Sedimentologie der Sedimente des Chile-Tsunami vom 27. Februar 2010. Analyse und Vergleich mit ähnlichen Ablagerungen der 2004 Sumatra, 2006 Java und anderer rezenter Tsunami
Final Report Abstract
Considering past tsunami events, respective deposits may be the only observable evidence, even though their preservation potential is limited. To understand how tsunami deposits form and how they can be identified in the geological record, it is of paramount importance to undertake detailed studies in the wake of such events as they occur. Here we report initial field data of a sedimentological post-tsunami field survey undertaken in Central Chile between March 31 and April 18, 2010. On February 27, 2010, at 3:34 am local time, an earthquake with Mw 8.8 occurred off the town of Constitución and caused a major tsunami. Most affected were coastal towns between Duao in the north (c. 35°S) and Concepción/Talcahuano in the south (c. 36.45°S). Maximum runup heights of up to 10 m were measured on broad coastal plains. The cliff coast at Tirua (c. 38.20°S) recorded a runup height between 30 m and 40 m, indicated by eroded vegetation and soil. At Callipo beach (c. 38°S) c. 20 m high dunes on a spit separating the Paicavi river from the sea were overflown by the tsunami, which then inundated the river’s floodplain. At selected localities we measured detailed topographic profiles including runup heights and inundation distances, and recorded the flow direction, thickness, distribution, sedimentological structures and facies of the respective tsunami deposits. Field data will be combined with high resolution grain size analyses, the latter of which will be used to derive hydrodynamic conditions of transport and deposition. We found the most instructive and complete sedimentological record of the February 27 tsunami at the northern tip of Isla Mocha, a small island off the Chilean coast at c. 38.15°S. Runup distances varied between 400 m and 600 m, the flow depth exceeded 3 m at ca. 100 m from the coast. In a rare sedimentological case, deposits of tsunami runup and backwash could be distinguished on Isla Mocha. The runup phase was mainly documented by fields of boulders extending c. 200 m inland. Boulders had maximum weights of 12 t. They were oriented with their long axis parallel to the wave front which indicates their transport by rolling. Algal veneers and barnacles on the boulder faces give evidence of entrainment in intertidal water depths. The boulders were embedded in mostly structureless coarse shelly sand of up to 30 cm thickness. These sands were entrained at least in parts during near shore supratidal erosion by the tsunami and transported inland during runup. At the maximum runup position thin veneers of sand were deposited. To the best of our knowledge ours is the first report of transport of sand to and its deposition at maximum runup. Flow structures indicate that the sands were then re-deposited during backwash. Downcurrent of terrace steps the tsunami backwash produced large erosional gullies which supplied additional volumes of coarse grained shelly sand from older terrace deposits. Further seawards from the gullies the backwash sands form fan-shaped braided deposits prograding over denuded soil surfaces. At Bucalemu bay (c. 34°40°S) the tsunami inundated the coastal plain and redeposited boulders with weights around 0.7 t. The boulders had been transported there from nearby quarries prior to the tsunami event for construction purposes. The redeposited boulders are runup deposits, showing two depositional clusters that may be related to transport by different waves, and are associated with thin veneers of silt and clay most likely deposited during maximum inundation.
Publications
- 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.
- Boulder transport by the 2010 Chile tsunami (Bucalemu, Central Chile): A quasi-experimental setting in a natural environment. AGU Fall Meeting 2010, abstract OS31D-1450
Spiske, M., Bahlburg, H.
- The February 27, 2010 Chile Tsunami - Sedimentology of runup and backflow deposits at Isla Mocha. AGU Fall Meeting 2010, abstract OS42B-06
Bahlburg, H., Spiske, M.
- The February 27, 2010 Chile Tsunami – First results of a sedimentological study of tsunami deposits between 34° and 39°S.- International Sedimentological Congress, Mendoza, Argentina
Bahlburg, H., Spiske M.
- (2011): A quasi-experimental setting of boulder transport by the 2010 Chile tsunami (Bucalemu, Central Chile).- Marine Geology 289, 72-85
Spiske, M., Bahlburg, H.
- Impact and sedimentary record of the February 27, 2010, Chile tsunami in southern Chile.- 28th IAS Meeting of Sedimentology, Zaragoza, Spain, 05.- 08.07.2011
Bahlburg, H., Spiske, M.
- Impact and sedimentary record of the February 27, 2010, Chile tsunami in southern Chile.- Lateinamerika Kolloquium 2011, Heidelberg, GAEA heidelbergensis 18, 38
Bahlburg, H., Spiske M.
- Tsunami deposits past and present, the determination of recurrence intervals, and the estimation of long term tsunami hazards.- Geo-risk management – a German Latin American perspective. Field workshop organized by the Geo-Network of Latin American German Alumni GOAL, Heidelberg, April 2, 2011. Freiberger Forschungshefte C538, 31-33
Bahlburg, H., Spiske M.
- (2012): Sedimentology of runup and backflow deposits of the February 27, 2010 tsunami at Isla Mocha off the central Chilean coast.- Sedimentology 59, 1063-1086
Bahlburg, H., Spiske, M.
