Tsunami Wave Scattering In The North Pacific


H.O. Mofjeld, V.V. Titov, F.I. González and J.C. Newman
NOAA/Pacific Marine Environmental Laboratory
Seattle, WA, USA

A theoretical study is being carried out to understand how escarpments, ridges and seamounts affect deep-water tsunami propagation in the Pacific Ocean. The study is also designed to determine the accuracy and resolution of bottom topography that are needed in numerical models to accurately simulate the effects of small-scale (100 km) topographic features on tsunami propagation. The initial focus of the work is on tsunamis that are generated in the Alaska/Aleutian Subduction Zone (AASZ) and propagate southward to Hawaii. Analytic theory shows that the main effect of these features is to scatter energy from the tsunami waves. The amount of scattering depends on the heights of the features relative to the total depth, their spatial extent compared with tsunami wavelengths, and their orientation relative to the direction of wave propagation. 2-D wavelet analyses of the Sandwell/Smith topography (Topo 6.2) are used to identify the spatial scales and locations of scattering features in the North Pacific.

Numerical simulations based on the MOST model (with and without small-scale topography) show that deep-water scattering produces only a small amplitude decrease in the first waves of AASZ tsunamis propagating to Hawaii (a few percent in energy). This is compared with the geometric spreading and broad-scale refraction that occurs when the tsunami waves propagate southward from their sources. The primary reason for relatively little deep-water scattering is that the tsunami-scale topographic features between Alaska and Hawaii do not extend vertically over a substantial fraction of the total water depth. However, significant focusing of tsunami waves does occur near Hawaii; this is due to the seamount field north of Kauai, the Emperor Seamount Chain and the Hawaiian Ridge. While deep-water scattering has only a minor influence on the first waves in AASZ tsunamis, it does contribute to the complicated temporal patterns of the waves that occur later in Pacific-wide tsunamis.

Scattering processes are much stronger on the upper continental slope and shelf; hence there is a need for accurate topography in these regions. Future work will extend the analysis to shallower water, to other source regions (e.g., Kamchatka, Kuril/Japan and Cascadia) and to other impact regions, especially the U.S. West Coast.