Tsunami Hazard Assessment Research Articles

Three‐Dimensional Physical Modeling of Tsunamis Generated by Partially Submerged Landslides

AbstractMassive landslides can generate large tsunamis and endanger human lives and properties. While a number of laboratory experiments on either subaerial or submarine landslide‐tsunamis have been conducted, few studies have focused on partially submerged events. The present study aimed to investigate the characteristics of partially submerged landslide‐tsunamis and to develop a predictive equation that can estimate their heights through three‐dimensional laboratory experiments that used 114 different conditions. The experimental results showed that, while waves generated in the landslide sliding direction had around 1.7 times higher amplitude than those propagating laterally in the near‐field region, the wave period, celerity, and wavelength did not alter significantly according to the propagation angles. The effects of wave dispersion were also shown to be significant during the propagation process. Such wave characteristics were consistent with those reported in existing subaerial landslide tsunami experiments conducted by other researchers. Based on the data set obtained, the authors also derived a predictive equation which was confirmed to be able to estimate the results of other experimental studies on partially submerged landslide‐tsunamis with an error of around ±30%, and thus can be helpful for a rapid tsunami hazard assessment. The developed equation indicated that the volume of a landslide could have more than twice the impact on the height of a partially submerged landslide‐tsunami than other parameters. The effects of the landslide size were also shown to be more significant than in the case of a subaerial landslide‐tsunami, though the wave attenuation process was similar in both types.

Probabilistic, high-resolution tsunami predictions in northern Cascadia by exploiting sequential design for efficient emulation

Abstract. The potential of a full-margin rupture along the Cascadia subduction zone poses a significant threat over a populous region of North America. Previous probabilistic tsunami hazard assessment studies produced hazard curves based on simulated predictions of tsunami waves, either at low resolution or at high resolution for a local area or under limited ranges of scenarios or at a high computational cost to generate hundreds of scenarios at high resolution. We use the graphics processing unit (GPU)-accelerated tsunami simulator VOLNA-OP2 with a detailed representation of topographic and bathymetric features. We replace the simulator by a Gaussian process emulator at each output location to overcome the large computational burden. The emulators are statistical approximations of the simulator's behaviour. We train the emulators on a set of input–output pairs and use them to generate approximate output values over a six-dimensional scenario parameter space, e.g. uplift/subsidence ratio and maximum uplift, that represent the seabed deformation. We implement an advanced sequential design algorithm for the optimal selection of only 60 simulations. The low cost of emulation provides for additional flexibility in the shape of the deformation, which we illustrate here considering two families – buried rupture and splay-faulting – of 2000 potential scenarios. This approach allows for the first emulation-accelerated computation of probabilistic tsunami hazard in the region of the city of Victoria, British Columbia.

Fast High-Resolution S-PTHA Along the Western Mediterranean Sea Coastlines. Application to the Bay of Cannes

Probabilistic Tsunami Hazard Assessment (PTHA) is a fundamental framework for producing time-independent forecasts of tsunami hazards at the coast, taking into account local to distant tsunamigenic earthquake sources. If high resolution bathymetry and topography data at the shoreline are available, local tsunami inundation models can be computed to identify the highest risk areas and derive evidence-based evacuation plans to improve community safety. We propose a fast high-resolution Seismic-PTHA approach to estimate the tsunami hazard at a coastal level using the Bay of Cannes as test site. The S-PTHA process is firstly fastened by performing seismic and tsunami hazards separately to allow for quick updates, either from seismic rates by adding new earthquakes, or from tsunami hazard by adding new scenarios of tsunamis. Furthermore, significant tsunamis are selected on the basis of the extrapolation of a tsunami amplitude collected offshore from low-resolution simulations to an a priori amplitude nearshore using Green’s law. This allows a saving in computation time on high-resolution simulations of almost 85%. The S-PTHA performed in the Bay of Cannes exhibits maximum expected tsunami waves that do not exceed 1 m in a 2500-year period, except in some particular places such as the Old Port of Cannes. However, the probability to experience wave heights of 30 cm in this same period exceeds 50% along the main beach of Cannes and these results need to be considered in risk mitigation plans given the high touristic attraction of the area, especially in summer times.

Geological evidence of an unreported historical Chilean tsunami reveals more frequent inundation

Assessing tsunami hazards commonly relies on historical accounts of past inundations, but such chronicles may be biased by temporal gaps due to historical circumstances. As a possible example, 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 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, decimetric subsidence. Coupled dislocation-tsunami models place the causative fault slip mostly offshore rather than below land. 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 tsunami hazard assessment.

Introduction to “Sixty Years of Modern Tsunami Science, Volume 1: Lessons and Progress”

Twenty-three papers are included in this PAGEOPH topical issue “Sixty Years of Modern Tsunami Science, Volume I: Lessons and Progress”. The papers are grouped into four categories: historical tsunami events, studies on tsunami source models and case studies, tsunami hydrodynamics, and probabilistic tsunami hazard assessment and forecasting. Papers that reflect the current state of tsunami science and research are briefly introduced.

Reevaluation of the 11 November 1858 Earthquake and Tsunami in Setúbal: A Contribution to the Seismic and Tsunami Hazard Assessment in Southwest Iberia

Reevaluation of the 11 November 1858 Earthquake and Tsunami in Setúbal: A Contribution to the Seismic and Tsunami Hazard Assessment in Southwest Iberia

A Generic Method to Derive Coastal Bathymetry From Satellite Photogrammetry for Tsunami Hazard Assessment

AbstractIn this study, we use Pleiades satellite stereo imagery to map shallow‐water bathymetry specifically for the purpose of assessing tsunami hazard in coastal regions. In order to calculate the refraction correction factor, we independently measure the refractive indices of seawater mixed with a variety of phytoplankton, which was previously unquantified. We find that the refractive index of seawater increases with cell density of phytoplankton, ranging from 1.3410 to 1.3425. Through rigorous error analysis, we demonstrate that the change in refraction due to heterogeneous distribution of phytoplankton is negligible, suggesting that the technique is applicable to the global ocean. Using the Penghu Islands as a case example, we show that photogrammetric bathymetry is able to reveal tsunami impacts more accurately and reasonably than the commonly used General Bathymetric Chart of the Oceans data, highlighting the importance of high‐resolution and high‐accuracy bathymetry for tsunami hazard assessment.

Probabilistic Tsunami Hazard Assessment for the Southeast Coast of China: Consideration of Both Regional and Local Potential Sources

Probabilistic Tsunami Hazard Assessment for the Southeast Coast of China: Consideration of Both Regional and Local Potential Sources

A numerical model for the efficient simulation of multiple landslide-induced tsunamis scenarios

Submarine landslides can pose serious tsunami hazard to coastal communities. However, performing a comprehensive landslide tsunami hazard assessment for a given area is in general difficult in view of the large uncertainty associated with tsunamigenic source parameters, which are often only approximately defined, based on estimates of the landslide geometry, slide material properties, and resulting kinematics. Therefore, a Probabilistic Tsunami Hazard Analysis (PTHA) should be performed by considering a large number of cases, which is computationally demanding. Here, we present an efficient model based on solving the linear Mild-Slope Equation with a time-dependent source term representing the seafloor motion. This approach allows carrying out many computations, for a large number of landslide scenarios, in a Monte Carlo (MC) approach framework, at a reduced computational cost compared to other available methods, while still providing physically accurate simulations of most landslide tsunami generation and propagation processes. To further speed-up the MC simulations, a database of elementary solutions is first developed, for many landslide sources of unit amplitude motion over a small seafloor area within the possible landslide footprint. For each unit source, the resulting tsunami elevations are computed and saved at many locations of interest. In the MC simulations, a large number of landslide scenarios are defined by randomly selecting slide parameters within their statistical distributions and each is then simulated for their specific bottom motion using a linear combination of the pre-computed unit sources. Hence, each resulting tsunami is quickly computed at the locations of interest by linear superposition. The paper presents the model validation against two tests cases and describes its novel methodology to perform multiple landslide tsunami scenarios.

Framework for probabilistic tsunami hazard assessment considering the effects of sea-level rise due to climate change

Sea-level rise due to climate change could significantly exacerbate tsunami disasters since the sea level is a critical parameter affecting the intensity of tsunamis. Considering the impacts of future climate change on the ocean, a method to consider the effects of sea-level rise on the tsunami hazard intensity is needed to precisely predict future tsunami disasters. This paper presents a novel framework for probabilistic tsunami hazard assessment, considering the sea-level rise and associated uncertainties. A probabilistic assessment of the sea-level rise hazard is performed using the available data and climate models considering several climate change emission scenarios. Conditional tsunami hazard curves are estimated by conducting tsunami propagation simulations given a sea-level rise while considering the uncertainties associated with fault movement (i.e., rake angles and average stress drops). Radial-basis-function-based surrogate models and quasi-Monte-Carlo simulations are employed to obtain tsunami hazard curves. Finally, the tsunami hazard curves considering the effects of sea-level rise are estimated by convolving the corresponding regional sea-level rise hazard with the conditional tsunami hazard curves based on the total probability theorem. An illustrative example is provided, in which the proposed framework is applied to several municipalities in the Mie Prefecture of Japan that would be affected by tsunamis during the anticipated Nankai-Tonankai earthquake. The effects of sea-level rise on the tsunami hazard intensities associated with the municipalities are discussed.

Tsunami Hazard Assessment and Potential Risk Mitigation Requirement for Sea of Marmara Coastline

The westward movement of the seismic energy along the North Anatolian Fault (NAF) during the 20th century has increased the probability that the next activity will be offshore in the Sea of Marmara. According to historical records 35 tsunami events have impacted the Sea of Marmara coasts (Altınok, et al., 2011). Offshore seismic sources may trigger these tsunamis directly or through coseismic underwater failure. The 1999 İzmit Bay tsunami led to more comprehensive analyses of these events which are generally caused by underwater failures close to the target coastline. Waves so generated can arrive at nearby coastlines in minutes, causing extensive damage and loss of life. Here this paper proposes, on the basis of tsunami models in the Sea of Marmara and methodology used internationally, tsunami inundation maps for the areas along the Sea of Marmara coasts in the light of both remote sensing and DTMs data. This investigation highlights the information regarding the most vulnerable tsunamigenic areas. Such maps for selected areas help to understand the possible effects on those regions and should only be used for evacuation planning and reducing possible hazard

Tsunami resonance and standing waves in Hangzhou Bay

Earthquakes occurring in the Ryukyu Trench, Nankai Trough, and Japan Trench can trigger tsunamis, affecting Hangzhou Bay, China. The 2011 Tohoku tsunami reached the east China coast and resulted in prolonged oscillations in Hangzhou Bay. We used this event as an example and studied the resonance behavior in Hangzhou Bay by spectral and modal analyses. We simulated the wavefield of the 2011 Tohoku tsunami and validated the modeling results using the tide gauge records. The spectral analysis based on the fast Fourier transform of the entire bay area indicated that the resonance oscillations with periods of 66 and 54 min had the highest energy amplification, and the mode of 120 min had a large amplitude near Zhenhai and Fengxian. Additionally, we conducted a modal analysis that relied solely on bathymetry data to analyze the eigenmodes of natural oscillations inside the bay. The regional bay-scale oscillations had periods of up to 286 min. The energy amplification area was generally consistent with the spectral analysis. These findings have implications for tsunami hazard assessments in coastal communities.

Seismogenic structure along the deformation front off SW Taiwan revealed by the aftershocks of the 2017 Tainan near-shore earthquake with ocean bottom seismometers

Located in the northern end of the Manila Trench, the deformation front (DF) off southwestern Taiwan is considered as a potential area for generating hazardous earthquakes and tsunamis. However, this area has been relatively seismically quiet, and the seismogenic structure has been unclear, particularly in the middle-to-lower crust. On February 10, 2017, an Mw5.3 earthquake occurred in the near-shore area of Tainan in SW Taiwan, which provides us with a good opportunity to study the seismogenic structure of this area. Five ocean bottom seismometers (OBSs) were deployed to cover the mainshock source area from February 13 to March 1, 2017, to record the aftershock data. All the OBSs were successfully recovered and, the P- and S-wave arrival were manually identified. In collaboration with arrival times from the Central Weather Bureau of Taiwan, we simultaneously determined the 1-D optimal Vp and Vs velocity models and hypocenter locations. The inversion results show that the upper crust (<15 km) has a relatively low velocity, and most events are confined between 10 and 15 km depth. The relocated epicenters were all in areas with positive gravity anomalies, and most of them formed a NE-SW striking seismogenic structure that was approximately parallel to the surface trace of the DF and did not link to any known active faults in SW Taiwan. The hypocenters illustrate a conjugate fault system comprising of a SW-dipping NE-SW striking fault and an SE-dipping NW-SE striking fault. Based on focal mechanisms and aftershock distribution, we demonstrate that the 2017 Tainan near-shore earthquake may result from the reactivation of pre-existing faults, and the driving force comes from the bending stresses of the subducting Eurasia Plate at the Manila Trench. Given the active subduction processes offshore of Taiwan, such slab-flexural-related earthquakes must be considered in seismic and tsunami hazard assessments.

Probabilistic tsunami hazard and exposure assessment for the pacific islands- Fiji

Tsunami are known to be one of the most powerful and destructive natural hazards to impact low-lying coastal communities. The Republic of Fiji is situated in a complex tectonic setting at the boundary of two active plates in the South-west Pacific; the Indo-Australian and Pacific Plate, making the country highly vulnerable to earthquakes and tsunami. Local and regional source tsunami is a concern for emergency management. This is due to the limited warning time in receiving a tsunami alert and issuing warnings to the public. Fiji's greatest local and regional tsunami hazard risk is from the Tonga-Kermadec trench and South New Hebrides faults. This paper presents the first probabilistic tsunami hazard assessment of a credible 1 in 10,000 year event for Fiji's two largest populated islands, Viti Levu and Vanua Levu. Subsequent to the probabilistic tsunami hazard assessment, this paper presents the results of a tsunami exposure assessment of the population, assets and critical infrastructure vulnerable to tsunami inundation from Tonga-Kermadec and South New Hebrides tsunami sources. This study applies a mix methodology approach, using a combination of hydrodynamic models, probabilistic approaches, Geographic Information Systems and quantitative analysis to determine maximum tsunami heights and exposure to tsunami hazard. Results of the exposure assessment indicates provinces in Vit Levu and Vanua Levu are likely to be susceptible to various tsunami inundation heights, concluding that people and assets in provinces such as Macuata and Tailevuare more vulnerable to tsunami. The results of this study provides valuable information for the Government of Fiji specifically the National Disaster Management Office, Mineral Resources Department, the National Tsunami Warning Centre and at risk communities. This will increase tsunami risk knowledge and will encourage decisive warning times and allow emergency management to make informative decision-making in regards to emergency preparedness and response planning.

Paleotsunamis on the Southern Hikurangi Subduction Zone, New Zealand, Show Regular Recurrence of Large Subduction Earthquakes

Abstract Geological records of subduction earthquakes, essential for seismic and tsunami hazard assessment, are difficult to obtain at transitional plate boundaries, because upper-plate fault earthquake deformation can mask the subduction zone signal. Here, we examine unusual shell layers within a paleolagoon at Lake Grassmere, at the transition zone between the Hikurangi subduction zone and the Marlborough fault system. Based on biostratigraphic and sedimentological analyses, we interpret the shell layers as tsunami deposits. These are dated at 2145–1837 and 1505–1283 yr B.P., and the most likely source of these tsunamis was ruptures of the southern Hikurangi subduction interface. Identification of these two large earthquakes brings the total record of southern Hikurangi subduction earthquakes to four in the past 2000 yr. For the first time, it is possible to obtain a geologically constrained recurrence interval for the southern Hikurangi subduction zone. We calculate a recurrence interval of 500 yr (335–655 yr, 95% confidence interval) and a coefficient of variation of 0.27 (0.0–0.47, 95% confidence interval). The probability of a large subduction earthquake on the southern Hikurangi subduction zone is 26% within the next 50 yr. We find no consistent temporal relationship between subduction earthquakes and large earthquakes on upper-plate faults.

Wedge Plasticity and Fully Coupled Simulations of Dynamic Rupture and Tsunami in the Cascadia Subduction Zone

AbstractInelastic wedge deformation likely plays an important role in the generation of tsunami and ocean acoustic waves in accretionary subduction margins. In an elastic dislocation model, whether or not the fault breaks the trench has a significant effect on seafloor deformation and resulting tsunami. However, this boundary condition is less important when significant inelastic deformation in the overriding wedge occurs, because large seafloor uplift can occur with little or no slip at the trench. Here we incorporate wedge plasticity in fully coupled dynamic rupture and tsunami simulations for a buried fault in the Cascadia subduction zone with realistic fault geometry, bathymetry, and velocity structure. A linearized Eulerian approach is verified and used to simulate gravity waves in the ocean. Our coupled models show that the inelastic deformation of wedge sediments can significantly contribute to seafloor uplift, producing tsunami heights at least twice as large as in purely elastic simulations, whilst generating weaker ocean acoustic and seismic waves. Inelastic wedge deformation is therefore an important mechanism to consider in tsunami hazard assessment in the Cascadia subduction zone. These results have important implications for tsunami generation and early warning in accretionary and other sediment‐filled margins worldwide.

Geodetic Coupling Models as Constraints on Stochastic Earthquake Ruptures: An Example Application to PTHA in Cascadia

AbstractCurrent stochastic rupture modeling techniques do not consider the potential influence of inter‐seismic coupling, a first‐order property of a megathrust, which can show correlation between areas of high coupling and areas of greater slip as seen in recent large ruptures globally. Therefore, it is reasonable to assume that it should be considered as prior information in rupture modeling. Here, we first present a mathematical formalism to introduce coupling models as prior information into stochastic rupture modeling. We then focus on how introducing slip deficit information into the stochastic rupture models influences slip distributions for the Cascadia subduction zone (CSZ). We compare rupture models created with two end member models of coupling, one with a shallow coupling and another with coupling deeper downdip. We also discuss the comparison to models created without assuming knowledge of the coupling distribution except for variation in the downdip limit of slip. Variations occur and correlate well with areas with the largest differences in slip deficit rates. The ruptures are then used for regional probabilistic tsunami hazard assessment. Overall, the tsunami amplitudes generated are much more hazardous in the northern extent of the CSZ where differences in coupling distribution are more prevalent. Models obtained from assuming a shallower downdip limit have tsunami amplitudes more similar to those from the geodetic coupling models. Although uncertainties are present in the accuracy of coupling, imposing either constraint created different hazard estimations when compared to those where no prior coupling information was used.

Time‐Dependent Probabilistic Tsunami Inundation Assessment Using Mode Decomposition to Assess Uncertainty for an Earthquake Scenario

AbstractThis study presents an innovative probabilistic tsunami inundation assessment for an earthquake scenario to randomly generate tsunami inundation depth distributions by quantitatively evaluating the spatial correlation of tsunami inundation depths using singular value decomposition (SVD) derived from proper orthogonal decomposition and to evaluate the tsunami inundation depths considering the imminent occurrence of an earthquake. We found a good agreement between the evaluation results of the proposed surrogate model and the numerical results of the nonlinear long wave equations for the tsunami inundation depth distribution in Kamakura city, Japan, due to the Sagami Trough megathrust earthquake. Evaluating the spatial correlation using SVD has the advantage that the covariance matrix does not need to be defined in advance but can be defined from the data itself. We also achieved a significant reduction in the number of required tsunami propagation simulations for the probabilistic assessment and attained higher computational efficiency by extracting spatial correlations with SVD. Furthermore, we conducted a probabilistic tsunami inundation assessment focusing on a relatively short period (i.e., 50 years) considering the time‐dependent occurrence probability of the target earthquake. The proposed probabilistic assessment method with mode decomposition is applicable to the general probabilistic tsunami hazard assessment by integrating it with physical stochastic slip models.

Non‐Stationary Probabilistic Tsunami Hazard Assessments Incorporating Climate‐Change‐Driven Sea Level Rise

AbstractWe face a new era in the assessment of multiple natural hazards whose statistics are becoming alarmingly non‐stationary due to ubiquitous long‐term changes in climate. One particular case is tsunami hazard affected by climate‐change‐driven sea level rise (SLR). A traditional tsunami hazard assessment approach where SLR is omitted or included as a constant sea‐level offset in a probabilistic calculation may misrepresent the impacts of climate‐change. In this paper, a general method called non‐stationary probabilistic tsunami hazard assessment (nPTHA), is developed to include the long‐term time‐varying changes in mean sea level. The nPTHA is based on a non‐stationary Poisson process model, which takes advantage of the independence of arrivals within non‐overlapping time‐intervals to specify a temporally varying hazard mean recurrence rate, affected by SLR. The nPTHA is applied to the South China Sea (SCS) for tsunamis generated by earthquakes in the Manila Subduction Zone. The method provides unique and comprehensive results for inundation hazard, combining tsunami and SLR at a specific location over a given exposure time. The results show that in the SCS, SLR has a significant impact when its amplitude is comparable to that of tsunamis with moderate probability of exceedance. The SLR and its associated uncertainty produce an impact on nPTHA results comparable to that caused by the uncertainty in the earthquake recurrence model. These findings are site‐specific and must be analyzed for different regions. The proposed methodology, however, is sufficiently general to include other non‐stationary phenomena and can be exploited for other hazards affected by SLR.

Probabilistic Tsunami Hazard Assessment in Meso and Macro Tidal Areas. Application to the Cádiz Bay, Spain

Tsunami hazard can be analyzed from both deterministic and probabilistic points of view. The deterministic approach is based on a “credible” worst case tsunami, which is often selected from historical events in the region of study. Within the probabilistic approach (PTHA, Probabilistic Tsunami Hazard Analysis), statistical analysis can be carried out in particular regions where historical records of tsunami heights and runup are available. In areas where these historical records are scarce, synthetic series of events are usually generated using Monte Carlo approaches. Commonly, the sea level variation and the currents forced by the tidal motion are either disregarded or considered and treated as aleatory uncertainties in the numerical models. However, in zones with a macro and meso tidal regime, the effect of the tides on the probability distribution of tsunami hazard can be highly important. In this work, we present a PTHA methodology based on the generation of synthetic seismic catalogs and the incorporation of the sea level variation into a Monte Carlo simulation. We applied this methodology to the Bay of Cádiz area in Spain, a zone that was greatly damaged by the 1755 earthquake and tsunami. We build a database of tsunami numerical simulations for different variables: faults, earthquake magnitudes, epicenter locations and sea levels. From this database we generate a set of scenarios from the synthetic seismic catalogs and tidal conditions based on the probabilistic distribution of the involved variables. These scenarios cover the entire range of possible tsunami events in the synthetic catalog (earthquakes and sea levels). Each tsunami scenario is propagated using the tsunami numerical model C3, from the source region to the target coast (Cádiz Bay). Finally, we map the maximum values for a given probability of the selected variables (tsunami intensity measures) producing a set of thematic hazard maps. 1000 different time series of combined tsunamigenic earthquakes and tidal levels were synthetically generated using the Monte Carlo technique. Each time series had a 10000-year duration. The tsunami characteristics were statistically analyzed to derive different thematic maps for the return periods of 500, 1000, 5000, and 10000 years, including the maximum wave elevation, the maximum current speed, the maximum Froude number, and the maximum total forces.