trans-Pacific Tsunamis Research Articles

A re-evaluation of the great Aleutian and Chilean earthquakes of 1906 August 17

SUMMARY We investigate two great earthquakes that occurred in the Aleutian Islands and Chile, within 30 min of each other, on 1906 August 17, based on a collection of seismograms compiled shortly after the events by scientists at Strasbourg. The method of Preliminary Determination of Focal Mechanisms (PDFM) is applied to 14 mantle waves from seven stations, in order to resolve the moment tensors of the two shocks. It is complemented by examination of body wave polarities at Japanese stations to lift the remaining indeterminacy in focal mechanism. The Chilean earthquake, occurring second, is a regular subduction event, whose moment (2.8 × 1028 dyn cm) is revised downwards from previous estimates (except Kanamori's), suggesting that its rupture did not involve more than ∼200 km of fault. The Aleutian earthquake, occurring first, has a larger moment (3.8 × 1028 dyn cm), but features mantle wave radiation patterns and body wave polarities incompatible with both underthrusting at the Aleutian subduction zone and tensional buckling at the outer rise. Rather, we suggest that it is an intraplate, somewhat deeper (∼50 km) earthquake: this is supported by tentative relocation of available arrival times north of it. The origin of the earthquake may be related to the presence of the Bowers ridge north of the Amchitka pass in the epicentral area. Finally, hydrodynamic simulations using our source mechanisms support the observation that the Chilean event was the source of the reported transpacific tsunami; the report of a 3.5-m wave at Maui constitutes a misassociation, as its timing is shown to be non-causal for both events.

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.

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.

Effects of bathymetry on tsunami propagation: Application of ray tracing to tsunamis

Ray tracing of seismic surface waves is applied to tsunami propagation to examine bathymetric effect along its propagation path. Computations are made for trans-Pacific tsunamis and for near-field tsunamis in the Japan Sea. For tsunamis across the Pacific Ocean, the comparison to a uniform ocean shows that focusing and defocusing, due to bathymetry, are significant for some combinations of source and receiver. For example, the refraction of rays is predominant at the East Pacific Rise for the tsunami from Chile. The tsunamis in the Japan Sea are strongly affected by the shallow Yamato Rise. The predicted arrival time and amplitude distribution generally agree with the observations from an actual tsunami. Since the computation can be made very quickly, the method is useful for preliminary analysis of tsunami propagation, such as in an operational warning system or in the determination of computational area for finite-difference computation.