Tsunamis - Extreme Events: A Physical Reconstruction and Risk Assessment - page 121

Return flow
Cross-beds
Onshore flow
Figure 5.6. Diagrammatic example of sedimentary structures (cross-bedded foresets)
showing both onshore and return (seaward) flow.
upon their studies of deposits from two tsunami inundations in Japan (Japan
Sea tsunami of 1983 and SW Hokkaido tsunami of 1993), they suggested that
tsunami run-ups of greater than 10 m were necessary to produce a continuous
sand sheet as opposed to more patchy deposits of sand.
Landward thinning of sand sheets is an important characteristic of tsunami
deposits, although the same occurs with storm deposits. But this characteristic
at least distinguishes between sediments laid down by marine inundations and
those by wind and rivers. The size of the tsunami (wave height at shore and run-
up height), along with local onshore topography, will also have some bearing
on the distance inland that a sheet will extend. Given a generally flat coastal
landscape, tsunamis that extend farther inland will produce more extensive
deposits, providing there is an adequate supply of sediment. The tsunami at
Sissano Lagoon, PNG, whilst very large, was unable to deposit an extensive sand
sheet because of the proximity of the lagoon at the rear of the sand barrier.
Dawson et al .(1988)foundasandsheet extending inland for over2kmineast
Scotland. The tsunami responsible was thought to have been generated by the
Storegga submarine landslide off the coast of Norway approximately 7950 years
ago (Fig. 5.7). This was the second of three massive submarine landslides which
also left landward thinning layers of sediments within tectonically raised lakes
on the east coast of Norway (Bondevik et al ., 1997). Bondevik et al ., (2003)have
also identified the Storegga slide tsunami in the Shetland Islands where there
is evidence that the tsunami attained a run-up height of 20--25 m above sea-
level at the time. There is evidence for two waves in the tsunami train here.
The first ripped up clasts of peat in coastal lakes and left these behind after the
wave drained the land as backwash. The second wave incorporated the ripped
up clasts into a sand layer that thins inland and decreases in grain size in the
same direction (Fig. 5.8).