DISTINGUISHING INDIVIDUAL WAVES IN A TSUNAMI DEPOSIT

Tsunamis are composed of multiple waves, but in most cases it is difficult to distinguish waves within a deposit. It is likely that different waves of a tsunami will vary in how they erode the various offshore, nearshore, and onshore sediment sources. We expect that source will change little within a wave, but may change between waves.

To test this hypothesis, we will use tsunami deposits from two localities in Aceh, Indonesia. At Busung Bay, Simeulue, the December 26, 2004 and March 28, 2005 tsunami deposits overlie each other. The deposit reflects two separate tsunamis, and thus clearly reflect separate waves. Further north, in Langi Bay, Simeulue, the 2004 tsunami left a deposit that includes cross laminae that indicate multiple reversals in flow. Again, deposits at this locality records separate waves, this time within one tsunami.

At each field location we collected sand samples to quantify sand size distributions. These distributions reflect both the sand source and the depositional process. To detect source influence, we will assume that deposition by the tsunami reflects a combination of mixing and simple settling. Simple settling predicts that the rate at which a particular size sediment leaves the water column is equal to its settling velocity times its concentration near the bed. Mixing predicts sand size distributions are a linear combination of some small number (2-3) of distributions. Using these simplifications we hope to detect when new sand populations appear in the deposit, allowing us to test whether different waves carry sediment from different sources.

If this analysis distinguishes separate waves in Busung and Langi Bays, we will extend the analysis to other Aceh deposits. This technique may provide a way of distinguishing single pulse local tsunamis from more complex trans-oceanic tsunamis. Also it may provide a basis for distinguishing tsunami and storm deposits.