Tsunami Signatures

From Geo Science, Tsunami Along the South Coast of NSW

The first event probably occurred concomitantly with the rise of Holocene sea-level near modern levels around 7000 BP. ...

The impact of these tsunami upon the coastal landscape has been profound. Several signatures provide estimates of the magnitude of run-up of these events. The height to which chaotic mixes of sediment and imbricated boulder stacks have been deposited and the height of headlands that have had a smear of clay, sand and shell plastered across them give general estimates of the run-up height. The elevation of eroded landscape features on headlands gives information about the depth and velocity of flow. The presence of sand laminae and splayed sand units within deltaic sediments permit the landward limit of tsunami impact to be determined.

This geomorphic evidence indicates that the largest tsunami waves swept sediment across the continental shelf and obtained flow depths of 15-20 m at the coastline with velocities in excess of 10 meters per second. Along cliffs, and especially at Jervis Bay, waves reached elevations of 40-100 m with evidence of flow depths in excess of 15 m. Preliminary evidence on the Shoalhaven delta indicates that waves penetrated 10 km inland for at least one event. This geomorphic evidence suggests that the New South Wales south coast is subject to tsunami waves an order of magnitude greater than that indicated by historic tide gauge records.

Recent work indicates that the southeast coast of Australia may not be the only coast to be affected by catastrophic tsunami. The geomorphic signatures of such events have been found on Lord Howe Island in the mid-Tasman Sea, along the north Queensland coast and along the northwest coast of Western Australia. At the latter location, there is good evidence that a recent wave swept more than 30 km inland, in the process topping 60 m high hills more than 2 km from the coast. Finally bedrock sculpturing features have been identified on the islands of Hawaii and along the east coast of Scotland. The latter location is within the zone affected by the tsunami generated by a large submarine landslide near Storegga, Norway also 7,000 years ago.