Observation Satellite and were compared to each other to show potential ground deformation patterns. A lack of any dislocation found on the images along the Opak River fault made evident the lack of movement along that fault, and though the aftershocks were occurring at a depth of , the deformation was distinct at the surface. The observed ground deformation that was detailed by the differential satellite images and Global Positioning System measurements was roughly east of (and parallel to) the Opak River Fault, along a zone that passed through the USGS' epicenter, and delineated a NE trending vertical fault (a dip of 89°). The displacements were not more than and indicated left-lateral strike-slip motion as well as a component of reverse slip, and to the west of the Opak River Fault (and closer to the areas of damage) strong ground motion triggered subsidence of volcanic deposits from Mount Merapi. Strong motion 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. Liquefaction A separate post-event study looked at the relationship with the layer of sediment and the occurrence of soil liquefaction during earthquakes near Bantul. Researchers stated that the Yogyakarta region is seismically active, with four known events in the 19th century and three in the 20th century, with peak ground acceleration values of 0.038–0.531g. The type and properties of sediment control the occurrence and distribution of liquefaction, and other environmental conditions (like the water table) also play a part, as well as the peak ground acceleration of the earthquake. The Bantul-Klaten plain consists of alluvium (sand, silt, clay, and gravel)