Abstract
Abstract The 2004 Sumatra-Andaman tsunami flooded coastal northern Sumatra to a depth of over 20 m, deposited a discontinuous sheet of sand up to 80 cm thick, and left mud up to 5 km inland. In most places the sand sheet is normally graded, and in some it contains complex internal stratigraphy. Structures within the sand sheet may record the passage of up to 3 individual waves. We studied the 2004 tsunami deposits in detail along a flow-parallel transect about 400 m long, 16 km southwest of Banda Aceh. Near the shore along this transect, the deposit is thin or absent. Between 50 and 400 m inland it ranges in thickness from 5 to 20 cm. The main trend in thickness is a tendency to thicken by filling low spots, most dramatically at pre-existing stream channels. Deposition generally attended inundation—along the transect, the tsunami deposited sand to within about 40 m of the inundation limit. Although the tsunami deposit contains primarily material indistinguishable from material found on the beach one month after the event, it also contains grain sizes and compositions unavailable on the current beach. Along the transect we studied, these grains become increasingly dominant both landward and upward in the deposit; possibly some landward source of sediment was exposed and exploited by the passage of the waves. The deposit also contains the unabraded shells of subtidal marine organisms, suggesting that at least part of the deposit came from offshore. Grain sizes within the deposit tend to fine upward and landward, although individual units within the deposit appear massive, or show reverse grading. Sorting becomes better landward, although the most landward sites generally become poorly sorted from the inclusion of soil clasts. These sites commonly show interlayering of sandy units and soil clast units. Deposits from the 2004 tsunami in Sumatra demonstrate the complex nature of the deposits of large tsunamis. Unlike the deposits of smaller tsunamis, internal stratigraphy is complex, and will require some effort to understand. The Sumatra deposits also show the contribution of multiple sediment sources, each of which has its own composition and grain size. Such complexity may allow more accurate modeling of flow depth and flow velocity for paleotsunamis, if an understanding of how tsunami hydraulics affect sedimentation can be established.
Highlights
The 2004 Sumatra-Andaman tsunami represents the first trans-oceanic tsunami in more than 40 years, and the only such tsunami to have been studied other than the 1960 Chilean event
Of Japan (Konno, 1961), but only a few words have been written about tsunami deposition by a large tsunami near its source (e.g., Cisternas et al, 2005; Wright and Mella, 1963)
Understanding the sedimentary deposits of large tsunamis close to their source is important because several large paleoseismic events have been recognized based, in part, on their tsunami deposits (e.g., Atwater et al, 2005; Cisternas et al, 2005; Nanayama et al, 2003; Pinegina et al, 2003)
Summary
The 2004 Sumatra-Andaman tsunami represents the first trans-oceanic tsunami in more than 40 years, and the only such tsunami to have been studied other than the 1960 Chilean event. The area was selected for a number of reasons, including: the presence of a sand sheet visible on satellite images, relatively few buildings to complicate flow patterns, and the presence of abundant flow depth and direction indicators. In this location, we characterized the tsunami sedimentation by establishing a measured transect in the direction of flow, extending from the shoreline to the limit of inundation. The sedimentology team traveled primarily to the southeast of Banda Aceh, from approximately Lampuuk south to Leupung (Fig. 1).
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