Aleutian Tsunami Research Articles

Landslide-induced tsunami simulation based on progressive landslide-shallow water equation coupling model: 1946 Aleutian tsunami case

Landslide-induced tsunami simulation based on progressive landslide-shallow water equation coupling model: 1946 Aleutian tsunami case

Geological evidence and sediment transport modelling for the 1946 and 1960 tsunamis in Shinmachi, Hilo, Hawaii

The Japanese community of Shinmachi, established on low-lying land between downtown Hilo and Waiakea, Hawaii, was obliterated by the 1946 Aleutian tsunami but was rebuilt, only to be destroyed again by the 1960 Chilean tsunami. The aim of this study was to find out if any geological evidence of these well documented events had been preserved in the sedimentary record in Wailoa River State Park, which replaced Shinmachi after the 1960 tsunami. This was achieved by collecting cores in the park and performing sedimentological, chronological and geochemical analyses, the latter also processed by principal component analysis. Sediment transport modelling was carried out for both tsunamis, to infer the source of the sediment and areas of deposition on land. The field survey revealed two distinct units within peat and soil, a thin lower unit composed of weathered basalt fragments within mud (Unit 1) and an upper unit dominated by fine volcanic sand within fine silt exhibiting subtle upward fining and coarsening (Unit 2, consisting of Unit 2A and Unit 2B), although these two anomalous units only occur on the western shore of Waiakea Mill Pond. Analysis with an ITRAX core scanner shows that Unit 1 is characterised by high Mn, Fe, Rb, La and Ce counts, combined with elevated magnetic susceptibility. Based on its chemical and sedimentological characteristics, Unit 1 is attributed to a flood event in Wailoa River that occurred around 1520–1660CE, most probably as a result of a tropical storm. The sharp lower contact of Unit 2 coincides with the appearance of arsenic, contemporaneous with an increase in Ca, Sr, Si, Ti, K, Zr, Mn, Fe, La and Ce. In this study, As is used as a chronological and source material marker, as it is known to have been released into Wailoa River Estuary and Waiakea Mill Pond by the Canec factory between 1932 and 1963. Thus, not only the chemical and sedimentological evidence but also sediment transport modelling, corroborating the historical record, suggest that Unit 2A was deposited by the 1946 tsunami, and the sediment most likely originated from Wailoa River Estuary, beach and nearshore seafloor. The upper part of this unit, Unit 2B, is believed to have been deposited by the 1960 tsunami, as suggested by sediment transport modelling, although limited accommodation space is likely to have resulted in the thin deposit (3cm thickness) present at that site. Limited accommodation space on the island of Hawaii has led to only rare locations where tsunami deposits are preserved, despite the repeated occurrence of tsunamis affecting the island.

Aleutian tsunami likely swamped Hawaii centuries ago

Aleutian tsunami likely swamped Hawaii centuries ago

Paleotsunami evidence on Kaua‘i and numerical modeling of a great Aleutian tsunami

The Hawaiian Islands' location in the middle of the Pacific Ocean is threatened by tsunamis from great earthquakes in nearly all directions. Historical great earthquakes Mw > 8.5 in the last 100 years have produced large inundations and loss of life in the islands but cannot account for a substantial (≤ 600 m3) paleotsunami deposit in the Makauwahi sinkhole on the Island of Kaua‘i. Using high‐resolution bathymetry and topography we model tsunami inundation of the sinkhole caused by an earthquake with a moment magnitude of Mw ~9.25 located in the eastern Aleutians. A preponderance of evidence indicates that a giant earthquake in the eastern Aleutian Islands circa 1425–1665 A.D.—located between the source regions of the 1946 and 1957 great tsunamigenic earthquakes—created the paleotsunami deposit in Kaua‘i. A tsunami deposit in the Aleutians dated circa 1530–1660 A.D. is consistent with this eastern Aleutian source region.

Multi-proxy evidence for trans-Pacific tsunamis in the Hawai'ian Islands

The origin of two sand layers buried in Pololū wetland, on the northeast coast of the island of Hawai'i, was investigated using a multi-proxy approach, including radiometric dating, sedimentology, geochemistry, micropalaeontology, palynology and historical records. Based on 210Pb, 137Cs and pollen data, the lower sand unit extending 250 to 350m inland is attributed to the 1946 Aleutian tsunami, while the upper sand unit was laid down by the 1957 Aleutian tsunami. Chronological information does not appear to support any link between these deposits and historical storm events. Furthermore, sedimentological, geochemical, diatom and pollen results suggest deposition of marine and near-beach sediments under high energy conditions typically associated with tsunami inundation. This work presents the first conclusive sedimentary evidence for distantly-generated tsunamis on the Hawai'ian Islands, opening the way for more comprehensive historical and palaeotsunami studies. Indeed, a brief review of archaeological data suggests that the Hawai'ian coast may have experienced similar large magnitude events in the past, from both local and distant sources.

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.

MTSU : Recovering Seismic Moments from Tsunameter Records

We define a new magnitude scale, M TSU, allowing the quantification of the seismic moment M0 of an earthquake based on recordings of its tsunami in the far field by ocean-bottom pressure sensors (“tsunameters”) deployed in ocean basins, far from continental or island shores which are known to affect profoundly and in a nonlinear fashion the amplitude of the tsunami wave. The formula for M TSU, M TSU=log10 M O‒20=log10 X (ω) + C D TSU + C S TSU +C O, where X(ω) is the spectral amplitude of the tsunami, C D TSU a distance correction and C S TSU a source correction, is directly adapted from the mantle magnitude M m introduced for seismic surface waves by Okal and Talandier. Like M m, its corrections are fully justified theoretically based on the representation of a tsunami wave as a branch of the Earth's normal modes. Even the locking constant C 0, which may depend on the nature of the recording (surface amplitude of the tsunami or overpressure at the ocean floor) and its units, is predicted theoretically. M TSU combines the power of a theoretically developed algorithm, with the robustness of a magnitude measurement that does not take into account such parameters as focal geometry and exact depth, which may not be available under operational conditions in the framework of tsunami warning. We verify the performance of the concept on simulations of the great 1946 Aleutian tsunami at two virtual gauges, and then apply the algorithm to 24 records of 7 tsunamis at DART tsunameters during the years 1994–2003. We find that M TSU generally recovers the seismic moment M 0 within 0.2 logarithmic units, even under unfavorable conditions such as excessive focal depth and refraction of the tsunami wave around continental masses. Finally, we apply the algorithm to the JASON satellite trace obtained over the Bay of Bengal during the 2004 Sumatra tsunami, after transforming the trace into a time series through a simple ad hoc procedure. Results are surprisingly good, with most estimates of the moment being over 1029 dyn-cm, and thus identifying the source as an exceptionally large earthquake.

Sedimentology of rocky shorelines: 3.Hydrodynamics of megaclast emplacement and transport on a shore platform, Oahu, Hawaii

Megaclasts of well cemented coralgal limestone have been emplaced onto a shore platform by large waves on the North Shore of Oahu, HI. Emplacement and movements of the largest, ca. 96 ton megaclast have been dated using aerial photographs. Hydrodynamic forces at the low submerged shoreline cliff are computed using design wave characteristics based on linear wave theory and experimental results, considering the local wave climate and near-shore bottom topography. Force exerted by a tsunami has been estimated based on the 1946 Aleutian Tsunami. The computed forces are evaluated in terms of the initial fracturing along a given failure plane that is required in order for the detachment to occur. The analysis shows that tsunami, as well as large swell waves, are capable of quarrying the megaclast, provided that sufficient initial fracturing is present. Dislodgement and emplacement most likely occurred in a sequence during impact of a single wave on the shoreline cliff. Swell waves, however, are seldom capable of emplacing large blocks onto the platform due to their rapid disintegration after breaking, so that most blocks quarried from the cliff edge fall back onto the submarine terrace. Emplacement of large boulders seems to require extreme sea waves with periods longer than storms or North Pacific swells usually produce. Transport mechanisms on the shore platform vary, depending on megaclast shape. Sliding is a common mechanism of transport for larger and irregular megaclasts, whereas somewhat smaller and platy megaclasts are occasionally found in overturned positions.

Sedimentology of rocky shorelines: 3.: Hydrodynamics of megaclast emplacement and transport on a shore platform, Oahu, Hawaii

Megaclasts of well cemented coralgal limestone have been emplaced onto a shore platform by large waves on the North Shore of Oahu, HI. Emplacement and movements of the largest, ca. 96 ton megaclast have been dated using aerial photographs. Hydrodynamic forces at the low submerged shoreline cliff are computed using design wave characteristics based on linear wave theory and experimental results, considering the local wave climate and near-shore bottom topography. Force exerted by a tsunami has been estimated based on the 1946 Aleutian Tsunami. The computed forces are evaluated in terms of the initial fracturing along a given failure plane that is required in order for the detachment to occur. The analysis shows that tsunami, as well as large swell waves, are capable of quarrying the megaclast, provided that sufficient initial fracturing is present. Dislodgement and emplacement most likely occurred in a sequence during impact of a single wave on the shoreline cliff. Swell waves, however, are seldom capable of emplacing large blocks onto the platform due to their rapid disintegration after breaking, so that most blocks quarried from the cliff edge fall back onto the submarine terrace. Emplacement of large boulders seems to require extreme sea waves with periods longer than storms or North Pacific swells usually produce. Transport mechanisms on the shore platform vary, depending on megaclast shape. Sliding is a common mechanism of transport for larger and irregular megaclasts, whereas somewhat smaller and platy megaclasts are occasionally found in overturned positions.

Source discriminants for near-field tsunamis

We present numerical simulations of tsunami run-up in the near field for a data set of 72 models of sources, involving both seismic dislocations and landslides. By varying one by one the parameters describing the source and the receiving beach, we are able to separate their individual influence on the amplitude and distribution of run-up, which we characterize by forming several dimensionless parameters, principally the ratio I2 of its maximum amplitude to its lateral extent along the beach. We find that I2 remains less than 10−4 for seismic dislocation sources but is greater than this threshold for all physically realistic models of underwater landslides. Thus, it can be used as a discriminant for the nature of the source of a near-field tsunami. For seismic dislocations, we also consider the ratio I1 of maximum run-up to seismic slip on the fault plane, and validate numerically the previously suggested ‘rule of thumb’ that this ratio cannot be much greater than 1. In the case of underwater landslides, we show that the distribution of near-field run-up is primarily controlled by the 2-D ‘wall of water’ displaced on the ocean surface as an initial condition to the simulation, and is practically independent of its extension in the third dimension, perpendicular to the beach. We apply this approach to nine profiles of tsunami run-up obtained experimentally during recent field surveys, and show that our method successfully identifies the 1998 Papua New Guinea tsunami as having been generated by an underwater landslide, thus confirming the results of shipboard and hydroacoustic investigations. It also strongly suggests a similar mechanism for the generation of the near-field Aleutian tsunami of 1946 April 1.

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.

Near-Field Survey of the 1946 Aleutian Tsunami on Unimak and Sanak Islands

The 1946 Aleutian earthquake stands out among tsunamigenic events because it generated both very high run-up near the earthquake source region and a destructive trans-Pacific tsunami. We obtained new data on the distribution of its tsunami in the near field along south-facing coasts between Unimak Pass on the west and Sanak Island on the east by measuring the height of driftwood and beach materials that were deposited by the tsunami above the extreme storm tide level. Our data indicate that (1) the highest measured run-up, which is at the Scotch Cap light-house, was 42 m above tide level or about 37 m above present storm tide elevation; (2) run-up along the rugged coast from Scotch Cap for 12 km northwest to Sennett Point is 12–18 m, and for 30 km east of Scotch Cap to Cape Lutke it is 24–42 m; (3) run-up along the broad lowlands bordering Unimak Bight is 10–20 m, and inundation is locally more than 2 km; (5) run-up diminishes to 8 m or less at the southeast corner of Unimak Island; (6) no evidence was found for run-up above present storm tides (about 4–5 m above MLLW) on the Ikatan Peninsula or areas along the coast to the west; and (7) run-up above storm tide level in the Sanak Island group is restricted to southwest-facing coasts of Sanak, Long, and Clifford Islands, where it is continuous and locally up to 24 m high. Generation of the tsunami by one or more major earthquake-triggered submarine landslides near the shelf edge south of Unimak Island seems to be the only viable mechanism to account for the data on wave arrival time, run-up heights, and distribution, as well as for unconfirmed anecdotal reports of local postquake increases in water depth and diminished bottom-fisheries productivity. A preliminary hydrodynamic simulation of the local tsunami propagation and run-up using a dipolar model of a possible landslide off Davidson Bank provides an acceptable fit to the characteristics of the distribution of local run-up, with a value at 34 m at the Scotch Cap lighthouse.

A Field Survey of the 1946 Aleutian Tsunami in the Far Field

This paper reports the interviews of 48 elderly witnesses to the Aleutian tsunami of 1 April 1946, which resulted in the measurement of a database of 54 values of runup and inundation in 31 valleys of the Marquesas, Easter, and Juan Fernandez Islands. The 1946 Aleutian tsunami remains one of the most enigmatic such events of the past century. It resulted in catastrophic destruction both locally (with the annihilation of the Scotch Cap lighthouse on Unimak Island, where runup reached 42 m) and in the far field, where it claimed a total of 162 lives in California, the Marquesas, and Hawaii (where runup reached 16 m) and wrought destruction as far away as Winter Island, Antarctica, 15,000 km from its epicenter (Fuchs, 1982). Yet the parent earthquake featured a relatively low conventional magnitude of only M = 7.4 as assigned at Pasadena by Gutenberg and Richter (1954). Kanamori (1972) defined the 1946 Aleutian shock as a “tsunami earthquake”, a class of events whose tsunamis have much greater amplitude than would be predicted from their seismic magnitudes. Later work by Fukao (1979), Newman and Okal (1998), Pelayo and Wiens (1992), and Polet and Kanamori (2000) has shown that several tsunami earthquakes feature a slow moment release (most probably involving rupture in sedimentary material), which leads to the underestimation of the earthquake's size if measured at the relatively short periods characteristic of conventional magnitudes. In the case of the 1946 Aleutian event, a number of studies have suggested that the seismic moment does indeed increase at longer periods, to values as high as 3.7 × 1028 dyne-cm (Kanamori, 1972) or even 7.6 × 1028 dyne-cm (Okal, 1992). Pelayo (1990) suggested 8.5 × 1028 dyne-cm and speculated that the moment could be even ten times larger. In very general …

Sedimentology of rocky shorelines: 2

Abstract Limestone megaclasts, up to 96 t in weight, rest on a coastal rock platform near Sunset Beach on the north shore of Oahu, Hawaii. They occur in seaward-dipping, imbricate clusters and as solitary clasts. The rock platform is subject to large swell waves and very large waves associated with winter storms and tsunami. A series of photographs, covering the period from ca. 1905 to 1996, show episodic rearrangement of megaclasts on the platform and the appearance of new megaclasts near the platform edge. The largest solitary megaclast (ca. 96 t) was emplaced on the platform between 1940 and 1950, possibly by the 1946 Aleutian tsunami and has moved twice since then in ca. 30-m jumps along two different trajectories. Larger clasts have been broken from the platform edge and emplaced on the platform only by very large wave events, whereas formation and movement of smaller tabular, clasts (ca. 30–40 t) appear to be a continuously ongoing process on the surface of the rock platform. Partial detachment of clasts by marine erosion processes along joints and cracks in the limestone bedrock facilitates their complete detachment from both the platform edge and platform surface. The larger numbers of megaclasts emplaced on the north and northwest sectors of the platform relative to other sectors are attributed to exertion of maximum wave power on these sectors due to a combination of steep slope and ramp-like submarine topography.

Detailed analysis of tsunami waveforms generated by the 1946 Aleutian tsunami earthquake

Abstract. The 1946 Aleutian earthquake was a typical tsunami earthquake which generated abnormally larger tsunami than expected from its seismic waves. Previously, Johnson and Satake (1997) estimated the fault model of this earthquake using the tsunami waveforms observed at tide gauges. However, they did not model the second pulse of the tsunami at Honolulu although that was much larger than the first pulse. In this paper, we numerically computed the tsunami waveforms using the linear Boussinesq equation to determine the fault model which explains the observed tsunami waveforms including the large second pulse observed at Honolulu. The estimated fault width is 40–60 km which is much narrower than the fault widths of the typical great underthrust earthquakes, the 1957 Aleutian and the 1964 Alasuka earthquakes. A previous study of the 1896 Sanriku earthquake, another typical tsunami earthquake, suggested that the additional uplift of the sediments near the Japan Trench had a large effect on the tsunami generation. In this study, we also show that the additional uplift of the sediments near the trench, due to a large coseismic horizon-tal movement of the backstop, had a significant effect on the tsunami generation of the 1946 Aleutian earthquake. The estimated seismic moment of the 1946 Aleutian earthquake is 17–19 × 1020 20 Nm (Mw 8.1).

日本沿岸における1996年6月10日アリューシャン津波の様相

日本沿岸における1996年6月10日アリューシャン津波の様相

Estimation of seismic moment and slip distribution of the April 1, 1946, Aleutian tsunami earthquake

The 1946 Aleutian earthquake produced a tsunami of tsunami magnitude Mt=9.3, but the surface wave magnitude is only Ms=7.4, making it a tsunami earthquake. The discrepancy between the apparent size of the earthquake based on the seismic data and the size of the tsunami has been explained by several mechanisms, including a landslide and a slow earthquake, but there are few seismic data available to determine the correct mechanism. We study the generating mechanism of the tsunami using tsunami waveforms recorded on tide gauges. We have modeled the source of the 1946 Aleutian tsunami as the result of an underthrusting earthquake. We performed both forward and inverse modeling of the data using a finite difference calculation to compute synthetic tsunamis. We include both vertical and horizontal deformation of the ocean bottom due to faulting in the computation. The results of the inversion of the tsunami waveforms show that the slip on the fault is mainly concentrated in the shallow section of the fault near the epicenter. There is little slip in the area of the aftershocks, indicating that the aftershocks may represent an area of afterslip or triggered seismicity. The moment estimate of the earthquake is 23×1020 N m, or Mw=8.2. The majority of the waveforms are well‐explained by this fault model and slip distribution, including the Honolulu waveform, but the Hawaiian data also suggest that other effects, such as a landslide, may be necessary for explaining the abnormal tsunami amplitudes in Hawaii. The 1946 earthquake is similar to other tsunami earthquakes, such as the 1896 Sanriku and 1992 Nicaragua earthquakes, indicating that slow, shallow rupture may explain the disproportionally large tsunamis.

Long Waves in Ocean and Coastal Waters

Long Waves in Ocean and Coastal Waters

TSUNAMI INUNDATION PREDICTION

This paper concerns the run-up and inundation characteristics of tsunami surges. The forces and moments produced by the waves are not discussed, however, the proposed technique does provide the necessary information for their determination. The method relies on the knowledge of the wave elevation at the coast (as determined from historical data or other means) and an estimation of the bed roughness. The considerations and calculations involved in determining these parameters are discussed in detail. Twenty-four observed run-ups on the island of Hawaii in the case of the 1946 Aleutian tsunami, and 18 run-ups on the island of Maui for the Chilean 1960 tsunami are used to illustrate the technique. Methods are also presented to predict the shoreline heights and extent of inundation of tsunami surges where historical data is not available.

Statistical studies on the wave-form and maximum height of large Tsunamis

The tide-gauge records of large tsunamis are classified into three types, A, B and C. The “A” type record is made up of one or a few large waves near the wave front. The “B” type record consists of one or a few wave groups. The “C” type is the combination of the “A” and “B” types. The data used are; the Kamchatka Tsunami of Nov. 4, 1952, the Aleutian Tsunami of March 9, 1957, the Chilean Tsunami of May 22, 1960 and the Alaska Tsunami of March 28, 1964.