Tsunami Source Research Articles

Diverse tsunamigenesis triggered by the Hunga Tonga-Hunga Ha'apai eruption.

On the evening of 15 January 2022, the Hunga Tonga-Hunga Ha’apai volcano1 unleashed a violent underwater eruption, blanketing the surrounding land masses in ash and debris2,3. The eruption generated tsunamis observed around the world. An event of this type last occurred in 1883 during the eruption of Krakatau4, and thus we have the first observations of a tsunami from a large emergent volcanic eruption captured with modern instrumentation. Here we show that the explosive eruption generated waves through multiple mechanisms, including: (1) air–sea coupling with the initial and powerful shock wave radiating out from the explosion in the immediate vicinity of the eruption; (2) collapse of the water cavity created by the underwater explosion; and (3) air–sea coupling with the air-pressure pulse that circled the Earth several times, leading to a global tsunami. In the near field, tsunami impacts are strongly controlled by the water-cavity source whereas the far-field tsunami, which was unusually persistent, can be largely described by the air-pressure pulse mechanism. Catastrophic damage in some harbours in the far field was averted by just tens of centimetres, implying that a modest sea level rise combined with a future, similar event would lead to a step-function increase in impacts on infrastructure. Piecing together the complexity of this event has broad implications for coastal hazards in similar geophysical settings, suggesting a currently neglected source of global tsunamis.

Historical tsunamis of Taiwan in the 18th century: the 1781 Jiateng Harbor flooding and 1782 tsunami event

Abstract. This research aims to study two historical tsunamis that occurred in Taiwan during the 18th century and to reconstruct the incidents. The 1781 Jiateng Harbor flooding, recorded by the Chinese historical document entitled “Taiwan Interview Catalogue” took place on the southwest coast of Taiwan. In contrast, the 1782 tsunami was documented in foreign languages, with uncertainties about the actual time. These two events seem to be close enough in time and location that, to some researchers, they are considered as the same event. Reasoning these historical events requires carefully examining the literature records and performing the scenarios that match the descriptions. The impact intensity analysis (IIA) is employed to locate possible regions of tsunami sources in order to reproduce the events. Numerical simulations based on the Cornell Multi-grid Coupled Tsunami Model (COMCOT) analyze the influence of different types of tsunamis generated by both submarine mass failures and seismic activities. Numerical results indicate that the source of the 1781 Jiateng Harbor flooding is located very possibly on the south-southwestern side of Taiwan. However, simulation results and historical records put the existence of the 1782 tsunami in doubt, and the possibility of storm surges could not be ruled out.

Numerical Modeling of Tsunami‐Generated Acoustic‐Gravity Waves in Mesopause Airglow

AbstractNumerical simulations of mesopause airglow (MA) fluctuations induced by tsunami‐generated acoustic and gravity waves (TAGWs) are performed. Simulated tsunamis over realistic bathymetry are used to excite atmospheric waves at the surface level of a three‐dimensional nonlinear and compressible neutral atmospheric model. The model incorporates the dynamics and chemistry of hydroxyl OH(3,1) MA under nighttime assumptions. We report case study results of eight recent large tsunami events and demonstrate that TAGW‐induced MA fluctuations are readily detectable with modern ground‐ and space‐based imagers, and may provide quantitative insight. The amplitudes of MA fluctuations reflect the evolution of ocean surface displacements, enhancing or decreasing accordingly, and revealing the tsunami's lobes and local wave focusing. The results suggest that MA observations have potential to supplement early‐warning systems, providing information on spatial and temporal evolution of tsunami waves of ∼10 cm and higher for the cases shown. They may find applications in tsunami tracking over large open ocean areas, as well as in the investigation or reconstruction of tsunami source characteristics.

Upper-plate structure and tsunamigenic faults near the Kodiak Islands, Alaska, USA

AbstractThe Kodiak Islands lie near the southern terminus of the 1964 Great Alaska earthquake rupture area and within the Kodiak subduction zone segment. Both local and trans-Pacific tsunamis were generated during this devastating megathrust event, but the local tsunami source region and the causative faults are poorly understood. We provide an updated view of the tsunami and earthquake hazard for the Kodiak Islands region through tsunami modeling and geophysical data analysis. Using seismic and bathymetric data, we characterize a regionally extensive seafloor lineament related to the Kodiak shelf fault zone, with focused uplift along a 50-km-long portion of the newly named Ugak fault as the most likely source of the local Kodiak Islands tsunami in 1964. We present evidence of Holocene motion along the Albatross Banks fault zone, but we suggest that this fault did not produce a tsunami in 1964. We relate major structural boundaries to active forearc splay faults, where tectonic uplift is collocated with gravity lineations. Differences in interseismic locking, seismicity rates, and potential field signatures argue for different stress conditions at depth near presumed segment boundaries. We find that the Kodiak segment boundaries have a clear geophysical expression and are linked to upper-plate structure and splay faulting. The tsunamigenic fault hazard is higher for the Kodiak shelf fault zone when compared to the nearby Albatross Banks fault zone, suggesting short wave travel paths and little tsunami warning time for nearby communities.

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.

Analysis of Faster-Than-Real-Time (FTRT) Tsunami Simulations for the Spanish Tsunami Warning System for the Atlantic

Real-time local tsunami warnings embody uncertainty from unknowns in the source definition within the first minutes after the tsunami generates. In general, Tsunami Warning Systems (TWS) provide a quick estimate for tsunami action from deterministic simulations of a single event. In this study, variability in tsunami source parameters has been included by running 135 tsunami simulations; besides this, four different computational domains in the northeastern Atlantic ocean have been considered, resulting in 540 simulations associated with a single event. This was done for tsunamis generated by earthquakes in the Gulf of Cadiz with impact in the western Iberian peninsula and the Canary Islands. A first answer is provided after one minute, and 7 min are required to perform all the simulations in the four computational domains. The fast computation allows alert levels all along the coast to be incorporated into the Spanish National Tsunami Early Warning System. The main findings are that the use of a set of scenarios that account for the uncertainty in source parameters can produce higher tsunami warnings in certain coastal areas than those obtained from a single deterministic reference scenario. Therefore, this work shows that considering uncertainties in tsunami source parameters helps to avoid possible tsunami warning level underestimations. Furthermore, this study demonstrates that this is possible to do in real time in an actual TWS with the use of high-performance computing resources.

A Database for Tsunamis and Meteotsunamis in the Adriatic Sea

In the frame of the Interreg Italy-Croatia program, the EU has funded the PMO-GATE project, focusing on the prevention and mitigation of the socioeconomic impact of natural hazards in the Adriatic region. The Database of Adriatic Tsunamis and Meteotsunamis (DAMT) is one of the deliverables of this project. DAMT is a collection of data documenting both meteotsunami and tsunami effects along the Eastern and Western Adriatic coasts, and it was realized by starting from the available database and catalogues, with the inclusion of new data gained from recent studies, newspapers and websites. For each tsunami and meteotsunami, the database provides an overview of the event and a detailed description of the effects observed at each affected location and gives a picture of the geographical distribution of the effects. The database can be accessed through a GIS WebApp, which allows the user to visualize the georeferenced information on a map. The DAMT WebApp includes three layers: (1) Adriatic Tsunami Sources, (2) Adriatic Tsunami Observation Points and (3) Adriatic Meteotsunamis Observation Points. The database contains 57 observations of tsunami effects related to 27 tsunamis along the Italian, Croatian, Montenegrin and Albanian coasts and 102 observations of meteotsunami effects related to 33 meteotsunamis.

Tsunami Occurrence 1900–2020: A Global Review, with Examples from Indonesia

We present an overview of tsunami occurrences based on an analysis of a global database of tsunamis for the period 1900–2020. We evaluate the geographic and statistical distribution of various tsunami source mechanisms, high-fatality tsunamis, maximum water heights (MWHs) of tsunamis, and possible biases in the observation and recording of tsunami events. We enhance a global statistical overview with case studies from Indonesia, where tsunamis are generated from a diverse range of sources, including subduction zones, crustal faults, landslides, and volcanic islands. While 80% of global recorded tsunamis during 1900–2020 have been attributed to earthquake sources, the median MWH of earthquake tsunamis is just 0.4 m. In contrast, the median water height of landslide tsunamis is 4 m. Landslides have caused or contributed to 24% of fatal tsunamis. During 1900–2020, more tsunamis with water heights > 1 m occurred in Indonesia than in any other country. In this region fatal tsunamis are caused by subduction zone earthquakes, landslides, volcanos, and intraplate crustal earthquakes. Landslide and volcano tsunami sources, as well as coastal landforms such as narrow embayments have caused high local maximum water heights and numerous fatalities in Indonesia. Tsunami hazards are increased in this region due to the densely populated and extensive coastal zones, as well as sea level rise from polar ice melt and local subsidence. Interrelated and often extreme natural hazards in this region present both an opportunity and a need to better understand a broader range of tsunami processes.

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.

Interactions between Dongsha Atoll and Mw 9 Tsunamis and their Impacts in South China Sea Region

Tsunamis induced by megathrust in the Manila subduction zone impose alarming threats to the coastal cities in the northern South China Sea (SCS), and risk assessment of tsunami hazards in this region becomes demanding. One distinguishable geographic feature in this region is Dongsha Atoll, which is situated between the tsunami source zone and the China coastline. This study discusses the role of the Dongsha Atoll in modifying the tsunami impacts through numerical simulations of a group of synthetic tsunami events with [Formula: see text]. Three types of representations for the Atoll in the numerical simulations are employed, specifically (i) the real topo-bathymetry of the Atoll is fully resolved (Model-1), (ii) the Atoll is removed (Model-2), and (iii) the Atoll is artificially represented by a cylinder (Model-3). The results show that without the Dongsha Atoll presence (Model-2), the tsunamis can induce a substantial increment of water level behind the Atoll and in the vicinity of the Pearl River Delta (PRD) region in China. While the results obtained by realistically modeling the Atoll (Model-1) show that a large amount of tsunami wave energy/momentum can be entrapped by the lagoon and slowly radiated to the ocean. The interactions between the tsunamis and the Atoll will lead to severe flooding on the Dongsha Island on the west bank of the Atoll. However, the peak water levels behind the Atoll and at the selected mainland coastal sites are reduced, indicating that the Dongsha Atoll can offer some degree of protection for the region in its behind. Meanwhile, a slightly smaller reduction of tsunami heights is reported in the simulations with the Atoll being represented by a cylinder (Model-3), implying that the reduction effects are sensitive to the representations of the Atoll. Therefore, the full model of the topo-bathymetry of the Dongsha Atoll is recommended for future tsunami risk assessments for both Dongsha Atoll and the mainland in the SCS region.

Local amplification of tsunami waves along the west coast of Negros Island and Panay Island

Panay Island and Negros Island in the east of the Sulu Sea face hazard of near-field earthquake-generated tsunami. This study aims to assess the tsunami hazard along the west coast of the two islands caused by near-field earthquakes with the largest possible magnitude, and focuses on the high hazard spots which are always attacked by higher tsunami waves. The Negros Trench is considered to be the potential source of seismic tsunami based on the historical tsunami and earthquake data, and special attention is given to the hazard associated with edge waves caused by near-field earthquake and other wave energy convergence mechanisms. A series of tsunami scenarios in this area are numerically simulated based on the widely-used ocean dynamic model FVCOM (an unstructured grid, Finite-Volume Community Ocean Model). The largest historical tsunami in the recent century is simulated and reproduced, and the results are in reasonable agreement with the historical observation. Further, a set of possible scenarios considering earthquakes with largest historical magnitude are simulated, and other seismic parameters are determined through the statistical analysis of the regional historical seismic data and the previous studies. The maximum tsunami wave along the coast in each scenario is analyzed and given. The results show that the near-shore tsunami is greatly affected by the epicenter location and other seismic parameters. The influences of seismic parameters on each coast location are different and complex, due to the winding shoreline and local topography. However, the spatial distribution of tsunami waves along the coast has certain regularity. It is found that there are some spots which always face a higher tsunami hazard. From the analysis of wave propagation, the high hazard along the west coast of Negros Island is ascribed to edge waves formed by the trap of wave energy in shallow water. Moreover, a variety of interesting phenomena associating with other high hazard spots can be found in this area, including wave convergence by bell-shaped mouth, the superposition of incident and reflected waves, and wave refraction around a cape. More attention in coastal protection should be paid on those high hazard spots.

Reexamination of tsunami source models for the twentieth century earthquakes off Hokkaido and Tohoku along the eastern margin of the Sea of Japan

Large earthquakes around Japan occur not only in the Pacific Ocean but also in the Sea of Japan, and cause both damage from the earthquake itself and from the ensuing tsunami to the coastal areas. Recently, offshore active fault surveys were conducted in the Sea of Japan by the Integrated Research Project on Seismic and Tsunami Hazards around the Sea of Japan (JSPJ), and their fault models (length, width, strike, dip, and slip angles) have been obtained. We examined the causative faults of M7 or larger earthquakes in the Sea of Japan during the twentieth century, comprising events of 1940, 1964, 1983, and 1993, using seismic and tsunami data. The 1940 off Shakotan Peninsula earthquake (MJMA 7.5) appears to have been caused by the offshore active faults MS01, MS02, ST01, and ST02 as modeled by the JSPJ. The 1993 off the southwest coast of Hokkaido earthquake (MJMA 7.8) likely occurred on the offshore active faults OK03a, OK03b, and OK05, while the 1983 Central Sea of Japan earthquake (MJMA 7.7) probably related to MMS01, MMS04, and MGM01. For these earthquakes, the observed tsunami waveforms were basically reproduced by tsunami numerical simulation from the offshore active faults with the slip amounts obtained by the scaling relation with three stages between seismic moment and source area for inland earthquakes. However, the observed tsunami runup heights along the coast were not reproduced at certain locations, possibly because of the coarse bathymetry data used for the simulation. The 1983 west off Aomori (MJMA 7.1) and the 1964 off Oga Peninsula (MJMA 6.9) earthquakes showed multiple faults near the source area that could be used to reproduce the observed tsunami waveforms; therefore, we could not identify the causative faults. Further analysis using near-field seismic waveforms is required for their identification of their causative faults and their parameters. The scaling relation for inland earthquakes can be used to obtain the slip amounts for offshore active faults in the Sea of Japan and to estimate the coastal tsunami heights and inundation area which can be useful for disaster prevention and mitigation of future earthquakes and tsunamis.Graphical

Submarine Landslides in the West Continental Slope of the South China Sea and Their Tsunamigenic Potential

The 109 meridian fault is located in the west of the South China Sea (SCS) connecting to the offshore Red River Shear Zone. Seismic data from the central Vietnamese shelf indicates that many submarine landslides were developed along the steep continental slope in this offshore region. Here, we analyze the potential for such landslides to trigger damaging tsunamis based on the local geological background and sedimentary environment. We assess their tsunamigenic potential along the coast of Southern Central Vietnam (SCV). We point out that the evolutionary processes of the 109° meridian fault: striking-subsidence of the adjacent basin, combined with the high sediment input from numerous montane rivers of the hinterland generate conditions that likely favor the development of submarine landslides along the well-defined and steep continental slope near SCV. To estimate the impact of tsunami waves on the SCV coastline, we conducted a pilot study using two numerical models: NHWAVE and FUNWAVE-TVD to model 4 representative landslides with volumes ranging between 1.3 and 14 km3 and water depth of 300–1000 m. The submarine landslides were treated as rigid slump and deformable slide corresponding to two different sedimentary environments. Our results show that the tsunami waves generated by rigid slump can reach up to 20 m height in the landslide source area and ∼5 m when arriving at the closest coastline. Tsunami waves could arrive at the central Vietnam coast within 30 min in eight scenarios. Our initial results also suggest that seafloor topography, i.e., waveguide effects of ocean ridges, shelf resonance and the potential bay resonance cause significant variability in potential run-up. We note that ocean ridges located in the deep basin of the SCS focus the tsunami energy propagating towards the northwest coast of Luzon Island, Philippines where tsunami wave heights of ∼2.3 m wave height are modeled. These findings underscore the importance of tsunami hazard assessments that account for both earthquake generated and earthquake triggered tsunamis. Our work also highlights a continued need to examine tsunami sources in the region as mitigation and preparedness for the socio-economically important and heavily populated coastlines of the SCS is reliant on a detailed understanding of the hazard.

Risk Assessment of Tsunamis Along the Chinese Coast Due to Earthquakes

China’s coastal areas are densely populated, economically developed, and located in close proximity to several potential tsunami sources; therefore, tsunami risk cannot be ignored. This study assessed tsunami risk in coastal areas of China by developing a framework for tsunami risk assessment from the perspectives of hazards, vulnerability, and exposure. First, a probabilistic tsunami hazard assessment (PTHA) model was applied to estimate the potential tsunami sources in both local crustal faults and circum-Pacific subduction zones based on numerical simulations. The output of the PTHA includes tsunami wave height distributions along the coast. Then, an indicator system reflecting exposure and vulnerability to tsunamis in the coastal areas of China was established by using the entropy method and analytic hierarchy process. The PTHA findings show that the tsunami wave height is close to 3 m on the southern coast of the Bohai Sea, the Pearl River Estuary, and the Yangtze River Delta and exceeds 2 m near the Taiwan Strait for the 2000-year return period. The results of the tsunami risk assessment show that the cities at the highest risk level (level I) include Tangshan, Yantai, and Hong Kong, while cities at the high risk level (level II) include Fuzhou, Xiamen, and Quanzhou near the Taiwan Strait and many cities on the Yangtze River Delta, the Pearl River Estuary, and the southern coast of the Bohai Sea. Our findings can provide an understanding of differences in tsunami risk between Chinese coastal cities that may be affected by tsunamis in the future.

Scenario-based tsunami hazard assessment for the coast of Vietnam from the \(109^{o}\) Meridian fault zone

Located on the continental shelf and stretching approximately 1,000 km along the Central and Southern Central coasts of Vietnam, the 109o Meridian Fault system has long been recognized as seismically active. Empirical evidence of the seismic, volcanic, and submarine landslide activities also suggests that this fault might be capable of originating near-field tsunamis, which can harm the Vietnamese coastal zone. This paper investigates the possibility of the near-field tsunamis from the 109o Meridian Fault and their impact on the Vietnamese coasts. Location and segmentation of the 109o Meridian fault were determined using seismological, geophysical data, and geomorphologic evidence. The fault’s seismotectonic and geodynamic characteristics were used to model the tsunami source. The COMCOT model was used for simulating three worst-case tsunami scenarios excited by earthquakes with Mw = 8.0 originated in two different segments of the 109o Meridian Fault source. The relationship between tsunami wave height and the source-to-coast travel time is investigated in detail at the virtual sea-level stations distributed along the Vietnamese coastline. The simulation results are presented in the deep-ocean tsunami amplitude maps for the entire East Vietnam Sea region and the coastal tsunami amplitude maps for the Vietnamese coasts. The simulation results show the highest tsunami hazards concentrated along the Central and Southern Central Vietnam coastal zones (from Quang Nam province to Ba Ria-Vung Tau province), with the highest tsunami waves not exceeding 4 m observed off-shore the Quang Ngai, Phu Yen and Ninh Thuan provinces. The shortest tsunami travel time from the source to the Southern Central coast is 35 minutes. Due to its strike-slip mechanism, there is a low possibility of the 109o Meridian fault generating earthquake-triggered tsunamis. Moreover, even if they occurred, the near-field tsunamis generated from the 109o Meridian fault source can hardly cause severe damage and losses for the coastal zones of Vietnam, as shown by the simulation results. Nevertheless, the worst-case scenarios simulation results provide the highest risk that the near-field tsunamis from the 109o Meridian fault source might affect the Vietnamese coasts, which is helpful for the national tsunami warning and response purpose.

Reconstructing the 26 June 1917 Samoa Tsunami Disaster

The 1917 Samoa tsunamigenic earthquake is the largest historical event to impact this region. Over a century later, little is known about the tsunami magnitude and its implications for modern society. This study reconstructs the 1917 tsunami to understand its hazard characteristics in the Samoan region and assesses the risk implications of tsunamis sourced from different locations along the subduction zone bend of the Northern Tonga Trench (NTT). We model the event from its origin to produce outputs of tsunami inundation extent and depth at spatially flexible grid resolution, which are validated using available runup observations and Apia harbour tide gauge records. We then combine the inundation model with digital distributions of buildings to produce exposure metrics for evaluating the likely impacts on present-day coastal assets and populations if a similar tsunami were to occur. Results exhibit recorded and modelled wave arrival time discrepancies in Apia harbour of between 30–40 min, with runup underestimated in southeast Upolu Island compared with the rest of the country. These differences could reflect complexities in the tsunami source mechanism that are not represented in our modelling and require further investigation. Nevertheless, our findings suggest that if a characteristic 1917-type event were to occur again, approximately 71% of exposed people would reside in Savai’i. Overall, this study provides the first detailed inundation model of the 1917 tsunami that supports an appreciation of the regional risk to local tsunamis sourced at the subduction zone bend of the NTT in Samoa.

Source Properties and Resonance Characteristics of the Tsunami Generated by the 2021 M 8.2 Alaska Earthquake

AbstractA tsunami was triggered in the North Pacific Ocean by the 2021 Alaska earthquake (M 8.2) on July 29. We studied the source properties and resonance characteristics of the tsunami event using observed records from offshore tsunameters and coastal tide gauges as well as numerical simulation. Spectral analyses were conducted on records at tsunameters and tide gauges. We reconstructed the tsunami source spectrum by calculating the ratio of tsunami spectra to the background spectra of these stations. Based on the source spectrum, we estimated the source size of the 2021 Alaska earthquake to be 332 km (length) 256 km (width), which was generally consistent with the source model proposed by the United States Geological Survey using seismological approaches. In addition, we also performed spectral analyses of the tsunami wavefield in a region of interest, including the Aleutian Islands and the Alaska Peninsula. Standing wave systems were found south of the Alaska Peninsula and the Aleutian Islands. The Aleutian Islands prevent the propagation of tsunami oscillations. Only a small portion of energy propagates through the archipelago from the main part of the Pacific Ocean to the Bering Sea.

Multilayer modelling of waves generated by explosive subaqueous volcanism

Abstract. Theoretical source models of underwater explosions are often applied in studying tsunami hazards associated with subaqueous volcanism; however, their use in numerical codes based on the shallow water equations can neglect the significant dispersion of the generated wavefield. A non-hydrostatic multilayer method is validated against a laboratory-scale experiment of wave generation from instantaneous disturbances and at field-scale subaqueous explosions at Mono Lake, California, utilising the relevant theoretical models. The numerical method accurately reproduces the range of observed wave characteristics for positive disturbances and suggests a relationship of extended initial troughs for negative disturbances at low-dispersivity and high-non-linearity parameters. Satisfactory amplitudes and phase velocities within the initial wave group are found using underwater explosion models at Mono Lake. The scheme is then applied to modelling tsunamis generated by volcanic explosions at Lake Taupō, New Zealand, for a magnitude representing an ejecta volume of 0.1 km3. Waves reach all shores within 15 min with maximum incident crest amplitudes around 0.2 m at shores near the source. This work shows that the multilayer scheme used is computationally efficient and able to capture a wide range of wave characteristics, including dispersive effects, which is necessary when investigating subaqueous explosions. This research therefore provides the foundation for future studies involving a rigorous probabilistic hazard assessment to quantify the risks and relative significance of this tsunami source mechanism.

Deterministic tsunami hazard assessment and zoning approach using far-field and near-field sources: Study of Cixi County of Zhejiang Province, China

Investigating the potential tsunami hazard for coastal regions is helpful for urban development and emergency evacuation. In this paper, we present a deterministic method for tsunami hazard assessment (DTHA) in terms of tsunami amplitude and tsunami-induced currents. 21 extreme far-field transoceanic tsunami sources located in the Pacific Rim and, 14 near-field sources in Ryukyu Trough and Manila Trench are suggested. The Digital Elevation Model (DEM) data of 50 m combined with coastal breakwater data is adopted. The tsunami hazard is analyzed based on tsunami amplitude and tsunami-induced velocity. The coastal risk of Cixi County is presented and discussed. The results highlight the necessity of combining the tsunami amplitude and tsunami-induced currents in hazard assessment with the protective role of breakwaters. Further study manifests the effects of astronomical tide on tsunami hazard. The submarine topography consisted of ridges and trenches could trap tsunami energy and change propagation direction. The heterogeneous effect of slip distribution on tsunami hazard assessment is investigated, indicating the feasibility of using uniform slip source in DTHA. This study provides a comprehensive and quantitative assessment method incorporating tide, DEM, breakwater, and multiple sources to evaluate the tsunami risk for hazard reduction and urban planning.

The Mw = 6.6 earthquake and tsunami of south Crete on 2020 May 2

SUMMARY On 2020 May 2, an Mw = 6.6 earthquake struck about 63 km south of Ierapetra in Crete, Greece. The earthquake generated a small tsunami which agitated local harbours. We studied this event in the context of earthquakes with seismic records in 1908, 1910, 1923, 1952, 2009 and 2013, all of similar magnitudes located south of Crete. Based on an energy-to-moment ratio, our analysis suggests that this event was neither slow nor fast, hence appropriate for using scaling laws to infer seafloor deformations. We also performed a field survey, three days after the event and present field observations from seven locations, including the island of Chrisi, where our highest measurement of 0.95 m was located. Runup along the coast of southern Crete ranged from 0.24 to 0.87 m. One tide gauge record is available for this event, and we did image analysis to obtain accurately timed water surface elevations from eyewitness videos and images. We undertook high-resolution hydrodynamic simulations using published moment tensor solutions to identify the source of the tsunami. Simulations were performed with two models, MOST (a nonlinear shallow water model) and COULWAVE (a Boussinesq-type model), to infer how different approximations of the parent equations of motion affect predictions for tsunamis of this size, which are fairly common in the Eastern Mediterranean and routinely trigger Tsunami Service Providers to issue warning messages. Based on the inter-model comparison, we conclude that the shallow-water equations are adequate in modelling this event at the distances considered, suggesting that such codes can be used to infer the tsunami source and to estimate tsunami impacts. Last, our field work revealed lack of knowledge of tsunami hazards, as most eyewitnesses remained near the waterfront, filming the associated unusual water motions instead of taking shelter on high ground.