GET TO KNOW ABOUT OPAK FAULT
Several geological studies have revealed that Southeast Asia is prone to earthquakes. This is inseparable from the fact that Southeast Asia is at the crossroads of three tectonic plates: the Pacific Ocean/Philippine Ocean Plate to the east, the Indo-Australian Plate to the south, and the Eurasian Plate to the northwest.
Opak Fault refers to a subsurface fault beneath the Opak River, 30 km southeast of Yogyakarta, and is covered by deposits of the young Mount Merapi, located in Bantul regency, southeast of Yogyakarta, within the southern Mount Merapi area. Early concerns about the existence of the Opak Fault arose when Dr. S. W. Visser reported an earthquake in Yogyakarta in 1867, with its epicenter located near the Opak River. It is also thought to have caused the formation of the Yogyakarta Basin, where the city of Yogyakarta is located.
The Opak Fault has an average depth of 55-82m, while its displacement ranges from 5-10m. Opak Fault is a normal fault that runs from southwest (SW) to northeast (NE); the west of the Opak Fault shows an uplift movement, while the east of the Opak Fault shows a subsidence movement. The fault runs through the Yogya Lowlands and the Wonosari Plateau, with old andesite rocks (OAF) as structural elements. This fault system also includes the Semilir Formation and the Nglanggran Formation to the east of Opak.
There is a significant surface deformation along the Opak Fault, which could be driven by the postseismic effect following the 2006 Yogyakarta earthquake or by aseismic creep. This long-term surface deformation can also affect various aspects of earthquake-affected land, including infrastructure and environmental damage.
The earthquake in Yogyakarta on 27 May 2006 was one of the worst disasters experienced by the people of Yogyakarta and Indonesia. Over 5,700 people were killed, 37,927 were injured, 240,396 homes were destroyed, and local infrastructure and economic activities were severely affected.
Located in the southern part of Java, Yogyakarta is prone to experiencing moderate to high-magnitude earthquakes (M > 6.0) with return periods of 50–100 years, making it one of Indonesia’s most seismically active cities. Of the 12 earthquakes in Yogyakarta from 1804-2006, three earthquakes, including the 2006 earthquake, were related to the movement of the Opak fault.
More recently, on 30 June 2023, Yogyakarta was hit by an earthquake measuring 5.8 (USGS) or 6.4 on the magnitude scale, although it only caused minor damage. This could also be attributed to the public’s preparedness and the strong structural integrity of buildings – lessons learned following the 2006 earthquake.
The position of the fault plane that cuts through the soil layer with a thickness of about 50 cm – 5 m indicates that this fault is active. Furthermore, the Opak Fault, which is still active, is increasingly showing signs of increased seismic activity based on BMKG monitoring. Communities around the Opak Fault often feel small earthquakes, or what they call “lindu”.
In addition, several areas traversed by the Opak Fault have high liquefaction potential. This is due to the geological environment in the form of alluvium deposits, groundwater table conditions, the path of the active Opak fault zone, and the Bantul basin (Bantul Graben).
Sustainable and resilient countermeasures are needed to deal with various potential disasters in a region historically prone to earthquake disasters, both in Yogyakarta and Southeast Asia. Therefore, the scenario of a 6.6 magnitude earthquake caused by the Opak Fault for the 2023 ASEAN REGIONAL DISASTER SIMULATION EXERCISE (ARDEX) offers a valuable opportunity for local and regional humanitarian actors to prepare for and respond to such a scenario, enhancing their readiness to mitigate the impact of such disasters effectively. The AHA Centre is pleased to participate in ARDEX-23, the ASEAN region’s biggest disaster and emergency response simulation exercise.
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Written by: M. Nadhif Achyansyah