Far-field Tsunami Research Articles

Analysis of Later-Phase Tsunamis by Seamount Chains and Implications to Far-Field Tsunami Warning

Focusing on the scattering waves excited at seamount chains, this study aims to investigate the delay of maximum tsunami to Pacific coast of Japan by integration of numerical analysis and analytical solution by Longuet-Higgins(1967). The results imply that Emperor seamount chain is one of major factors to excite scattering waves to Japan, and the later-phase tsunamis by the magnitude 8 class earthquakes from Hokkaido, Kuril and Alutian islands possibly affect to Pacific coast of Japan. Throughout the results, we discuss the issues to improve the accuracy of tsunami forecasting.

Historic tsunami in Britain since AD 1000: a review

Abstract. The British coast is not considered at particular risk from tsunami, a view that is supported by a number of recent government reports. However, these reports largely ignore some written historic records that suggest southern Britain has experienced a number of events over the past 1000 yrs. This study briefly assesses these records and recognises four groups of events: 1) sea disturbance and coastal floods in southeast England linked to earthquakes in the Dover Straits (e.g. 1382 and 1580), 2) far-field tsunami reaching the coast of the British Isles, for example, from earthquakes along the Azores-Gibraltar Fault Zone offshore Portugal (e.g. 1755), 3) tsunami associated with near-coastal low magnitude earthquakes (e.g. 1884 and 1892), and 4) a flood event in AD 1014 that has been linked to comet debris impact. The seismogenic events range from minor water disturbance, through seismic seiching, to small and "giant" waves, suggesting near-coastal, low-magnitude, shallow earthquakes may be capable of triggering disturbance in relatively shallow water, as supported by similar occurrences elsewhere, and that the British tsunami risk requires a more careful evaluation.

Far-field tsunami hazard from mega-thrust earthquakes in the Indian Ocean

We evaluate far-field tsunami hazard in the Indian Ocean Basin based on hydrodynamic simulations of ten case studies of possible mega earthquakes at the major seismic zones surrounding the basin. They represent worst-case scenarios of seismic rupture along the full extent of seismogenic faults having supported large earthquakes in the historical record. In a series of numerical experiments in which the source parameters of the 2004 Sumatra tsunami are allowed to vary one by one, while keeping the seismic moment and the fault orientation unchanged, we document that the main patterns of far-field tsunami amplitudes are remarkably robust with respect to nominal variations in such parameters as hypocentral depth, exact centroid location, and slip distribution on the fault plane. These results validate the concept of modelling case scenarios of potential future earthquakes whose source is by definition imprecise. We consider seismic sources located at the extremities of the 2004 Sumatra–Andaman rupture, namely along the southern coast of Sumatra and in the Andaman–Myanmar province; along the Makran coast of Pakistan and Iran; and also along the southern coast of Java, where the possibility of a large interplate thrust earthquake cannot be entirely dismissed. The results of our hydrodynamic simulations indicate that the distribution of maximum amplitudes in the Indian Ocean Basin is primarily controlled by the classical effect of source directivity, and additionally by refraction and focusing along bathymetric features. As a result, many provinces in the basin could be threatened by higher tsunami amplitudes than in 2004. This pattern is particularly important along the coast of East Africa, from Somalia to and including South Africa, in Madagascar and the Mascarene Islands, especially under a South Sumatra scenario involving an earthquake comparable to, or even possibly larger than, the 1833 event, whose epicentral area is widely believed to be under enhanced seismic risk as a result of stress transfer from the 2004 and 2005 ruptures to the northwest, possibly even in the wake of the 2007 Bengkulu earthquakes.

Far-field simulation of the 1946 Aleutian tsunami

SUMMARY We present hydrodynamic far-field simulations of the Aleutian tsunami of 1946 April 1, using both a dislocation source representing a slow earthquake and a dipolar one modelling a large landslide. The earthquake source is derived from the recent seismological study by Lopez and Okal, while the landslide source was previously used to explain the exceptional run-up at Scotch Cap in the near field. The simulations are compared to a field data set previously compiled from testimonies of elderly witnesses at 27 far-field locations principally in the Austral and Marquesas Islands, with additional sites at Pitcairn, Easter and Juan Fernandez. We find that the data set is modelled satisfactorily by the dislocation source, while the landslide fails to match the measured amplitudes, and to give a proper rendition of the physical interaction of the wavefield with the shore, in particular at Nuku Hiva, Marquesas. The emerging picture is that the event involved both a very slow earthquake, responsible for the far-field tsunami, and a major landslide explaining the near-field run-up, but with a negligible contribution in the far field.

Evaluation of far-field tsunami hazard in French Polynesia based on historical data and numerical simulations

Abstract. The first tsunami hazard map of French Polynesia is presented here on the basis of historical data, and numerical simulations. French Polynesia, because of its central position in the most tsunami prone ocean, the Pacific, is strongly exposed to far-field tsunamis. As no previous study on the area had been conducted, we compiled catalogues of all the historical observations (14 events), and tide gauges records (69 marigrams). The catalogues emphasise the higher hazard associated to the Marquesas archipelago, but also the deficiency of robust data in most other parts of French Polynesia. The recourse to numerical simulations allowed us to complement the existing records, and to test tsunami scenarii over different bathymetry and topography configurations, representative of the diversity of islands in French Polynesia. The tsunami hazard map assigns a high exposure level to the Marquesas and the island of Rurutu. Other islands of the Austral, and the Gambier archipelago have a elevated level of exposure, as well as three islands of Society: Tahiti, Moorea, and Huahine. All other islands of French Polynesia are considered as moderately exposed.

Forecast of Tsunamis from the Japan–Kuril–Kamchatka Source Region

This paper describes an investigation of the subfault distribution along the Japan–Kuril–Kamchatka subduction zone for the implementation of a far-field tsunami forecast algorithm. Analyses of seismic data from 1900 to 2000 define the subduction zone, which in turn is divided into 222 subfaults based on the fault characteristics. For unit slip of the subfaults, a linear long-wave model generates a database of mareograms at water-level stations along the subduction zone and at warning points in the North Pacific. When a tsunami occurs, an inverse algorithm determines the slip distribution from near-source water-level records and predicts the waveforms at the warning points using the pre-computed mareograms. A jackknife resampling scheme uses combinations of input water-level records to provide a series of waveform predictions for the computation of the confidence-interval bounds. The inverse algorithm is applied to hindcast two major tsunamis generated from the Japan–Kuril–Kamchatka source and the computed tsunami heights show good agreement with recorded water-level data.

Application of Analytical Modeling to the Far-field Investigation of Tsunami and Oceanic Rayleigh Waves from the 1998 Papua New Guinea Earthquake

We analyze far-field Rayleigh and tsunami waves generated by the 1998 Papua New Guinea (PNG) earthquake. Using the normal mode theory and Thomson-Haskell matrix formalism we calculate synthetic mareograms of oceanic surface waves excited by finite-dimensional line source and propagated in a flat, multilayered oceanic structure. Assuming that the source of destructive sea waves was the main shock of the PNG event and based on the expression for seismic wave displacement in the far-field zone, we compute the energy of the seismic and tsunami waves and the E z /E ts ratio. The results of our modeling are generally consistent with those obtained empirically for events with magnitude 7. Also, treating the results of a submarine slide as a single solitary wave and using the theoretical arguments of Striem and Miloh (1976) we estimate the energy of the tsunami induced by a landslide. The difference between the energy of the seismic tsunami and of the aseismic one is about one order of magnitude. The results of our theoretical modeling show that surface sea waves in the far-field zone account well for seismic origin, although additional tsunami energy from a landslide source could be required to explain the local massive tsunami in the Sissano Lagoon.

The earthquake and tsunami of 1865 November 17: evidence for far-field tsunami hazard from Tonga

SUMMARY Historical reports of an earthquake in Tonga in 1865 November identify it as the only event from that subduction zone which generated a far-field tsunami observable without instruments. Run-up heights reached 2 m in Rarotonga and 80 cm in the Marquesas Islands. Hydrodynamic simulations require a moment of 4 × 1028 dyn cm, a value significantly larger than previous estimates of the maximum size of earthquake to be expected at the Tonga subduction zone. This warrants an upwards re-evaluation of the tsunami risk from Tonga to the Cook Islands and the various Polynesian chains, which had hitherto been regarded as minor.

環太平洋での津波対策の過去と現状―我が国の防災対策への教訓―

The paper introduces the activity for disaster mitigation of tsunami in Pacific ocean after the 1960 Chilean tsunami, which should be lessons to improve the research and public work for the mitigation in Japan. The Japan Meteorological Agency (JMA) failed to issue an effective tsunami warning for the tsunamis of the 1960 Chile Earthquake, being aware that international cooperation is essential. Through the experience of the 1960 tsunami, the two international cooperation organizations; ICG/ITSU and PTWC were started to establish the far-field tsunami warning system in the Pacific ocean. JMA also exchanges of tidal data with the United States of America and other states, monitoring the data of foreign eighteen stations in this area. DART, TIME in US, and Pacific museum are introduced for the early time warning system with high accuracy, making hazards maps to increase people awareness and evacuation system, and public education by providing documentation, exhibition, workshops, and lectures.

A re-analysis of the Late Bronze Age eruption and tsunami of Santorini, Greece, and the implications for the volcano–tsunami hazard

The paroxysmal eruption of Santorini (ca. 3500 BP), referred to as the Late Bronze Age (LBA) eruption, probably generated multiple tsunami; their occurrence and impacts being cited frequently in scientific papers and articles. This paper examines what is known about any LBA tsunami, noting possible mechanisms of generation and identifying sedimentological traces. Firstly, the eruption sequence is outlined providing the context in which tsunami genesis may have occurred. Secondly, the arguments forwarded for the tsunami and a summary of the evidence is given. Thirdly, results of a new geological investigation for LBA tsunami deposits at 41 coastal sites from Crete and Kos are presented. The data are used to test the hypothesis that the LBA eruption generated an east Mediterranean-wide tsunami. It will be seen that no terrestrial geological evidence is identified. The paper re-examines the original arguments presented for LBA tsunami, challenging them because their founding assumptions are flawed. Together, the new data and the re-analysis of the original tsunami hypothesis indicate that there is insufficient evidence to demonstrate that any significant far-field tsunami propagated throughout the entire east Mediterranean as frequently purported. Some terrestrial evidence exists to suggest localised near-field tsunami inundation. There is good submarine evidence however, to suggest that a tsunami was focused to the W and WSW. The results have important implications for understanding the volcano-related tsunami hazard within the region and elsewhere.

Normal Mode Energetics for Far-field Tsunamis Generated by Dislocations and Landslides

We apply the normal mode representation of tsunami waves, as introduced by WARD (1980) to the systematic study of the excitation of far-field tsunamis by both dislocation sources (represented by double-couples of moment M o), and landslides (represented by single forces). Using asymptotic representations of the continuation of the tsunami eigenfunction into the solid Earth, we derive analytical expressions of the spectral amplitude generated by both systems. We show that the quadrupolar corrections defined by DAHLEN (1993) in the case of landslides can result in an increase of 1 to 2 orders of magnitude of the effective force. Even so, the spectrum of tsunami waves generated by landslides is found to be offset significantly to relatively high frequencies (10 mHz), where dispersion becomes important and eventually diminishes time-domain amplitudes. We proceed to calculate the total energy delivered into the tsunami modes by integrating the energy of multiplets for an average source geometry. In the case of dislocation sources, and taking into account the corner frequency of the source, we reproduce the scaling with M o 4/3, which was derived from purely static arguments by KAJIURA (1981). We compare the directivity patterns of far-field tsunami waves by dislocations and landslides, and conclude that the latter cannot give rise to pronounced lobes of directivity for physically acceptable values of the velocity of the slide. Directivity thus constitutes a robust discriminant of the nature of the source which, when applied to the 1946 Aleutian tsunami in the far-field, requires generation by a dislocative source.

Some Aspects of Energy Balance and Tsunami Generation by Earthquakes and Landslides

— Tsunamis are generated by displacement or motion of large volumes of water. While there are several documented cases of tsunami generation by volcanic eruptions and landslides, most observed tsunamis are attributed to earthquakes. Kinematic models of tsunami generation by earthquakes — where specified fault size and slip determine seafloor and sea-surface vertical motion — quantitatively explain far-field tsunami wave records. On the other hand, submarine landslides in subduction zones and other tectonic settings can generate large tsunamis that are hazardous along near-source coasts. Furthermore, the ongoing exploration of the oceans has found evidence for large paleo-landslides in many places, not just subduction zones. Thus, we want to know the relative contribution of faulting and landslides to tsunami generation. For earthquakes, only a small fraction of the minimum earthquake energy (less than 1% for typical parameter choices for shallow underthrusting earthquakes) can be converted into tsunami wave energy; yet, this is enough energy to generate terrible tsunamis. For submarine landslides, tsunami wave generation and landslide motion interact in a dynamic coupling. The dynamic problem of a 2-D translational slider block on a constant-angle slope can be solved using a Green's function approach for the wave transients. The key result is that the largest waves are generated when the ratio of initial water depth above the block to downslope vertical drop of the block H 0 /W sin δ is less than 1. The conversion factor of gravitational energy into tsunami wave energy varies from 0% for a slow-velocity slide in deep water, to about 50% for a fast-velocity slide in shallow water and a motion abruptly truncated. To compare maximum tsunami wave amplitudes in the source region, great earthquakes produce amplitudes of a few meters at a wavelength fixed by the fault width of 100 km or so. For submarine landslides, tsunami wave heights — as measured by b, block height — are small for most of the parameter regime. However, for low initial dynamic friction and values of H 0 /W sin δ less than 1, tsunami wave heights in the downslope and upslope directions reach b and b/4, respectively.Wavelengths of these large waves scale with block width. For significant submarine slides, the value of b can range from meters up to the kilometer scale. Thus, the extreme case of efficient tsunami generation by landslides produces dramatic hazards scenarios.

Complex earthquake rupture and local tsunamis

In contrast to far‐field tsunami amplitudes that are fairly well predicted by the seismic moment of subduction zone earthquakes, there exists significant variation in the scaling of local tsunami amplitude with respect to seismic moment. From a global catalog of tsunami runup observations this variability is greatest for the most frequently occurring tsunamigenic subduction zone earthquakes in the magnitude range of 7 < Mw < 8.5. Variability in local tsunami runup scaling can be ascribed to tsunami source parameters that are independent of seismic moment: variations in the water depth in the source region, the combination of higher slip and lower shear modulus at shallow depth, and rupture complexity in the form of heterogeneous slip distribution patterns. The focus of this study is on the effect that rupture complexity has on the local tsunami wave field. A wide range of slip distribution patterns are generated using a stochastic, self‐affine source model that is consistent with the falloff of far‐field seismic displacement spectra at high frequencies. The synthetic slip distributions generated by the stochastic source model are discretized and the vertical displacement fields from point source elastic dislocation expressions are superimposed to compute the coseismic vertical displacement field. For shallow subduction zone earthquakes it is demonstrated that self‐affine irregularities of the slip distribution result in significant variations in local tsunami amplitude. The effects of rupture complexity are less pronounced for earthquakes at greater depth or along faults with steep dip angles. For a test region along the Pacific coast of central Mexico, peak nearshore tsunami amplitude is calculated for a large number (N = 100) of synthetic slip distribution patterns, all with identical seismic moment (Mw = 8.1). Analysis of the results indicates that for earthquakes of a fixed location, geometry, and seismic moment, peak nearshore tsunami amplitude can vary by a factor of 3 or more. These results indicate that there is substantially more variation in the local tsunami wave field derived from the inherent complexity subduction zone earthquakes than predicted by a simple elastic dislocation model. Probabilistic methods that take into account variability in earthquake rupture processes are likely to yield more accurate assessments of tsunami hazards.

Tsunami risk assessment in the Marquesas Islands (French Polynesia) through numerical modeling of generic far-field events

Abstract. Earthquakes occurring at the Pacific Rim can trigger tsunamis that propagate across the ocean and can produce significant damages far away from the source. In French Polynesia, the Marquesas Islands are the most exposed to the far-field tsunami hazards, since they are not protected by any outer coral reef and since submarine slopes are less steep than in other islands. Between 1994 and 1996, four tsunamis have reached the bays of the archipelago, among them, the tsunami initiated by the Chilean Mw 8.1 earthquake, produced up to 3 m high waves in Tahauku Bay. Numerical modeling of these recent events has already allowed us to validate our method of resolution of hydrodynamics laws through a finite-difference scheme that simulates the propagation of the tsunamis across the ocean and computes the inundation heights (run-up) in remote bays. We present in this paper the simulations carried out to study potentially threatening areas located at the Pacific Rim, on the seismogenic Aleutian and Tonga subduction zones. We use a constant seismic moment source (that of the Mw 8.1 Chile 1995 earthquake, M0 = 1.2 1021 N.m) located at several potential epicenters, with the fault strike adapted from the regional seismotectonics pattern. Our results show that the sources chosen in the Aleutian trench do not produce large inundations in the Marquesas bays, except for the easternmost source (longitude 194° E). Sources located in the Tonga trench do not produce high amplifications either, except for the northernmost one (latitude 16° S). We also discuss the behaviour of the tsunami waves within the archipelago, and evidence contrasting responses depending on the arrival azimuths. These results show that, for a given initial seismic energy, the tsunami amplification in remote bays is highly dependent on the source location and fault strike.

Numerical simulation of far-field tsunamis using the linear Boussinesq equation - The 1998 Papua New Guinea Tsunami.

The dispersion effect is not negligible in the numerical simulation of far-field tsunamis propagating through deep oceans. Imamura et al. (1990) introduced a technique in which the discretization error in the finite difference equation of the linear long wave equation was used to approximate the physical dispersion term. The technique is widely accepted to compute trans-Pacific tsunamis caused by great earthquakes (Mw> 8). However, the technique has never been applied to compute tsunamis caused by smaller earthquakes (Mw< 7) because the approximation may break down. In order to compute the tsunami caused by the 1998 Papua New Guinea earthquake (Mw 7.1), we numerically solve the linear Boussinesq equation, which includes the physical dispersion term, using an implicit scheme. For comparison, we also compute the tsunami using Imamura's technique. The comparison of the computed waveforms at the ocean bottom pressure gauge off Boso (BS3-OBP) from the two numerical simulations indicates that the linear Boussinesq equation should be used to simulate the tsunami waveform more accurately, especially the later phase of tsunami waveforms. We also found that the observed tsunami that was originally generated by the 1998 Papua New Guinea earthquake and recorded at BS3-OBP was a ridge wave. The ridge wave was enhanced by the shallow water region around the Izu-Bonin Islands.

Analysis of the far‐field tsunamis generated by the 1998 Papua New Guinea Earthquake

The tsunami generated by the 1998 Papua New Guinea earthquake was observed at several tide gauges and ocean bottom pressure gauges in Japan. The fault model of the 1998 Papua New Guinea earthquake was estimated using those tsunami waveforms observed in Japan and coseismic subsidence observed along the coast of Papua New Guinea near the source region. The numerical simulation of the tsunami using the linear Boussinesq equation was carried out. The tsunami waveforms in Japan are explained by the fault model which is consistent with the Harvard CMT solution including the seismic moment of 3.7 × 1019 Nm. The additional tsunami source, such as a sediment slump, is not needed to explain the observed tsunami in Japan. The mechanism of the earthquake was likely to be a steeply dipping reverse fault. The 1998 Papua New Guinea earthquake was likely to occur within a small accretionary prism near the New Guinea trench.

The Aleutian Earthquake of June 10, 1996 (Mw 7.9) ruptured parts of both the Andreanof and Delarof Segments

The epicenter of the Aleutian earthquake of June 10, 1996, is located in the Delarof segment where a large subduction earthquake is expected. However, the aftershock area of the earthquake is not only in the Delarof segment but also in the Andreanof segment where the 1986 Andreanof earthquake occurred previously. We estimate the fault parameters of the 1996 earthquake by numerically computing the tsunami and comparing the waveforms with that recorded at Adak. The fault width is 30 km or less, and the average slip is about 4 m. The observed far‐field tsunamis around the Pacific are also explained by the synthetic tsunami computed using the above fault parameters. The results indicate that the 1996 earthquake ruptured parts of both the Delarof and Andreanof segments. However, the 1996 earthquake did not rupture the region of the largest moment release by the 1986 earthquake.

Source characteristics of historic earthquakes along the central Chile subduction Askew et alzone

We have analyzed four large to great historic earthquakes that occurred along the central Chile subduction zone from north to south on November 11, 1922 (M s=8.3), April 6, 1943 (M s=7.9), December 1, 1928 (M s=8.0) and January 25, 1939 (M s=7.8). Waveform modeling and P-wave first motions indicate that the 1922, 1928 and 1943 earthquakes are shallow and consistent with underthrusting of the Nazca Plate beneath the South American plate. In contrast, the 1939 earthquake is not an underthrusting event but rather a normal fault event within the down-going slab. The 1922 earthquake is by far the largest event with a complex source time function showing three pulses of moment release and a duration of 75 s. The 1943 earthquake has a simple source time function with one pulse of moment release and a duration of 24 s. This event had a local tsunami of 4 m and a far-field tsunami height in Japan of 10–30 cm. The 1928 earthquake also has a simple source time function with a duration of 28 s. The aftershocks and highest intensities are south of the epicenter indicating a southward rupture with most of the seismic moment release occurring 50–80 km south of the 1928 epicenter but still north of the adjacent 1939 earthquake region. The 1939 Chillan earthquake was not an underthrusting but rather a complex normal fault earthquake. Our preferred model is a normal fault mechanism at a depth of 80 to 100 km with two pulses of moment release and a total duration of approximately 60 s. The high intensities, lack of a tsunami, and inland location associated with the 1939 event are all consistent with an intraplate event within the down-going slab. The 1939 earthquake was clearly more destructive than the other similar size or larger events. This may in part be due to the intraplate nature of the event but also due to high amplification of the sites in the Central Valley of south central Chile.

The 1965 Rat Islands earthquake: A critical comparison of seismic and tsunami wave inversions

Abstract We use the 1965 Rat Islands earthquake to compare seismic wave inversion results for moment-release distributions to tsunami wave inversion results for slip distribution. Inversion using the two different datasets shows a high correlation between the areas of high moment release and areas of high slip. These areas of high slip correspond to the tectonic features of the western Aleutians, namely, the Rat, Buldir, and Near blocks. Less slip is observed between the blocks. The correlation indicates that far-field tsunami data is as useful as seismic data for studying moment distributions.

A numerical model for far-field tsunamis and its application to predict damages done to aquaculture

The 1960 Chilean tsunami which traveled the Pacific Ocean and caused much damages to Japan is simulated from its generation to the terminal effects on coastal areas. In the computation of ocean propagation by the linear longwave theory, a new technique is introduced to keep the same accuracy as the linear Boussincsq equation and reduce the CPU time as well as the computer memory. In the coastal transformation computation, the energy dissipation due to sea-bottom scouring is suggested to be included, particularly in the case of long bays. To obtain accurate results, the current velocity requires finer spatial grids than the water surface elevation. Damage done to pearl culture rafts are explained in terms of the computed current velocity.