In 2006, Mount Merapi had not been active for more than four years, but on May 11 a pyroclastic flow triggered the evacuation of more than 20,000 people from the northern sector of Yogyakarta. While authorities expected a larger eruption to follow, the earthquake occurred instead. The volcano's previous eruptions deposited loosely bound sedimentary material in the valley during lahar flows and this material was found to have played a significant role in the effects of the shock. For example, German and Indonesian scientists set up instruments at several locations situated on different soil types to measure aftershocks. Of nine events that were analyzed, it was found that the station at Imogiri (a heavily affected village that was built on of sediment) showed signs of local amplification when compared to a location that was built on bedrock, and that the deposits amplified the impact of the shallow crustal rupture.