Maximum Tsunami Wave Heights Research Articles

Possible Maximum Earthquake Tsunami Along the South Coast of China Inferred From GPS‐Derived Surface Velocities

AbstractThis study aims to assess the tsunami hazard of the maximum possible earthquake along the south coast of China. GPS‐derived surface velocities around the Manila Trench have been used to clarify the active tectonics and estimate the seismic potential of Manila subduction, where catastrophic tsunamis may originate and cause devastating damage to the surrounding area of the South China Sea. Based on the maximum potential earthquake magnitude estimated from the accumulated seismic moment, more than 100,000 possible tsunami scenarios with varying epicenters have been simulated considering the slip heterogeneity. The impact of several uncertain parameters including the release period and release ratio of accumulated seismic moment are investigated, as well as some assumptions in the model of tsunami generation. Although different release conditions for accumulated seismic moment result in a significant impact on the value of tsunami height, similar trend in spatial distribution has been shown. The results show that the tsunami wave height along the south coast of China has an increasing trend from west to east, with the maximum tsunami wave height exceeding 10 m corresponding to the potential earthquake Mw = 9.1 with a 1,000‐year release period. The impact of fault slip heterogeneity on tsunami wave height is significant. Compared to uniform slip, considering stochastic slip will increase the average tsunami wave height by about 15%. Considering locking distribution will increase the average tsunami wave height by about 20%–30% and also increase the maximum tsunami wave height by about 60%–70%.

Probabilistic Tsunami Hazard Assessment of Offshore Wave Heights for Southern Region of Pingtung County in Southern Taiwan

This study applies the unit tsunami method and probabilistic tsunami hazard analysis to investigate the tsunami hazard for the southern region of Pingtung County in southern Taiwan. The FUNWAVE-TVD model is used to calculate the aleatory and epistemic uncertainties. The aleatory uncertainty is analyzed using a comparison of simulated and observed maximum tsunami wave heights and the epistemic uncertainty is evaluated using a scenario caused by earthquakes in the Manila and Ryukyu subduction zones according to probabilistic seismic hazard analysis in Taiwan using Senior Seismic Hazard Analysis Committee Level 3 methodology. A unit tsunami method database is established for the Manila and Ryukyu subduction zones and used to carry out a probability-based offshore tsunami wave height simulation for the southern region of Pingtung County. A hazard map for tsunamis caused by earthquakes in the Manila and Ryukyu subduction zones is also derived.

Great earthquake and tsunami potential in the eastern Makran subduction zone: New insights from geodetic and structural constraints

The Makran subduction zone ranks one of the highest potentials for seismic and tsunami hazards. However, the rupture style and tsunami-wave dynamics remain ambiguous due to incomplete historical recordings and the lack of modern seismo-geodetic measurements. In this study, we integrate geomorphological, seismic structural models based on seismic images, InSAR measurement, high-resolution multibeam bathymetry, and the Slab-2 model of the subducted oceanic lithosphere to propose a series of geodetic-structural-constrained slip deficit models for tsunami hazard assessment in the northwestern Indian Ocean with a focus on Gwadar port in Pakistan. We show that the accumulated strain on the megathrust could generate an Mw 8.1–8.4 earthquake assuming current steady-state coupling ratio and a 30 GPa rigidity. We reveal the imbricate thrusts of the outer wedge are more efficient to excite tsunamis than the sub-horizontal megathrust (reference model) by increase in wave height at least 1 m (50%) when included, and these have not been considered in tsunami hazard assessments previously. In the worst-case of Mw 9 megathrust model, can occur waves >5 m, strong currents reaching 6 m/s, and inundation distance (>1 km) in the Gwadar port when rupturing to the trench. Additionally, if the updip area of the 1945 event source region ruptured, it could amplify wave height by 0.2-1 m in the Gwadar port and generate a strong tsunami current (>2 m/s) compared with the reference model. These new findings are crucial for seismic and tsunami hazard mitigation and preparedness in the northwestern Indian Ocean. We also demonstrate that modeled maximum tsunami wave heights based on the multibeam, SRTM15+ and GEBCO_2020 bathymetry differ by 0.2-1 m in the source and by 0.1–0.3 m in the far-field. Our study provides better-constrained source models for inundation studies at coastal mega-cities for hazard preparedness and highlights that the tsunami hazard potential is notably higher than previously recognized in this region.

Analysis of the tsunami amplification effect by resonance in Yeongil Bay

Predicting tsunami hazards based on the tsunami source, propagation, runup patterns is critical to protect humans and property. Potential tsunami zone, as well as the historical tsunamis in 1983 and 1993, can be a threat to the east coast of South Korea. The Korea Meteorological Administration established a tsunami forecast warning system to reduce damage from tsunamis, but it does not consider tsunami amplification in the bay due to resonance. In this study, the Numerical model, Cornell Multi-grid Coupled Tsunami model, was used to investigate natural frequency in the bay due to coastal geometry. The study area is Yeongill bay in Pohang, southeast of South Korea, because this area is a natural bay and includes three harbors where resonance significantly occurs. This study generated a Gaussian-shaped tsunami, propagated it into the Yeongill bay, and compared numerical modeling results with data from tide gauge located in Yeongill bay during several storms through spectral analysis. It was found that both energies of tsunamis and storms were amplified at the same frequencies, and maximum tsunami wave height was amplified about 3.12 times. The results in this study can contribute to quantifying the amplification of tsunami heights in the bay.

Five Great Tsunamis of the 20th Century as Recorded on the Coast of British Columbia

The five great trans-Pacific tsunamis of the 20th century that occurred in 1946, 1952, 1957, 1960 and 1964 were accurately recorded by analogue tide gauges on the coast of British Columbia. All available pen-and-paper records of these events were collected, digitised, de-tided and analysed. The 1946 Aleutian Islands event was recorded at two stations, Tofino and Victoria, where maximum trough-to-crest tsunami wave heights were 55 and 27 cm, respectively. These two gauged stations, as well as Prince Rupert, Alert Bay and Kitimat, also recorded the 1952 Kamchatka tsunami, which generated a maximum wave height of 77 cm at Tofino. The 1957 Andreanof Islands tsunami was recorded at six primary stations and the 1960 Chile tsunami by 17 primary and temporary tide gauges. For both of these events, the maximum tsunami wave heights also occurred at Tofino: 48 cm (1957) and 132 cm (1960). The 1964 Alaska tsunami remains the strongest tsunami yet instrumentally recorded on the coast of British Columbia. Our examination of 16 records from this event shows that maximum wave heights at eight stations were higher than 1 m, including Port Alberni (770 cm), Ocean Falls (376 cm), Tofino (237 cm) and Alert Bay (222 cm). We also find that the maximum wave at all stations for this event was among the first three waves. Subsequent wave heights rapidly attenuated following this group of waves. Frequency-time (f–t) analyses of the tsunami waveforms reveal that, for each station, the dominant frequencies of the waves and their evolution with time were very similar for different tsunamis, but differed considerably among sites for a particular tsunami, indicating the strong influence of local/regional topography on the incoming waves. From the latter point of view, the 1964 tsunami was exceptional. The epicentre of the 1964 Alaska earthquake was located much closer to the BC coast than for the other events and, therefore, the influence of the source was much stronger. The “ringing” of this tsunami was substantially shorter (< 1.5 days), the energy decay much faster, and the f–t diagrams at all sites more similar than for the other major events. During the 1964 event, energy associated with the dominant period of 2 h rapidly decayed. All of the tsunamis examined penetrated deep inside the narrow channels, fjords and inlets, typical of the BC coast. Waves also propagated far into some of the rivers. The 1952 Kamchatka tsunami was recorded at Kitimat, located at the head of Kitimat Arm about 80 km from Hecate Strait, while the 1957 Andreanof Islands tsunami was measured at Bella Coola, located at the remotest part of Burke Channel, a distance of ~110 km from the open ocean (Queen Charlotte Sound). The 1960 Chile tsunami was recorded at five stations located in the complex network of inlets and channels that make up the Seymour–Belize Inlet system on the central mainland coast of British Columbia, while the 1964 Alaska tsunami was observed at many stations in the Fraser River, including Pitt Lake that is connected to the Strait of Georgia through a 64-km route upstream in the Fraser and Pitt rivers. Our results further show that occasionally the fjords and inlets of the BC coast not only do not hinder the incoming waves but can strongly enhance them through resonant amplification. This was the case for the 1964 tsunami waves in Alberni Inlet, where the resonant response to the incoming waves resulted in severe damage to Port Alberni at the head of the inlet. The results of our analyses make it possible to isolate the tsunami signal and evaluate the principal parameters of tsunami waves. This, in turn, is of considerable value for three major problems: (1) Cataloguing of tsunami events; (2) estimation of the potential tsunami risk to the BC coast; and (3) verification and calibration of numerical tsunami models.

An agent-based vertical evacuation model for a near-field tsunami: Choice behavior, logical shelter locations, and life safety

In the event of near-field tsunamis, vertical evacuation can be an alternative protective action to horizontal evacuation. The objective of this paper is to present an agent-based modeling (ABM) framework to evaluate vertical evacuation behavior and shelter locations for a near-field tsunami hazard from a Magnitude 9.0 Cascadia Subduction Zone (CSZ) earthquake. The expected mortality rate depending on the location of the vertical evacuation shelter (VES) has been chosen as the primary criterion to assess the effectiveness of the vertical evacuation. In addition, maximum tsunami wave height and the vertical evacuation behavior changes with changes in the placement of the VES have been assessed from a constraint point of view. The results revealed that (1) the choice of VES locations will directly impact the proportion of the people who evacuate vertically; (2) The percentage of people who evacuate vertically exponentially drops as the shelter gets farther from the population centroid; (3) the location of shelter significantly impacts the total mortality rates; (4) improvements in evacuees’ mobility, such as faster walking speed or shorter milling time, will significantly reduce mortality rate and expand the area of choices for VES locations; and (5) when more people choose to evacuate vertically, the total mortality rate reduces notably. However, wrong placement of VES, and at the same time promoting vertical evacuation behavior can result in catastrophic mortality rates. In addition, a study on the impact of the distance of the VES to the ocean, to the population centroid, and to the horizontal shelters outside of the inundation zone, on the total mortality rate and the evacuation efficiency has been performed. This work reveals the non-linear correlation between the aforementioned characteristics of the VES on the expected mortality rate. The results of this research provide an evidence-driven vertical evacuation modeling framework to guide decision makers at city, state, and federal level to understand the dynamics of vertical evacuation behavior and choice of vertical evacuation shelter locations for a community.

The 2004 Sumatra tsunami in the Southeastern Pacific Ocean: New Global Insight from Observations and Modeling

AbstractThe 2004 Sumatra tsunami was an unprecedented global disaster measured throughout the world oceans. The present study focused on a region of the southeastern Pacific Ocean where the “westward” circumferentially propagating tsunami branch converged with the “eastward” branch, based on data from fortuitously placed Chilean DART 32401 and tide gauges along the coast of South America. By comparison of the tsunami and background spectra, we suppressed the influence of topography and reconstructed coastal “spectral ratios” that were in close agreement with a ratio at DART 32401 and spectral ratios in other oceans. Findings indicate that even remote tsunami records carry spectral source signatures (“birth‐marks”). The 2004 tsunami waves were found to occupy the broad frequency band of 0.25–10 cph with the prominent ratio peak at period of 40 min related to the southern fast‐slip source domain. This rupture “hot‐spot” of ∼350 km was responsible for the global impact of the 2004 tsunami. Data from DART 32401 provided validation of model results: the simulated maximum tsunami wave height of 2.25 cm was a conservative approximation to the measured height of 2.05 cm; the computed tsunami travel time of 25 h 35 min to DART 32401, although 20 min earlier than the actual travel time, provided a favorable result in comparison with 24 h 25 min estimated from classical kinematic theory. The numerical simulations consistently reproduced the wave height changes observed along the coast of South America, including local amplification of tsunami waves at the northern stations of Arica (72 cm) and Callao (67 cm).

Challenges in Defining Tsunami Wave Heights

The National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Information (NCEI) and co-located World Data Service for Geophysics maintain the global tsunami archive consisting of the historical tsunami database, imagery, and raw and processed water level data. The historical tsunami database incorporates, where available, maximum wave heights for each coastal tide gauge and deep-ocean buoy that recorded a tsunami signal. These data are important because they are used for tsunami hazard assessment, model calibration, validation, and forecast and warning. There have been ongoing discussions in the tsunami community about the correct way to measure and report these wave heights. It is important to understand how these measurements might vary depending on how the data were processed and the definition of maximum wave height. On September 16, 2015, an 8.3 M w earthquake located 48 km west of Illapel, Chile generated a tsunami that was observed all over the Pacific region. We processed the time-series water level data for 57 coastal tide gauges that recorded this tsunami and compared the maximum wave heights determined from different definitions. We also compared the maximum wave heights from the NCEI-processed data with the heights reported by the NOAA Tsunami Warning Centers. We found that in the near field different methods of determining the maximum tsunami wave heights could result in large differences due to possible instrumental clipping. We also found that the maximum peak is usually larger than the maximum amplitude (½ peak-to-trough), but the differences for the majority of the stations were <20 cm. For this event, the maximum tsunami wave heights determined by either definition (maximum peak or amplitude) would have validated the forecasts issued by the NOAA Tsunami Warning Centers. Since there is currently only one field in the NCEI historical tsunami database to store the maximum tsunami wave height for each tide gauge and deep-ocean buoy, NCEI will consider adding an additional field for the maximum peak measurement.

Converting Tsunami Wave Heights to Earthquake Magnitudes

There is a fairly strict relation between maximum tsunami wave heights and causation earthquake magnitudes. This provides a new tool for estimating the magnitude of past earthquakes from the observed wave heights of related paleo-tsunami events. The method is subjected to a test versus two paleoseismic events with multiple independent estimates of corresponding earthquake magnitude. The agreement to the tsunami wave height conversion is good, confirming very high magnitudes of M 8.5 - 9.0 and M 8.4 - 8.5. Applying the same method to two Late Holocene events of methane venting tectonics indicates a ground shaking of forces equivalent to a M 8.0 earthquake, seriously changing previous long-term crustal hazard assessments.

Tsunami Hazard Analysis of Future Megathrust Sumatra Earthquakes in Padang, Indonesia Using Stochastic Tsunami Simulation

This study assesses the tsunami hazard potential in Padang, Indonesia probabilistically using a novel stochastic tsunami simulation method. The stochastic tsunami simulation is conducted by generating multiple earthquake source models for a given earthquake scenario, which are used as input to run Monte Carlo tsunami simulation. Multiple earthquake source models for three magnitude scenarios, i.e. Mw 8.5, Mw 8.75, and Mw 9.0, are generated using new scaling relationships of earthquake source parameters developed from an extensive set of 226 finite-fault models. In the stochastic tsunami simulation, the effect of incorporating and neglecting the prediction errors of earthquake source parameters is investigated. In total, 600 source models are generated to assess the uncertainty of tsunami wave characteristics and maximum tsunami wave height profiles along coastal line of Padang. The results highlight the influence of the uncertainty of the scaling relationships on tsunami simulation results and provide a greater range of tsunamigenic scenarios produced from the stochastic tsunami simulation. Additionally, the results show that for the future major earthquakes in the Sunda megathrust, the maximum tsunami wave height in Padang areas can reach 20 m and therefore, significant damage and loss may be anticipated in this region.

Assessment of tsunami hazard for coastal areas of Shandong Province, China

Shandong province is located on the east coast of China and has a coastline of about 3100km. There are only a few tsunami events recorded in the history of Shandong Province, but the tsunami hazard assessment is still necessary as the rapid economic development and increasing population of this area. The objective of this study was to evaluate the potential danger posed by tsunamis for Shandong Province. The numerical simulation method was adopted to assess the tsunami hazard for coastal areas of Shandong Province. The Cornell multi-grid coupled tsunami numerical model (COMCOT) was used and its efficacy was verified by comparison with three historical tsunami events. The simulated maximum tsunami wave height agreed well with the observational data. Based on previous studies and statistical analyses, multiple earthquake scenarios in eight seismic zones were designed, the magnitudes of which were set as the potential maximum values. Then, the tsunamis they induced were simulated using the COMCOT model to investigate their impact on the coastal areas of Shandong Province. The numerical results showed that the maximum tsunami wave height, which was caused by the earthquake scenario located in the sea area of the Mariana Islands, could reach up to 1.39m off the eastern coast of Weihai city. The tsunamis from the seismic zones of the Bohai Sea, Okinawa Trough, and Manila Trench could also reach heights of >1m in some areas, meaning that earthquakes in these zones should not be ignored. The inundation hazard was distributed primarily in some northern coastal areas near Yantai and southeastern coastal areas of Shandong Peninsula. When considering both the magnitude and arrival time of tsunamis, it is suggested that greater attention be paid to earthquakes that occur in the Bohai Sea. In conclusion, the tsunami hazard facing the coastal area of Shandong Province is not very serious; however, disasters could occur if such events coincided with spring tides or other extreme oceanic conditions. The results of this study will be useful for the design of coastal engineering projects and the establishment of a tsunami warning system for Shandong Province.

Did an underwater landslide trigger the June 22, 1932 tsunami off the Pacific coast of Mexico?

On June 22, 1932, a 10- to 12-m-high tsunami wave struck ~60 km off the Mexican Pacific coast. The associated earthquake that apparently produced this tsunami is questionable because of its relatively small magnitude (Ms = 6.9) to produce such tsunami heights. Historical documents, survivor testimony, tsunami catalogs, a post-tsunami survey report, together with geomorphological interpretation of the continental shelf and slope, and numerical modeling were combined to characterize the tsunami parameters. Our results suggest that recorded maximum tsunami wave height, horizontal inundation, arrival time, directivity, effects, and damage are compatible with those characteristics related to an underwater landslide tsunami. The associated landslide (slump) is 4.2 km long, 3.9 km wide, 0.448 km thick, and is located in the upper continental shelf of the Armeria Canyon. Elucidating the cause and mechanisms of the near-field 1932 tsunami would aid in considering a wider spectrum of tsunami sources in hazard mitigation programs of the Mexican Pacific coast.

Propagation mechanisms of incident tsunami wave in Jiangsu coastal area, caused by eastern Japan earthquake on March 11, 2011

At 13:46 on March 11, 2011 (Beijing time), an earthquake of Mw=9.0 occurred in Japan. By comparing the tsunami data from Guanhekou marine station with other tsunami wave observation gathered from southeast coastal area of China, it was evident that, only in Guanhekou, the position of the maximum wave height appeared in the middle part rather than in the front of the tsunami wave train. A numerical model of tsunami propagation based on 2-D nonlinear shallow water equations was built to study the impact range and main causes of the special tsunami waveform discovered in Jiangsu coastal area. The results showed that nearly three-quarters of the Jiangsu coastal area, mainly comprised the part north of the radial sand ridges, reached its maximum tsunami wave height in the middle part of the wave train. The main cause of the special waveform was the special underwater topography condition of the Yellow Sea and the East China Sea area, which influenced the tsunami propagation and waveform significantly. Although land boundary reflection brought an effect on the position of the maximum wave height to a certain extent, as the limits of the incident waveform and distances between the observation points and shore, it was not the dominant influence factor of the special waveform. Coriolis force’s impact on the tsunami waves was so weak that it was not the main cause for the special phenomenon in Jiangsu coastal area. The study reminds us that the most destructive wave might not appear in the first one in tsunami wave train.

Earthquake Scenario-Based Tsunami Wave Heights in the Eastern Mediterranean and Connected Seas

We identified a set of tsunami scenario input parameters in a 0.5° × 0.5° uniformly gridded area in the Eastern Mediterranean, Aegean (both for shallow- and intermediate-depth earthquakes) and Black Seas (only shallow earthquakes) and calculated tsunami scenarios using the SWAN-Joint Research Centre (SWAN-JRC) code (Mader 2004; Annunziato 2007) with 2-arcmin resolution bathymetry data for the range of 6.5—Mwmax with an Mw increment of 0.1 at each grid in order to realize a comprehensive analysis of tsunami wave heights from earthquakes originating in the region. We defined characteristic earthquake source parameters from a compiled set of sources such as existing moment tensor catalogues and various reference studies, together with the Mwmax assigned in the literature, where possible. Results from 2,415 scenarios show that in the Eastern Mediterranean and its connected seas (Aegean and Black Sea), shallow earthquakes with Mw ≥ 6.5 may result in coastal wave heights of 0.5 m, whereas the same wave height would be expected only from intermediate-depth earthquakes with Mw ≥ 7.0 . The distribution of maximum wave heights calculated indicate that tsunami wave heights up to 1 m could be expected in the northern Aegean, whereas in the Black Sea, Cyprus, Levantine coasts, northern Libya, eastern Sicily, southern Italy, and western Greece, up to 3-m wave height could be possible. Crete, the southern Aegean, and the area between northeast Libya and Alexandria (Egypt) is prone to maximum tsunami wave heights of >3 m. Considering that calculations are performed at a minimum bathymetry depth of 20 m, these wave heights may, according to Green’s Law, be amplified by a factor of 2 at the coastline. The study can provide a basis for detailed tsunami hazard studies in the region.

Time series modelling of maximum tsunami wave heights recorded in Sibolga (Sumatra)

This study reports univariate modeling methodologies applied to the maximum tsunami wave height over Sibolga, Sumatra. The univariate time series models fitted are autoregressive model (AR), autoregressive integrated moving average (ARIMA) and autoregressive neural network (AR-NN). Goodness of fit of the models to the time series of maximum tsunami wave height has been assessed using percentage of prediction error, Pearson correlation coefficient, and Willmott's indices. After rigorous skill assessment using the above three models, the AR-NN model with seven previous values as predictor has been identified as the best predictive model for the time series under study.

Numerical Modeling of Generation and Propagation of Tsunami Waves along the Southern Coast of Iran

The main goal of this study is to increase the software capabilities of Iran towards the development of a tsunami warning system for its southern coast bordering the Indian Ocean. The devastating tsunami of 26 December 2004 in the Indian Ocean region emphasized the necessity for development of tsunami warning systems for the world’s vulnerable coastlines, especially for those neighboring the Indian Ocean. Studies showed that there are two main tsunamigenic zones in the Indian Ocean region, namely the Sumatra subduction zone in the east and the Makran subduction zone at the northwest of this ocean. The Makran subduction zone is located offshore Iran and Pakistan and produced some destructive tsunamis in the past. The most recent tsunami in the Makran region occurred on 28 November 1945 which claimed more than 4000 lives along the coasts of Iran, Pakistan, Oman and India. As the southern coast of Iran is at risk of tsunami from the Makran subduction zone, Iran is committed to work towards the development of a tsunami warning system, and to cooperate with the countries in the region in this field. All activities towards the development of a tsunami warning system can be classified into two categories: 1) hardwares like tsunameters, tide gauges, seismic networks, and etc., and 2) softwares. Numerical modeling of tsunami is among the second group and is a vital part of any tsunami warning system. Here, we discuss the modeling of tsunami generation and propagation. At first, the algorithm for tsunami generation modeling is presented and then, the generation phase of a possible tsunami in the region was modeled. The results of generation modeling were compared with international experiences. In the next section, the propagation of tsunami was modeled using a well-validated international numerical model for long waves. Results of the tsunami generation modeling showed that in the case of an earthquake with a moment magnitude of 8, the maximum vertical uplift of the seafloor will be about 1.5 m, and an area with the dimensions of 150 and 250 km will be deformed due to the earthquake. Tsunami propagation modeling showed that the first tsunami waves will hit the nearest coastline within 15-20 min after the earthquake. The maximum tsunami wave height was estimated about 3 m. It is showed that how the results of the numerical modeling of tsunami can help to develop a tsunami warning system. Our preliminary investigation shows that at least 891 case-scenarios should be simulated to develop the tsunami warning system in the Makran region.

Double jeopardy: Concurrent arrival of the 2004 Sumatra tsunami and storm‐generated waves on the Atlantic coast of the United States and Canada

A detailed analysis of over one hundred tide gauge records from the Atlantic coast of North America reveals that the arrival of the 26 December 2004 Sumatra tsunami on this coast coincided with the presence of tsunami‐like waves being generated by a major storm tracking northward along the eastern seaboard of the United States. According to the tide gauge records, waves from the two events coalesced along the shores of Maine and Nova Scotia on 27 December where they produced damaging waves with heights in excess of 1 m. Tsunami waves were identified in almost all outer tide gauges from Florida to Nova Scotia with maximum tsunami heights for the northern regions estimated to be 32–39 cm. In the south, maximum tsunami wave heights were in the range of 15 to 33 cm.

Effects of Tides on Maximum Tsunami Wave Heights: Probability Distributions*

Abstract A theoretical study was carried out to understand how the probability distribution for maximum wave heights (ηm) during tsunamis depends on the initial tsunami amplitude (A) and the tides. It was assumed that the total wave height is the linear sum of the tides and tsunami time series in which the latter is decaying exponentially in amplitude with an e-folding time of 2.0 days, based on the behavior of observed Pacific-wide tsunamis. Direct computations were made to determine the statistics of maximum height for a suite of different arrival times and initial tsunami amplitudes. Using predicted tides for 1992 when the lunar nodal f factors were near unity during the present National Tidal Datum Epoch 1983–2001, the results show that when A is small compared with the tidal range the probability density function (PDF) of the difference ηm − A is closely confined in height near mean higher high water (MHHW). The ηm − A PDF spreads in height and its mean height ηo − A decreases, approaching the PDF of the tides and MSL, respectively, when A becomes large compared with the tidal range. A Gaussian form is found to be a close approximation to the ηm − A PDF over much of the amplitude range; associated parameters for 30 coastal stations along the U.S. West Coast, Alaska, and Hawaii are given in the paper. The formula should prove useful in probabilistic mapping of coastal tsunami flooding.

Potential Tsunami Hazard Associated With the Kerepehi Fault, Firth of Thames, New Zealand

Geophysical data have identified four submarine segments of the Kerepehi Fault, roughly bisecting a back-arc rift (Hauraki Rift). These segments have been traced through the shallow waters of the Firth of Thames, which lies at the southern end of the Hauraki Gulf, New Zealand. No historical or paleotsunami data are available to assess the tsunami hazard of these fault segments. Analysis of the fault geometry, combined with paleoseismic data for three further terrestrial segments of the Fault, suggest Most Credible Earthquake (MCE) moment magnitudes of 6.5-7.1. Due to the presence of thick deposits of soft sediment, and the semi-confined nature of the Firth, the MCE events are considered capable of generating tsunami or tsunami-like waves. Two numerical models (finite element and finite difference), and an empirical method proposed by Abe (1995), were used to predict maximum tsunami wave heights. The numerical models also modelled the tsunami propagation. The MCE events were found not to represent a major threat to the large metropolitan centre of Auckland City (New Zealand's largest population centre). However, the waves were a threat to small coastal communities around the Firth, including the township of Thames, and 35,000 ha of low-lying land along the southern shores of the Firth of Thames. The Abe method was found to provide a quick and useful method of assessing the regional tsunami height. However, for sources in water depths <25 m the Abe method predicted heights 2-4 times larger than the numerical models. Since the numerical models were not intended for simulating tsunami generation in such shallow water, the Abe results are probably a good guide to the maximum wave heights.