Using ground penetrating radar to investigate tsunami deposits

Buck, Lucy Katharine (2022) Using ground penetrating radar to investigate tsunami deposits. PhD thesis, Birkbeck, University of London.

Abstract

An understanding of tsunami deposits allows an understanding of the tsunami risk of a coastal area, both in terms of inundation extent and frequency of events. Typical investigations of these deposits use boreholes and trenches. Though techniques are destructive as well as time and labour intensive. Ground penetrating radar (GPR) provides a non-destructive technique to study tsunami deposits while allowing a larger area to be surveyed in the same period of time. This allows a greater understanding of the size of deposits and potentially the size of the risk. In this PhD project GPR is used to survey tsunami deposits from the Storegga Slide tsunami in Northern Scotland and the Shetland Islands and the 2004 Indian Ocean tsunami. GPR will be used to image the erosional signature created by the tsunami in Indonesia. Knowledge will be gained of how the tsunami wave interacted with the coast as well as how the beach has recovered since the tsunami. This achieves an understanding of beach recovery aiding in disaster risk reduction and coastal protection methods. GPR has previously been to map tsunami deposits in Thailand, Sri Lanka, Greece, Portugal, Spain, the USA, Japan, Oman, Australia and New Zealand. However it has not been used to study any deposits created by the Storegga Slide tsunami or in Indonesia to study tsunami deposits. Two events have been studied in three locations. The first is the Storegga Slide tsunami. The Storegga Slide is the largest known submarine landslide in the North Atlantic and occurred off the Norwegian coast 8,000 years ago, with two smaller failures 5,000 and 1,500 years ago. All three failures triggered tsunamis and deposits from these events have been found in Scotland and the Shetland Islands as well as several other countries around the North Sea basin. The second event is the 2004 Indian Ocean tsunami in Aceh, Indonesia. This was triggered by a 9.1 Mw earthquake that occurred on the 24th December 2004. Over 250,000 people were killed in 18 countries. Aceh, the northernmost province in Indonesia on the island of Sumatra, was the closest land to the epicentre of the earthquake and had the highest death toll. GPR surveys were undertaken in four sites in Northern Scotland, five sites in the Shetland Islands and four sites in Indonesia. Several different frequencies of antenna were used; 100 MHz, 200 MHz and 500MHz, to allow a balance between resolution and depth penetration. In Northern Scotland the GPR surveys expanded on previous bore hole based studies in the area and clarified and expanded previous results. They showed multiple events in an area where only one had been located previously. In the Shetland Islands the tsunami deposits constituted a sand layer interbedded within peat. The difference in dielectric properties of peat and sand produce a strong reflection on GPR and due to this the GPR results were excellent. Much thinner deposits were imaged than were expected and allowed the tsunami deposits to be traced further inland than has been described before. The results showed that both areas were more heavily effected by the tsunamis here than was previously thought. This has huge implications on coastal protection schemes, infrastructure and the local population. In Indonesia it was found that beach recovery occurred very quickly after the tsunami. This is because the beach processes that are occurring along the coast are the same before and after the tsunami so the beach quickly returns to equilibrium. This also means that the part of the beach that eroded away in the 2004 event is the most likely to be eroded away in a future event indicating any rebuilding here should be avoided. The GPR surveys also revealed the history of tsunamis in the area. However two sites yielded null results due to ground conditions. It was found that GPR is a useful tool to study tsunami deposits and the results found have important implications for coastal management and disaster risk reduction.

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