Sedimentologische Analyse der Ablagerungen des Java-Tsunami vom 26.07.2006: Parameter der Strömungsprozesse während des Auflaufens und Rückstroms im Vergleich zum Sumatra-Tsunami vom Dezember 2004
Zusammenfassung der Projektergebnisse
With the help of tsunami deposits, tsunami run-up characteristics and tsunami depositional processes can be inferred in order to understand tsunami as a geological process, but also to recognise tsunami deposits within sedimentary deposits of different sedimentary environments. Unique features of such deposits have not been identified to distinguish them from, for example, storm or other deposits indicating high-energy regimes within sandy beach deposits. It is sfill unresolved in which water depths and environments tsunami entrain sediment during run-up. We report the initial results of a study of the deposits of the July 17, 2006 Java tsunami. The Java tsunami was caused 200 km off the south coast of Java by a ca. 10 km deep Mw 7.8 earthquake at 15:19 local time. The quake caused a ca. 200 km trench-parallel rupture and struck the coast 20 minutes later. Depending on the geometry of coast and beaches, run-up heights exceeded 20 m at headlands, and were significantly lower, less than 2 m, along many straight beaches. The flow depth reached c. 165 cm at a distance of 150 m from the swash zone. We surveyed beaches along the central south coast of Java between Pangandaran and Parangtritis south of Yogyakarta, sampled the tsunami deposits, and measured the maximum run-up distance of 700 m to the west of Pandangaran. Quasi-linear debris accumulations document up to three waves of decreasing inundation and run-up. The tsunami deposits consist of sand, reach maximum thicknesses of 10 cm, display a weakly developed fining-inland trend, and taper out approximately at half the run-up distance. They commonly have erosive bases and consist of one to five parallel-laminated and in some cases graded layers. Bottom layers may contain soil rip-up clasts eroded from inundated agricultural fields. Sedimentary structures include small current ripples produced by backwash flow in morphological depressions. Larger palm trees were bent inland during tsunami run-up whereas the less energetic backwash bent smaller palm trees seaward. The beach sands underlying the tsunami deposits as well as the tsunami deposits themselves are rich in detrital magnetite and frequently show a weakly polymodal grain size distribufion. The two main modes differ by almost 1 phi unit. We assumed that these reflect light and heavy mineral abundances, respectively, representing hydraulically equivalent grain size fractions. To test this assumption we separated magnetic and non-magnetic light and heavy mineral fractions and analyzed them separately by digital-optical grain size analysis using a PartAn 2100 L particle analyzer. Contrary to expectations, the grain size distributions show very similar modes in all four layers of the sample and all subsamples (non-magnetic light minerals, non-magnetic heavy minerals, magnetic light minerals, magnetic heavy minerals), albeit with varying abundances between layers. This is evidence that conditions of transport and deposition during the formation of each layer did not permit the sorting of the detritus into hydraulically equivalent grains size fractions, and that settling equivalence was not attained. Furthermore, the similarity between the tsunami deposits and the pre-tsunami beach indicates that the grain size distribution of the tsunami layers may in fact be a feature inherited from the beach deposit.
Projektbezogene Publikationen (Auswahl)
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(2006): Tsunamisedimente und die Tsunamigeschichte von Ozeanrändern.- Department colloquium, Geowissenschaftliches Zentrum der Universität Göttingen, 06.12.2006
Bahlburg, H.
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(2007): Sedimentological characteristics of the July 17, 2006 tsunami in South Java.- Eos Transactions. American Geophysical Union, Fall Meeting Supplement
Piepenbreier, J., Bahlburg, H., Spiske, M., Amijaya, H., and Weiss, R.
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(2007): Sedimentology of the deposits of the July 17, 2006 Java Tsunami.- 25th Regional Meeting of the International Association of Sedimentologists IAS, Patras, Book of Abstracts, 47
Bahlburg, H. and Spiske, M.
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(2007): Sedimentology of tsunami deposits in light of the Indonesia tsunami of December 26, 2004 and July 17, 2006.- Department colloquium, GeoForschungsZentrum Potsdam, 16.01.2007
Bahlburg, H.
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(2007): Tsunami: Eine unterschätzte Naturgefahr.- Invited, Exhibition and colloquium series „Unruhige Erde", Geotechnologien, Münster-Osnabrück Airport, 11.01.2007
Bahlburg, H.
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(2008): Inversion of flow depth and speed from tsunami deposits using TsuSedMod.- Eos Transactions. American Geophysical Union, Fall Meeting Supplement
Spiske, M., Weiss. R. and Bahlburg, H.
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(2008): Sedimentological characteristics of the July 17, 2006 tsunami in South Java and the application of TsuSedMod.- Department of Earth and Space Science, University of Washington, Seattle, December 2008
Spiske, M., Weiss, R. and Bahlburg, H.
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(2008): Sedimentological characteristics of the July 17, 2006 tsunami in South Java.- 2nd International Tsunami Field Symposium, Puglia, Italia, 22.-28.09.2008, abstracts
Bahlburg, H., Spiske, M., Roskosch, J., Amijaya, H., Weiss, R. and Piepenbreier, J.
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(2009): A review of criteria for the distinction of high energy wave events.- 27. Jahrestagung des Arbeitskreises „Geographie der Meere und Küsten", Kiel, 24.-26.04.2009
Spiske, M., Bahlburg, H., Deicke, M., Steffahn, J.
