"The Earth is divided into plates, which are semi rigid, but they do move sometimes and interact with each other. Indonesia has the unlucky circumstance of being on a very major plate boundary and, particularly under Java, you have one plate diving directly another." - Katherine Cashman, Professor of Volcanology at Bristol Univesity
A summary of two earthquakes which occurred in Sumatra, Indonesia:
This is a summary of the 2012 Sumatra earthquake which shows that it is both the largest intra-plate and strike-slip earthquake with it being the 10th largest earthquake ever. This animation compares the 2012 Sumatran Earthquake to another earthquake of magnitude 9.1 which was a subduction earthquake which triggered a tsunami killing over 230,000 people. Although the earthquakes were similar in magnitude, they were caused by different tectonic processes related to the Indo-Australian plate.
Destruction caused by Indonesia's plate tectonics: