Earthquake risk assessment of the Opak and Merapi-Merbabu active faults to support mitigation program in Yogyakarta province and its vicinity

Based on historical records, Yogyakarta has a high seismic risk related to the earthquake events along active faults, such as the Opak and Merapi-Merbabu Faults. These faults were responsible for several destructive earthquakes in Yogyakarta City and its vicinity and caused fatalities and building damage in the area, e.g., the 2006 (Mw 6.3) Yogyakarta earthquake and earlier in 1943 and 1867. A previous study shows that the Opak Fault has a geodetic slip-rate of 5 mm/y and a potential magnitude Mw 6.6. In addition, the active Merapi-Merbabu Fault has a geodetic slip-rate of 1 mm/y and a potential magnitude Mw 6.6. We used scaling law relations of earthquake parameters and magnitude scenarios to estimate the recurrence time of each fault based on a kinematic model. Our results estimate that the earthquake return period (Tr) for the Opak Fault (Mw 6.6) is ∼162 years, the maximum intensity is ∼VII-VIII MMI scale, the Peak Ground Acceleration (PGA) is ∼36 % g, and Peak Ground Velocity (PGV) is ∼ 30 cm/s for a 5 km hypocentral depth. In the meantime, the earthquake return period for the Merapi-Merbabu Fault (Mw 6.6) is estimated to be ∼810 years, the maximum intensity is ∼ VI-VII MMI, the PGA is ∼ 30-36 % g, and the PGV is ∼ 21-24 cm/s for a 5 km hypocentre depth. Both faults potentially produce destructive earthquakes (Mw > 6.0) in Yogyakarta City and its vicinity. Therefore, assessments of (paleo) earthquakes are needed of both the Opak Fault and the Merapi-Merbabu Fault to support the long-term earthquake hazard mitigation program.