The main cause of the tsunami in Thailand was the sliding of part of the Earth's outer layer called the India plate below another segment known as the Burma plate. Geologically, subduction is the name given to the process that caused this Tsunami.
During the Thailand tsunami, the powerful movement of segments of the tectonic plates displaced a massive amount of water, sending strong shock waves next to the coastlines of several countries adjacent to the Indian Ocean. Subduction occurs when one of the Earth's crutal plates overrides another one, triggering tensions in the Earth's crust. The impact produces earthquakes and tsunamis.