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A Note on the Preservation of Offshore Tsunami Deposits

¹ Joint Institute for the Study of the Atmosphere and Ocean, University of Washington-NOAA Center for Tsunami Research, National Oceanographic and Atmospheric Administration, Sand Point Way NE, Seattle, Washington 98115, U.S.A; Geologisch-Paläontologisches Institut und Museum, Westfälische-Wilhelms Universität, Correnstrasse 24, 48149 Münster, Germany; weiszr{at}u.washington.edu
² Geologisch-Paläontologisches Institut und Museum, Westfälische-Wilhelms Universität, Correnstrasse 24, 48149 Münster, Germany

In this contribution we explore the preservation potential of offshore tsunami deposits. The application of linear wave theory and flat-bottom conditions allows a simplified representation of the physical environment. In such an environment it is possible to compute the boundary water depth, below which the influence of storm waves on tsunami deposits is negligible. The majority of the tsunami deposits described in the literature were deposited onshore where the tsunami wave is transformed into bores and rollers. These deposits differ from tsunami sediments of the deeper shelf, where the water-sediment interaction corresponds more to wave influences.

We define the boundary depth d_b as the depth below which tsunami deposits cannot be reworked due to storm waves. This is based on the assumption that the wave base coincides with the seabed. If the wave base is below the bed, a reworking of bed sediments occurs. If it is above, no energy is transferred to the bottom. As an example, we use tide-gauge records at Pointe de La Rue, Seychelles, and ca. 50 km offshore from Brisbane, Australia, at a water depth of 70 m, to derive the major wave parameters of the Sumatra tsunami and a Category 1 Cyclone that occurred from 4–6 March 2004 in Brisbane. The Brisbane storm wave reached the maximum wave height of 14.3 m. Wave periods varied between 6 s and 14 s and the computed maximum wavelength was 237 m. The largest height of the Sumatra tsunami was 3.01 m. Periods of between 24 min and 61 min were computed for the time between two subsequent peaks.

The combination of our hydrodynamic considerations with information from a simplified Hjulström-Sundborg diagram implies that the most powerful storm and tsunami waves produce conditions near and at the sea bed that allow the transport of sediment grains from decimeters to meters in diameter. That means a sandy tsunami deposit in the area of the Brisbane tide gauge would be reworked by storm waves. Assuming the highest wave amplitude during the 2004 March storm at Brisbane as such a boundary wave, the water depth below which preservation of tsunami deposits is most likely is greater than 65 m.