Tsunami Propagation Research Articles

Hydroacoustic Signals Originating from Marine Volcanic Activity at Kadovar Island, Papua New Guinea, Recorded by the Comprehensive Nuclear-Test-Ban Treaty International Monitoring System

Hydroacoustic signals originating from marine volcanic activity at Kadovar Island (Papua New Guinea), recorded by the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS) hydroacoustic (HA) station HA11 Wake Island (USA), are examined herein. Episodes of high volcanic activity were identified on two occasions, separated by a period of 1 month. The events studied pertain to an initial eruption series during a period between January and February 2018. Based on local visual observations, the Kadovar volcano began to erupt at the summit and then created a new vent spot near the coast. This series of events also included the collapse of a lava dome. Direction-of-arrival estimates for the hydroacoustic signals detected at HA11 were computed using a cross-correlation technique, which allowed for the discrimination between hydroacoustic signals originating from the Kadovar volcanic activity and numerous other hydroacoustic signals attributed to seismic activity in the Pacific Ocean. The Kadovar-related seismic signals could not be identified by regional IMS seismic stations, suggesting a submarine origin of these events. On the other hand, hydroacoustic signals originating from the Kadovar volcanic activity were identified by the seismometer at Manus Island, which is located between Kadovar and HA11. The study suggests that a series of explosive bursts followed by an unusual rumble and a broadband signal plus rumble may constrain the time of the lava dome collapse event at Kadovar Island to 00:30 UTC, 00:33 UTC, and 00:46 UTC on 09 February 2018. Given the compatibility of this observation with the tsunami generation reported by eyewitnesses on the nearby island of Blup Blup, the authors interpret this particular hydroacoustic signal as being a remote observation of this tsunamigenic event. The objective of this study was to assess the potential added value of IMS hydroacoustic data for remote surveillance of geohazards in otherwise sparsely monitored areas.

Parameterising Maximum Tsunami Amplitude with Earthquake Moment Magnitude for Trans-Oceanic Tsunamis

This study examines a relationship between earthquake size and maximum tsunami amplitude using large earthquakes of M_w> 7.5 that led to trans-Pacific and Indonesian tsunamis. The data were sampled from tide gauges or DART surface buoys for seven Pacific tsunamis (the 2006 Kuril, Russia, 2009 New Zealand, 2011 Tohoku-oki, Japan, 2013 Solomon Island, 2010 Maule, 2014 Iquique, and 2015 Illapel) and six Indonesian tsunamis (the 2004 Indian Ocean, 2006 Pangandaran, 2007 Bengkulu, 2010 Mentawai, 2010 Simeulue, and 2012 Northern Sumatera). We found that the size better scales with M_w instead of other measures when relating to the mean maximum amplitude η. The main finding for the trans-Pacific cases was that the M_w scale is a logarithmic function of the mean amplitude, M_w = 0.77 log η + 8.84, consistent with previous work. For the Indonesian events, it was found that M_w = 1.92 log η + 10.36, reflecting different tsunami dynamics in the Pacific and Indian Oceans. The apparent difference is thus attributable to differences in both the topographical complexity and tsunami directivity in the two oceans. This is vital as the results provide insight into the nature of tsunami propagation approaching shorelines hence useful for improved tsunami early warning.

Back‐Projection Imaging of a Tsunami Excitation Area With Ocean‐Bottom Pressure Gauge Array Data

AbstractA back‐projection method has been applied to many earthquakes in seismology due to its simple and low computational cost, and it can estimate complex fault rupture processes without any specific a priori information. In this study, we applied the back‐projection method to the tsunami records observed using an ocean‐bottom pressure gauge array and demonstrated it to be a powerful new tool other than the familiar waveform inversion. The obtained back‐projection image was consistent with the initial tsunami height distributions estimated by previous waveform inversions, and its spatial resolution appeared to be even better. Our result suggests that the fault size of the 2016 Off‐Fukushima earthquake was about half, different from the scaling law of standard earthquakes. The present tsunami back‐projection analysis can also estimate the feature of early tsunami propagations. In addition, the estimated image seems to be reliable even 30 min after the origin time, so the back‐projection analysis will be useful in an early detection of the location and spatial extent of a tsunami source. In the present case, the number of available stations in the analysis was found to be affected by the diffraction of tsunami propagation caused by the refraction by a high velocity zone near the Japan Trench. In other words, the further the source is from the coast, the more stations to be analyzed are available. Since most tsunami‐generating earthquakes occur near the subduction axis or its outer‐rise region, the back‐projection analysis should be effective for source estimation of the majority of tsunami‐generating earthquakes.

Disasters and Hazards: Risk Reduction, Mitigation and Management

Disasters and Hazards: Risk Reduction, Mitigation and Management

Tsunami Risk Assessment in Business Continuity Planning for Palu Special Economic Zone

On September 28, 2018, an earthquake with a magnitude of 7.4 Mw struck Central Sulawesi, Indonesia and triggered a major tsunami. At least 3475 people lost their lives as reported by the Indonesian National Board for Disaster Management (BNPB), while a large unknown number is still buried under the rubble and presumed dead. This event also damaged the facilities and infrastructure of the Palu City coastal area. Currently, The Government of Palu is developing the Palu Special Economic Zone (PSEZ) in Tawaeli District, Palu, Central Sulawesi to attract investors and spur economic growth in neighbouring areas. Thus, mitigation plans are urgently needed to reduce damage and losses due to the impact of future tsunamis on PSEZ. This study assesses the impact of tsunamis around the Palu Special Economic Zone and its supporting infrastructure. Hypothetical earthquake-induced tsunamis were simulated using Delft3D and DelftDashboard. The fault scenario is developed based on data from PUSGEN. Further parameters were developed with the empirical earthquake source-scaling laws. The model simulates the tsunami generation, propagation, and inundation from the source to the coastal area. A tsunami impact scenario is then developed as a basis for analysing the business impact to the transportation infrastructure and critical utilities serving the PSEZ, as well as to the operation of the industries and businesses within the PSEZ. The lessons learned from this study can assist stakeholders in developing an areawide business continuity plan as a commitment of the stakeholders for implementing areawide business continuity management for the PSEZ, which will improve the economic resilience of the Palu City as well as South Sulawesi region.

Time-Dependent Analytic Solutions for Water Waves above Sea of Varying Depths

We investigate a hydrodynamic equation system which—with some approximation—is capable of describing the tsunami propagation in the open ocean with the time-dependent self-similar Ansatz. We found analytic solutions of how the wave height and velocity behave in time and space for constant and linear seabed functions. First, we study waves on open water, where the seabed can be considered relatively constant, sufficiently far from the shore. We found original shape functions for the ocean waves. In the second part of the study, we also consider a seabed which is oblique. Most of the solutions can be expressed with special functions. Finally, we apply the most common traveling wave Ansatz and present relative simple, although instructive solutions as well.

Tsunami potential source in the eastern Sea of Marmara (NW Turkey), along the North Anatolian Fault system

Based on morphobathymetric and seismic reflection data, we studied a large landslide body from the eastern Sea of Marmara (NW Turkey), along the main strand of the North Anatolian Fault, one of the most seismically active geological structures on Earth. Due to its location and dimensions, the sliding body may cause tsunamis in case of failure possibly induced by an earthquake. This could affect heavily the coasts of the Sea of Marmara and the densely populated Istanbul Metropolitan area, with its exposed cultural heritage assets. After a geological and geometrical description of the landslide, thanks to high-resolution marine geophysical data, we simulated numerically possible effects of its massive mobilization along a basal displacement surface. Results, within significant uncertainties linked to dimensions and kinematics of the sliding mass, suggest generation of tsunamis exceeding 15–20 m along a broad coastal sector of the eastern Sea of Marmara. Although creeping processes or partial collapse of the landslide body could lower the associated tsunami risk, its detection stresses the need for collecting more marine geological/geophysical data in the region to better constrain hazards and feasibility of specific emergency plans.

Numerical Modeling of Generation of Landslide Tsunamis: A Review

Depth-integrated wave models are widely used for simulating large-scale propagation of landslide tsunamis, with the generation of tsunami being simulated separately by various generation models to provide the required initial conditions. For a given problem, the selection of a proper tsunami generation model is an important aspect for tsunami hazard analysis. The generation of tsunamis by submarine or subaerial landslides is a transient multiphase process which involves important fine-scale physics. Depth-integrated generation models, while relatively easy to use, cannot simulate these fine-scale physics. Depth-resolved generation models can overcome the shortcomings of depth-integrated generation models but are computationally demanding. This paper first reviews existing depth-integrated generation models to show the need for depth-resolved generation models. Four classes of depth-resolved generation models are reviewed: computational fluid dynamics (CFD) models, approaches coupling CFD and discrete element method, multiphase flow models, and meshless particle models. Multiphase flow models, which are relatively new, can consider complex interactions between landslide materials and its surrounding fluids. Meshless particle models are appealing for simulating landslide tsunamis because of their convenience to deal with the violent motion of the water surface and ability to run on graphics processing units. The main strengths, weaknesses, and future research directions of the reviewed models are briefly discussed. The literature reviewed, which is by no means complete, aims to provide researchers updated and practical guidelines on numerical modeling techniques for simulating the generation process of landslide tsunamis.

Systematic Comparison of Tsunami Simulations on the Chilean Coast Based on Different Numerical Approaches

Tsunami inundation estimates are of crucial importance to hazard and risk assessments. In the context of tsunami forecast, numerical simulations are becoming more feasible with the growth of computational power. Uncertainties regarding source determination within the first minutes after a tsunami generation might be a major concern in the issuing of an appropriate warning on the coast. However, it is also crucial to investigate differences emerging from the chosen algorithms for the tsunami simulations due to a dependency of the outcomes on the suitable model settings. In this study, we compare the tsunami inundation in three cities in central Chile (Coquimbo, Viña del Mar, and Valparaíso) using three different models (TsunAWI, Tsunami-HySEA, COMCOT) while varying the parameters such as bottom friction. TsunAWI operates on triangular meshes with variable resolution, whereas the other two codes use nested grids for the coastal area. As initial conditions of the experiments, three seismic sources (2010 Mw 8.8 Maule, 2015 Mw 8.3 Coquimbo, and 1730 Mw 9.1 Valparaíso) are considered for the experiments. Inundation areas are determined with high-resolution topo-bathymetric datasets based on specific wetting and drying implementations of the numerical models. We compare each model’s results and sensitivities with respect to parameters such as bottom friction and bathymetry representation in the varying mesh geometries. The outcomes show consistent estimates for the nearshore wave amplitude of the leading wave crest based on identical seismic source models within the codes. However, with respect to inundation, we show high sensitivity to Manning values where a non-linear behaviour is difficult to predict. Differences between the relative decrease in inundation areas and the Manning n-range (0.015–0.060) are high (11–65%), with a strong dependency on the characterization of the local topo-bathymery in the Coquimbo and Valparaíso areas. Since simulations carried out with such models are used to generate hazard estimates and warning products in an early tsunami warning context, it is crucial to investigate differences that emerge from the chosen algorithms for the tsunami simulations.

Discriminating the occurrence of inundation in tsunami early warning with one-dimensional convolutional neural networks

Tsunamis are natural phenomena that, although occasional, can have large impacts on coastal environments and settlements, especially in terms of loss of life. An accurate, detailed and timely assessment of the hazard is essential as input for mitigation strategies both in the long term and during emergencies. This goal is compounded by the high computational cost of simulating an adequate number of scenarios to make robust assessments. To reduce this handicap, alternative methods could be used. Here, an enhanced method for estimating tsunami time series using a one-dimensional convolutional neural network model (1D CNN) is considered. While the use of deep learning for this problem is not new, most of existing research has focused on assessing the capability of a network to reproduce inundation metrics extrema. However, for the context of Tsunami Early Warning, it is equally relevant to assess whether the networks can accurately predict whether inundation would occur or not, and its time series if it does. Hence, a set of 6776 scenarios with magnitudes in the range M_w 8.0–9.2 were used to design several 1D CNN models at two bays that have different hydrodynamic behavior, that would use as input inexpensive low-resolution numerical modeling of tsunami propagation to predict inundation time series at pinpoint locations. In addition, different configuration parameters were also analyzed to outline a methodology for model testing and design, that could be applied elsewhere. The results show that the network models are capable of reproducing inundation time series well, either for small or large flow depths, but also when no inundation was forecast, with minimal instances of false alarms or missed alarms. To further assess the performance, the model was tested with two past tsunamis and compared with actual inundation metrics. The results obtained are promising, and the proposed model could become a reliable alternative for the calculation of tsunami intensity measures in a faster than real time manner. This could complement existing early warning system, by means of an approximate and fast procedure that could allow simulating a larger number of scenarios within the always restricting time frame of tsunami emergencies.

A heuristic features selection approach for scenario analysis in a regional seismic probabilistic tsunami hazard assessment

Seismic Probabilistic Tsunami Hazard Analysis (SPTHA) is aimed at estimating the annual rate of exceedance of an earthquake-induced tsunami wave of a certain location with reference to a predefined height threshold. The analysis relies on computationally demanding numerical simulations of seismic-induced tsunami wave generation and propagation. A large number of scenarios needs to be simulated to account for uncertainties. However, the exceedance of tsunami wave threshold height is a rare event so that most of the simulated scenarios bring little statistical contribution to the estimation of the annual rate yet increasing the computational burden. To efficiently address this issue, we propose a wrapper-based heuristic approach to select the set of most relevant features of the seismic model, for deciding a priori the seismic scenarios to be simulated. The proposed approach is based on a Multi-Objective Differential Evolution Algorithm (MODEA) and is developed with reference to a case study whose objective is calculating the annual rate of threshold exceedance of the height of tsunami waves caused by subduction earthquakes that might be generated on a section of the Hellenic Arc, and propagated to a set of target sites: Siracusa, on the eastern coast of Sicily, Crotone, on the southern coast of Calabria, and Santa Maria di Leuca, on the southern coast of Puglia. The results show that, in all cases, the proposed approach allows a reduction of 95% of the number of scenarios with half of the features to be considered, and with no appreciable loss of accuracy.

Global Tonga tsunami explained by a fast-moving atmospheric source

Volcanoes can produce tsunamis by means of earthquakes, caldera and flank collapses, pyroclastic flows or underwater explosions1–4. These mechanisms rarely displace enough water to trigger transoceanic tsunamis. Violent volcanic explosions, however, can cause global tsunamis1,5 by triggering acoustic-gravity waves6–8 that excite the atmosphere–ocean interface. The colossal eruption of the Hunga Tonga–Hunga Ha’apai volcano and ensuing tsunami is the first global volcano-triggered tsunami recorded by modern, worldwide dense instrumentation, thus providing a unique opportunity to investigate the role of air–water-coupling processes in tsunami generation and propagation. Here we use sea-level, atmospheric and satellite data from across the globe, along with numerical and analytical models, to demonstrate that this tsunami was driven by a constantly moving source in which the acoustic-gravity waves radiating from the eruption excite the ocean and transfer energy into it by means of resonance. A direct correlation between the tsunami and the acoustic-gravity waves’ arrival times confirms that these phenomena are closely linked. Our models also show that the unusually fast travel times and long duration of the tsunami, as well as its global reach, are consistent with an air–water-coupled source. This coupling mechanism has clear hazard implications, as it leads to higher waves along land masses that rise abruptly from long stretches of deep ocean waters.

The enigmatic 1693 AD tsunami in the eastern Mediterranean Sea: new insights on the triggering mechanisms and propagation dynamics

The disastrous earthquake of 1693 AD caused over 60,000 causalities and the total destruction of several villages and towns in south-eastern Sicily. Immediately after the earthquake, a tsunami struck the Ionian coasts of Sicily and the Messina Strait and was probably recorded even in the Aeolian Islands and Malta. Over the last few decades, the event has been much debated regarding the location of the seismogenic source and the possible cause of the associated tsunami. The marine event has been related to both a submarine landslide and a coseismic displacement at the seafloor. To better define the most reliable sources and dynamics of the tsunami, we couple high-resolution marine seismic survey data with hydrodynamic modelling to simulate various scenarios of tsunami generation and propagation. Results from the simulations are compared with geomorphological evidence of past tsunami impacts, described in previous work along the coast of south-eastern Sicily, and within historical chronicles and reports. The most reliable scenario considers the 1693 event composed by two different tsunami waves: a first wave generated by the coseismic fault displacement at the seafloor and a second wave generated by a submarine landslide, triggered by the earthquake shaking. Tsunami modelling shows that a simultaneous movement between fault displacement and submarine mass movement could determine a destructive interference on the tsunami waves, resulting in a reduction in wave height. For this reason, the second tsunami wave probably occurred with a maximum delay of few minutes after the one generated by the earthquake and induced a greater flooding. The double-source model could explain the observation because in the course of other destructive earthquakes in south-eastern Sicily, such as that of 1169 AD, the associated tsunami caused less damages. This implies the need to better map, define and assess the hazard responsible for this type of tsunami events.

Supershear shock front contribution to the tsunami from the 2018 Mw 7.5 Palu, Indonesia earthquake

SUMMARY Hazardous tsunamis are known to be generated predominantly at subduction zones. However, the 2018 Mw 7.5 Palu (Indonesia) earthquake on a strike-slip fault generated a tsunami that devastated the city of Palu. The mechanism by which this tsunami originated from such an earthquake is being debated. Here we present near-field ground motion (GPS) data confirming that the earthquake attained supershear speed, i.e. a rupture speed greater than the shear wave speed of the host medium. We subsequently study the effect of this supershear rupture on tsunami generation by coupling the ground motion to a 1-D non-linear shallow-water wave model accounting for both time-dependent bathymetric displacement and velocity. With the local bathymetric profile of Palu bay around a tidal station, our simulations reproduce the tsunami arrival and motions observed by CCTV cameras. We conclude that Mach (shock) fronts, generated by the supershear speed, interacted with the bathymetry and contributed to the tsunami.

Tsunamis Generated and Amplified by Atmospheric Pressure Waves Due to an Eruption over Seabed Topography

Numerical simulations were generated using a nonlinear shallow-water model of velocity potential to study the fundamental processes of tsunami generation and amplification by atmospheric pressure waves. When an atmospheric pressure wave catches up with an existing tsunami that is propagating as a free wave over an abrupt change in water depth, the amplified tsunami propagates in the shallower water. An existing tsunami propagating as a free wave over a sloping seabed is also amplified by being passed by atmospheric pressure waves. When atmospheric pressure waves travel over an abrupt change in water depth, the water surface profile of tsunamis in the shallower water depends on both the interval of the atmospheric pressure waves and the phase of the tsunami-generation process over the change in water depth. Moreover, when atmospheric pressure waves travel over an abrupt change in water depth, the tsunami amplitude in the shallower water increases, as the water depth of the shallower area is decreased and the Proudman resonance is further reduced. When an atmospheric pressure wave train with positive pressure travels over a sloping seabed, the amplification of tsunami crests propagating as free waves is controlled by leaving the forced water waves following the atmospheric pressure waves. Conversely, the amplitudes of tsunami troughs propagating as free waves increase.

Occurrence of Local Tsunamis Along the Eastern Coast of the Korean Peninsula Based on Numerical Modeling of Historical Earthquakes

Korean historical literature records a major offshore earthquake with an associated tsunami in the East Sea of Korea in 1681. The event also generated strong ground motion and landslides over the Korean Peninsula. This study examined the occurrence and characteristics of the reported tsunami along the eastern coast of the peninsula using numerical modeling of tsunami propagation from submarine faults identified in a recently compiled marine fault map. Results from some scenarios indicated runup heights in good agreement with descriptions in historical records. We also examined the time required for tsunamis to travel from the causative faults to vulnerable areas along the eastern coast under various scenarios. Our results successfully hindcast the occurrence and effects of local tsunamis in the historical literature and have important implications for assessing tsunami hazard and risk for coastal areas of the Korean Peninsula.

Modeling of sediment transport in rapidly-varying flow for coastal morphological changes caused by tsunamis

Tsunamis can cause significant coastal erosion and harbor sedimentation that exacerbate the concomitant flood hazards and hamper recovery efforts. Coupling of the non-hydrostatic model NEOWAVE and the sediment transport model STM provides a tool to understand and predict these morphological changes. The non-hydrostatic model can describe flow fields associated with tsunami generation, wave dispersion as well as shock-related and separation-driven coastal processes. The sediment transport module includes non-equilibrium states under rapidly-varying flows with a variable exchange rate between bed and suspended loads. A previous flume experiment of solitary wave runup on a sandy beach provides measurements for a systematic evaluation of sediment transport driven by shock-related processes. The extensive impacts at Rikuzentakata, Iwate, Japan and Crescent City Harbor, California, USA from the 2011 Tohoku tsunami provide pertinent case studies for model benchmarking. We utilize a self-consistent fault-slip model to define the tsunami source mechanism and field survey data to determine the characteristic grain sizes and morphological changes. The near-field modeling at Rikuzentakata gives reasonable fits with observed large-scale erosion and sedimentation associated with transition of the incoming wave into a surge and formation of a hydraulic jump in the receding flow. The non-hydrostatic module becomes instrumental in resolving tsunami waves at the far-field shore of Crescent City. The results show good agreement with local tide-gauge records as well as observed scour around coastal structures and deposition in basins resulting from separation-driven processes. While the erosion patterns in the laboratory and field cases can be explained by suspended sediment transport in the receding flow, bed load transport can be a dominant mechanism in sediment laden flows and scour around coastal structures.

Towards the new Thematic Core Service Tsunami within the EPOS Research Infrastructure

Tsunamis constitute a significant hazard for European coastal populations, and the impact of tsunami events worldwide can extend well beyond the coastal regions directly affected. Understanding the complex mechanisms of tsunami generation, propagation, and inundation, as well as managing the tsunami risk, requires multidisciplinary research and infrastructures that cross national boundaries. Recent decades have seen both great advances in tsunami science and consolidation of the European tsunami research community. A recurring theme has been the need for a sustainable platform for coordinated tsunami community activities and a hub for tsunami services. Following about three years of preparation, in July 2021, the European tsunami community attained the status of Candidate Thematic Core Service (cTCS) within the European Plate Observing System (EPOS) Research Infrastructure. Within a transition period of three years, the Tsunami candidate TCS is anticipated to develop into a fully operational EPOS TCS. We here outline the path taken to reach this point, and the envisaged form of the future EPOS TCS Tsunami. Our cTCS is planned to be organised within four thematic pillars: (1) Support to Tsunami Service Providers, (2) Tsunami Data, (3) Numerical Models, and (4) Hazard and Risk Products. We outline how identified needs in tsunami science and tsunami risk mitigation will be addressed within this structure and how participation within EPOS will become an integration point for community development.

A Numerical Study on Distant Tsunami Propagation Considering the Strong Nonlinearity and Strong Dispersion of Waves, or the Plate Elasticity and Mantle Fluidity of Earth

Numerical simulations were generated to investigate the propagation processes of distant tsunamis, using a set of wave equations based on the variational principle considering both the strong nonlinearity and strong dispersion of waves. First, we proposed estimate formulae for the time variations of the tsunami height and wavelength of the first distant tsunami, by assuming that the initial tsunami profile was a long crest in a uniform bathymetry. Second, we considered the plate elasticity and upper-mantle fluidity of Earth, to examine their effects on the distant tsunami propagation. When the plate and upper mantle meet certain conditions with both a large depth and moderately large density of the upper mantle, the internal-mode tsunamis with a significant tsunami height propagated slower than the tsunamis in the corresponding one-layer problems, leading to the delay of the arrival time observed in distant tsunamis from that evaluated by the one-layer calculation.

Possible evidence of the 1755 CE transatlantic tsunami in Brazil

This work addresses the transatlantic impact of the 1755 CE Lisbon tsunami on the Brazilian coast using historical description, numerical modeling and sediment analysis. Historical documents are revisited, novel numerical modeling is provided and coupled with sedimentological data. The combination of these results attests to the potential presence of CE 1755 tsunami imprints on the Brazilian coast. Building upon the available historical records, numerical simulations of tsunami generation and transoceanic propagation are conducted to identify areas along the coast of Brazil more susceptible to be affected by the 1755 CE tsunami inundation. This procedure pinpointed areas along the northern and the northeastern coasts of Brazil as locations potentially impacted by the tsunami. The geomorphological analysis allowed the definition of locations with higher potential for the preservation of tsunami deposits and thus priorities for field investigations. The 270 km field survey performed along the coast revealed the presence of a peculiar coarse “marine” sand unit. The unit comprises two layers within a coastal aeolian sequence at Pontinhas (Pernambuco, Brazil). The grain size and foraminifera analysis allows speculation on a correlation with a high-energy marine inundation as responsible for its deposition. Detailed tsunami numerical modeling -consistent with historical data - also supports this hypothesis, showing potential inundation at the location where the peculiar deposits were found. The sedimentological analysis allows a tentative association of the deposit with the 1755 CE tsunami, despite the uncertainty on its age dating.1755 CE