What caused the Boxing Day tsunami?

The Boxing Day tsunami that occurred on Dec. 26, 2004, was caused by an earthquake of above 9 magnitude in the Indian Ocean. The earthquake was caused by the movement of the Indio-Australian tectonic plate as it subducted below the Eurasian plate, causing the seafloor above it to lift upwards.

While travelling through the open ocean, the tsunami measured a height of less than 1 meter. Upon reaching the shores of the countries around the Indian Ocean, the waves measured up to 15 meters tall. The impact of the Boxing Day tsunami resulted in approximately a quarter million deaths and two million people being made homeless.