Evidence for three North Sea tsunamis at the Shetland Islands between 8000 and 1500 years ago

Coastal fen- and lake deposits enclose sand layers that record at least three Holocene tsunamis at the Shetland Islands. The oldest is the well-known Storegga tsunami (ca 8100 cal yr BP), which at the Shetlands invaded coastal lakes and ran up peaty hillsides where it deposited sand layers up to 9.2 m above present high tide level. Because sea level at ca 8100 cal yr BP was at least 10-15 m below present day sea level, the runup exceeded 20 m. In two lakes, we also found deposits from a younger tsunami dated to ca 5500 cal yr BP. The sediment facies are similar to those of the Storegga tsunami-rip-up clasts, sand layers, re-deposited material and marine diatoms. Runup was probably more than 10 m. Yet another sand layer in peat outcrops dates to ca 1500 cal yr BP. This sand layer thins and fines inland and was found at two sites 40 km apart and traced to ca 5-6 m above present high tide. The oldest tsunami was generated by the Storegga slide on the Norwegian continental slope. We do not know what triggered the two younger events.