The people of Indonesia suffered a double whammy of natural disasters during October 2010. Mount Merapi, Indonesia's most volatile volcano erupted and this was closely followed by a magnitude 7.7 earthquake off the south-west of Sumatra that triggered a tsunami in which at least 40 people were killed. What makes this part of the world so susceptible to these sorts of natural hazards and are they related?
This concentration of seismic (earthquake) activity along the south-western coast of Indonesia indicates that this area is close to a plate margin.
When we look at the global distribution of earthquakes we see that these define plate boundaries. In this case, Indonesia is located on the Pacific 'Ring of Fire' at the boundary between the Indo- Australian plate and the Eurasian plate. Here the Indo-Australian plate is sinking beneath the Eurasian plate along what is termed a subduction zone. As can be seen from the more detailed diagram, there is a significant element of lateral slip. The process of one plate moving down beneath another, in this case, the Indo- Australian plate moving down beneath Indonesia, creates earthquakes that become deeper towards Indonesia. At the same time, this movement creates a deep oceanic trench, the Sumatran Trench.