The Opak Fault runs southwest to northeast across the region of Yogyakarta in central Java, Indonesia, roughly following the valley of the Opak River near the ancient city and the Prambanan temple complex. It lies in a densely populated area at the foot of the active Merapi volcano.
The fault is generally regarded as a strike-slip structure accommodating deformation in the crust behind the Sunda subduction zone, where the Australian Plate dives beneath Java. Its precise geometry has been debated by scientists, but it clearly plays a central role in the seismicity of the Yogyakarta region.
The Opak Fault zone is associated with the devastating 2006 Yogyakarta earthquake of magnitude 6.3, which killed around 5,700 people and destroyed hundreds of thousands of homes despite its moderate size. The catastrophe, amplified by shallow depth and vulnerable buildings, made the Opak Fault a stark reminder of the danger posed by shallow crustal faults beneath crowded regions.