR Lisboa 2020 and FCT, and by projects IF/01641/2015 MEGAWAVE and FCT/UIDB/50019/2020 - IDL, also funded by FCT.

How to cite: Ramalho, R. S., Madeira, J., Costa, P. J. M., Stewart, J. A., Robinson, L. F., Rebelo, A. C., Melo, C. S., Ryan, D. D., Rasser, M. W., Freitas, M. C., Cachão, M., Andrade, C., Hipólito, A., Rovere, A., and Ávila, S. P.: Chaotic conglomerates from Santiago Island (Cabo Verde): a review and insights into the proximal impacts of collapse-triggered megatsunamis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10397, https://doi.org/10.5194/egusphere-egu22-10397, 2022.

Assessing seismic and tsunami hazards commonly relies on historical accounts of past events, but aside from limitations where such chronicles are too short to account for variability in earthquake size, rupture style, tsunamigenesis and the existence of supercycles, even where long written histories exist, records may be biased by temporal gaps due to historical circumstances. We demonstrate that this is the case for the area affected by the magnitude 9.5 1960 Chile earthquake. Historical records document four great earthquakes (M8+) in the last 450 years in this region, but while devastating tsunamis are known to have accompanied earthquakes in 1575, 1837 and 1960 CE, there is no such record of inundation in 1737. The lack of reports of tsunami inundation from the 1737 south-central Chile earthquake has been attributed to either civil unrest or a small tsunami due to deep fault slip below land. Here we cross-check the historical record using a coastal sedimentary record from Chaihuín, a tidal marsh 15 km southwest of Valdivia, close to the region of maximum 1960 slip. Tidal marshes are low energy intertidal settings that may preserve evidence for abrupt co-seismic changes in land level and inundation by extreme waves. We conduct sedimentological and diatom analyses of tidal marsh sediments within the 1737 rupture area and find evidence for a locally-sourced tsunami consistent in age with this event. The evidence is a laterally-extensive sand sheet coincident with abrupt, decametric-scale subsidence. Coupled dislocation-tsunami models place the causative fault slip mostly offshore rather than below land, as had previously been assumed from the absence of historical accounts of a tsunami. Whether associated or not with the 1737 earthquake, our findings reduce the average recurrence interval of tsunami inundation derived from historical records alone, highlighting the importance of combining geological and historical records in order to obtain robust long-term patterns to inform seismic and tsunami hazard assessment.

How to cite: Hocking, E., Garrett, E., Aedo, D., Carvajal, M., and Melnick, D.: Evidence of an unreported Chilean tsunami highlights the importance of combining geological and historical records in tsunami hazard assessment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8768, https://doi.org/10.5194/egusphere-egu22-8768, 2022.

The assessment of seismic hazards along subduction zone coastlines provides important information regarding the frequency and magnitude of earthquakes and tsunamis that can be expected in the future.  Unlike many subduction zone coastlines that involve one tectonic plate subducting under another, seismic hazard assessments for the Kanto region of Japan are complicated by the presence of a nearby triple junction; where one continental (CON) and two oceanic plates (PHS, PAC) collide.   The CON/PHS (Sagami Trough) and CON/PAC (Japan Trench) boundaries are recognized earthquake sources.  However, historical and geological evidence of a large PHS/PAC (Izu-Bonin Trench) earthquake has been lacking and decades worth of instrumental data point to low seismicity along this boundary.  Here we show that two unusually large tsunamis are evidenced by sandy deposits preserved along 50 km of coastline in the Kanto region.  The oldest of them, deposited about 1,000 years ago, contains evidence consistent with tsunami deposits reported elsewhere (e.g., marine foraminifera, rip-up clasts, pebbles, erosional base) and represents a previously unknown prehistoric earthquake.  In computer simulations, this earthquake deposited sand that extended too far inland to represent any known historical earthquake originating from the CON/PHS and CON/PAC boundaries alone.  Rather, the greater inland inundation points to significantly greater displacement on the CON/PHS and CON/PAC boundaries, which may be unrealistic, or much smaller displacement along the previously unconsidered PHS/PAC megathrust.  This plate-boundary fault adds another source for earthquakes in Tokyo and tsunamis in the Pacific Ocean.

How to cite: Pilarczyk, J., Sawai, Y., Namegaya, Y., Tamura, T., Tanigawa, K., Matsumoto, D., Shinozaki, T., Fujiwara, O., Shishikura, M., Shimada, Y., Dura, T., Horton, B., Parnell, A., and Vane, C.: An unconsidered source of earthquakes and tsunamis from the Kanto region of Japan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13502, https://doi.org/10.5194/egusphere-egu22-13502, 2022.

Tsunamis can cause catastrophic impacts at the coastline. Australia’s NW continental margin displays abundant massive slope-failure deposits, but there is little evidence of associated coastal tsunami deposits. Here we report on investigations of an exposed field of cemented dunes and associated conglomero-breccias, located on Barrow Island. Preliminary OSL dating indicates that these deposits formed when relative sea level was around 30 to 50 m below present. If the deposits can be interpreted as having been formed by a mega-tsunami, then it was a very significant event that ran inland several km and achieved a maximum run-up of several tens of metres. A similar event today would directly impact thousands of people, multiple ports, and industrial facilities worth many billions of dollars, as well as impacting many unique ecological and cultural resources.

How to cite: Larcombe, P., Wilson, M., Whitley, T., Ward, I., Pirrie, D., Li, T., Hill, J., Florec, V., and Bateman, M.: Sedimentary evidence for a Quaternary mega-tsunami in NW Australia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3848, https://doi.org/10.5194/egusphere-egu22-3848, 2022.

Shark Bay Marine Park is a UNESCO World Heritage Property in a region of marginal tropical cyclone influence and its sustainability requires a deep consideration of cyclone hazards. Here, we analyse historical records of a large storm surge from a Tropical Cyclone in 1921 that generated remarkable overland flow leaving fish and sharks stranded over 9 km inland. We weight information from the historical archives in a new framework and model event scenarios to reconstruct its magnitude. The plausible event scenarios imply that the cyclone was a marginal Category 4 or 5 storm with a return interval equivalent or slightly greater than the regional planning level. The outcome underscores the importance of examining the pre-instrumental events in areas of marginal cyclone influence as they are commonly of key economic importance.  Our work also implies that TC risk affects marine conservation in the Shark Bay World Heritage Property and requires attention.

How to cite: Switzer, A. D., Christensen, J., Aldridge, J., Taylor, D., Churchill, J., Watson, H., Fraser, M. W., and Shaw, J.: A 1921 Western Australian tropical cyclone underscores the utility of historical records for hazard analysis in areas of marginal cyclone influence., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2435, https://doi.org/10.5194/egusphere-egu22-2435, 2022.

New field investigations along the East Tunisian coastline reveal sedimentary deposits and damaged localities that may account for a catastrophic event during late Holocene. North of Sfax - Thyna city (at Henchir El Majdoul site) ~3.4 m high cliff coastal marine and alluvial terraces show a 20 to 50-cm-thick chaotic layer with sandy coarse gravels mixed with limestone beach-rocks, reworked blocks, broken shells of marine and lagoon gastropods and lamellibranch mollusks, organic matter, and Roman pottery. The chaotic layer truncates a succession of sandy-silty paleosol, covers Roman settlements and is overlain by fire remains and a relatively thin (~10 cm) sandy-silty aeolian unit and ~1-m-thick alluvial deposits. Charcoal samples collected at 10 cm below and 4 cm above the catastrophic deposits provide radiocarbon dating that brackets a catastrophic event between 286 and 370 CE (2s). Beside the damaged Roman site of Thyna, other localities of the east Tunisian coastline such as Neapolis (Nabeul) near Tunis, Hadrumete (Sousse), Meninx-town in Girba (Djerba), Wadi Ennouili (Gulf of Gabes), and Sabratha (in Libya) experienced major damage and abandonment of sites in Fifth century. The extent of damage from northern Libya to northern Tunisia at the Fourth century and radiocarbon dating, added to the 2.6 m thick turbidite deposits west of Malta correlate with the major tsunamigenic earthquake of 21 July 365 (Mw ~ 8) in west Crete (Greece). Numerical modelling of the tsunami caused by an earthquake in the Hellenic Arc subduction zone suggests more than 3.5 m high tsunami waves propagation affecting the Tunisia coastline, resulting in a run-up consistent with the stratigraphic evidence presented here. The catastrophic deposits, offshore-onshore correlations, archeological damage and modelling of tsunami waves suggest a new, higher-resolution, assessment of the tsunami hazard leading to a better estimate of tsunami risk on the eastern coast of Tunisia.

How to cite: Bahrouni, N., Meghraoui, M., Bayraktar, H. B., Lorito, S., Zagrarni, M. F., Polonia, A., Bel Mabrouk, N., Kamoun, M., Khadraoui, A., and Kamoun, F.: Spread tsunami impact in East Tunisia contemporaneous of the CE 365 Crete earthquake, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9383, https://doi.org/10.5194/egusphere-egu22-9383, 2022.