Abstract
AbstractThe Shetland Isles represent an ideal field laboratory for tsunami geoscience research. This is due to the widespread preservation of Holocene tsunami sediments in coastal peat deposits. This study uses published accounts of the Holocene Storegga Slide tsunami to illustrate how two different approaches – mapping of tsunami sediments and numerical modelling – produce radically different run‐up heights. The Storegga Slide is one of the world largest submarine slides and took place ca 8150 cal yr bp on the continental slope west of Norway. The tsunami generated by the landslide deposited locally extensive sheets of marine sand and gravel, as well as redeposited clasts of peat across the contemporary land surface. These sediment accumulations have subsequently been buried by peat growth during the Holocene while exposures of the deposits are locally visible in coastal cliff sections. In several areas, the tsunami sediments can be traced upslope and inland within the peat as tapering sediment wedges up to maximum altitudes of between ca 8·1 m and 11·8 m above present sea level. Since reconstructions of palaeo‐sea level for Shetland for ca 8150 cal yr bp suggest an altitude of 20 m below high tide on the day that the tsunami struck, it has been inferred that the minimum tsunami run‐up was locally between 28·1 m (8·1 + 20 m) and 31·8 m (11·8 + 20 m). However, numerical models of the tsunami for Shetland suggest that the wave height may only have reached a highest altitude in the order of +13 m above sea level on the day the tsunami took place. In this paper a description is given of the sedimentary evidence for tsunami run‐up in the Shetland Isles. This is followed by an evaluation of where the palaeo‐sea level was located when the tsunami occurred. Significant differences are highlighted in tsunami inundation estimates between those based on the observed (geological) data and the theoretically‐modelled calculations. This example from the Shetland Isles may have global significance since it exemplifies how two different approaches to the reconstruction of tsunami inundation at the coast can produce radically different results with modelled wave height at the coast being considerably less than the geological estimates of tsunami run‐up.
Highlights
One of the most problematic areas of tsunami science is how to assess the tsunami risk posed by submarine landslide activity
The present study focuses principally on the maximum altitudes of the Shetland tsunami deposits above sea level and what this means in terms of former tsunami run-up, since this factor is critically important to the hydrodynamic modelling of the landslide-generated tsunami
The geological evidence of tsunami-deposited sand sheets that occur at several locations along the Shetland coastline show that the highest limit of these range between +8Á1 m and +11Á8 m above present sea level
Summary
One of the most problematic areas of tsunami science is how to assess the tsunami risk posed by submarine landslide activity. Storegga tsunami deposits have been recovered from sediment cores sampled from coastal lakes in Shetland located between 0Á5 to 3Á0 m above present high tide level (Bondevik et al, 2005a) Radiometric dating of these sediments has been. Storegga tsunami sediment exposures are widespread in coastal peat areas between the Sullom Voe oil and gas terminal and Scatsa airport (Fig. 2) In this area, numerous sediment exposures are visible in peat cuttings while extensive hand-augering has shown that a distinctive tsunami sediment unit is present nearly everywhere; rising in altitude from just below high tide sea level in coastal peat exposures to an altitude of 11Á8 m above sea level where the sheets of sediment have tapered and thinned to such an extent that only individual grains of sediment are visible within the peat (cf Smith et al, 2004). At the Houb the tsunami sediment unit is almost a continuous feature along the shoreline for ca 150 m in coastal peat exposures
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