"The Indian Ocean tsunami in 2004 was not the first of its size to hit the region, according to new research.
Two international collaborations have sampled the sediments in Thailand and Sumatra to examine tsunami history.
At both sites, there was evidence of sediment laid down by a large tsunami between 600 and 700 years ago, pre-dating written and oral records."http://news.bbc.co.uk/2/hi/science/nature/7697482.stm
you can see the layers of the repeated waves over time
They have very distinct features, so we know what they are. This is what real geology is, not the creationist nonsense that many people spout without understanding even the basics.