Brill, Dominik (2012). The tsunami history of southwest Thailand - Recurrence, magnitude and impact of palaeotsunamis inferred from onshore deposits. PhD thesis, Universität zu Köln.
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Abstract
The Indian Ocean tsunami (IOT) of December 2004 has uncovered a till then widely unacquainted tsunami risk for the west coast of southern Thailand. To limit loss of life and damage due to potential future events, data about the frequency and magnitude of tsunamis in Thailand over a longer time period is needed. Since modern and historical tsunami recording in the Bay of Bengal is limited to the last 300 years, details about the long-term hazard have to be drawn from geological evidence of prehistoric tsunami flooding by (i) identifying new palaeotsunamis, (ii) dating and (iii) correlating spatially distinct evidence, and (iv) establishing the magnitude and the intensity of these events. For this, sediments from near-shore geoarchives in the coastal areas of Phang-nga province and Phuket (Thailand) have been sampled and analysed sedimentologically (granulometry, geochemistry, mineralogy, fauna, sedimentation modelling) and chronologically (optically stimulated luminescence and radiocarbon dating). Evidence of multiple prehistoric coastal flooding events in the form of allochthonous sand beds was identified in the coastal plain of Ban Bang Sak. The layers are assigned to high-energy events of marine origin, either tsunamis or tropical storms, by means of the sediment characteristics. The distinction between tsunamites and tempestites is mainly based on comparison of the palaeoevent beds with the local deposit of the IOT 2004, which revealed similarities regarding spatial extent and sediment properties. Furthermore, the radiocarbon age of the youngest palaeoevent correlates with contemporaneous tsunami evidence from India and Sumatra. Hence, the youngest event, dated to 500-700 cal BP, is related to a basin-wide tsunami, while several older events that took place between 2000 and 1180 cal BP preliminary are treated as tsunami candidates. Since the application of radiocarbon for dating tsunamites on Phra Thong Island is limited by the scarcity of datable macro-remains and the contamination by young roots, here, the potential of optical dating (OSL) is evaluated by applying the method to a suite of tsunamigenic and littoral deposits for which independent age control is available. Small aliquots of coarse-grained quartz were used for measurements, and processed statistically by means of appropriate age models. Cross-checking with independent radiocarbon data showed good agreement for littoral deposits and only insignificant residuals of less than 40 years for the poorly bleached deposits of the IOT 2004. These results legitimate the use of OSL to determine ages of palaeotsunamis in the same setting. Therefore, the established luminescence procedure was applied as a tool to correlate the spatially distinct sand sheets of prehistoric tsunamis that are preserved in the swales of Phra Thong’s beach-ridge plain. At least three palaeoevents – being 490–550, 925–1035 and 1740–2000 years old – are distinguished and correlated between the swales of Phra Thong (as an indicator for inundation distances), as well as within the Bay of Bengal (as an indicator for the impact areas). For the interpretation of onshore tsunami deposits in terms of magnitude and impact on coastal systems, the boundary conditions at the time of prehistoric tsunami events have to be taken into account. Pre-existing sea-level curves for the Malay-Thai Peninsula postulate a mid-Holocene maximum of 1-5 m above present sea level, whereas the exact timing and magnitude of the Holocene highstand in Thailand remained to be defined. By the use of fixed biological and geomorphological sea-level indicators a Holocene maximum of +2.6 m in the south and +1.5-2.0 m in the north of the study area is assessed for c. 5500 years ago. In addition, Holocene changes of palaeogeography and their influence on local tsunami records are studied. While the patterns of palaeotsunamis recorded on Phra Thong and at Ban Bang Sak can be explained by the onset of back-barrier swamp formation, the existence of only one prehistoric tsunami deposit at Pakarang Cape is in conflict with the persistence of a back-reef lagoon, which provided favourable conditions for preservation over millennia. Although the IOT 2004 had a strong short-term impact on infrastructure, housing, ecosystems and coastal morphology in Thailand, the damaged systems have almost completely recovered after less than 10 years. For prehistoric tsunamis, the long-term impact is assessed based on variations of the sedimentary environments before and after the events. It can be shown that all recorded palaeotsunamis had only insignificant impact on the ecosystems and the geomorphology of Phra Thong Island, Ban Bang Sak and Pakarang Cape. To deduce transport processes and hydrodynamic parameters of tsunami events from the onshore deposits found at Ban Bang Sak, as an estimate of palaeotsunami magnitude, two different ap-proaches are applied. (i) The maximum offshore sediment source is determined using granulometry, geochemistry, mineralogy, and foraminifera of tsunamites and reference samples from various marine and terrestrial environments. (ii) By means of sedimentation modelling, onshore flow velocities and flow depths of the associated tsunami waves are estimated. The approach is evaluated by means of the IOT 2004, for which modelled flow velocities and onshore flow depths, as well as the deduced sediment source are in good agreement with quotations based on survivor videos and post-tsunami field surveys. For the 500-700 year old predecessor with modelled flow velocities and flow depths of 4.1-5.9 m/s and 7.0 m, respectively, a magnitude similar to the IOT 2004 is reconstructed. The deposits of the three tsunami candidates dated to 1180-2000 cal BP partly indicate source areas equal to the IOT 2004, suggesting events of comparable size, and partly indicate a much shallower sediment source, pointing to a storm origin or tsunamis of a lesser magnitude. In conclusion, a recurrence interval of 500-600 years for tsunamis in southwest Thailand is estimated on the basis of sedimentary evidence. Since all tsunamis recorded in Thailand are triggered by mega-ruptures of the Sunda Arc with similar magnitude and geometry as the 2004 earthquake, which have not only local but basin-wide impact, the data from Thailand provide valuable information for the assessment of future tsunami risk in other countries around the northern Indian Ocean, as well.
Item Type: | Thesis (PhD thesis) | ||||||||
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URN: | urn:nbn:de:hbz:38-49069 | ||||||||
Date: | 2012 | ||||||||
Language: | English | ||||||||
Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||
Divisions: | Faculty of Mathematics and Natural Sciences > Department of Geosciences > Geographisches Institut | ||||||||
Subjects: | Earth sciences Geography and travel |
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Date of oral exam: | 5 November 2012 | ||||||||
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Projects: | TRIAS ("Tracing Tsunami Impacts onshore and offshore in the Andaman Sea Region”), Deutsche Forschungsgemeinschaft (DFG, reference codes: BR 877/27-1, -2; KE 190/26-1) | ||||||||
Refereed: | Yes | ||||||||
URI: | http://kups.ub.uni-koeln.de/id/eprint/4906 |
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