Fault Instability and Its Relation to Static Coulomb Failure Stress Change in the 2016 Mw 6.5 Pidie Jaya Earthquake, Aceh, Indonesia

The earthquake was followed by aftershocks detected up to one month after the event. Regional and local networks in Indonesia recorded these aftershocks, including large aftershocks detected by the Agency for Meteorology, Climatology and Geophysics (BMKG) and smaller aftershocks detected by the Pijay-Net temporary local stations. The Coulomb failure criterion, introduced by Sibson (1985), states that failure on a plane of rock can occur if shear stress acting on the plane exceeds the failure resistance. The failure resistance components include the rock cohesive strength and internal friction multiplied by normal stress acting on the plane. This concept is widely used by researchers to obtain two main methods: fault reactivation potential or fault instability, and earthquake interaction. Fault reactivation potential estimates the ratio in which a fault plane is optimally oriented, based on the Mohr-Coulomb diagram. The ratio is high when a fault plane is optimally oriented and low when the fault plane is disoriented. The potential of fault reactivation was introduced by Sibson (1985) and further developed by other researchers, each using a unique name for this phenomenon. Earthquake interaction is explained using a Coulomb failure assumption in the Coulomb Failure Stress Change (ΔCFS) method, which calculates stress changes before and after an earthquake. The Pidie Jaya earthquake lies along the east side of a nearly 180° strike fault, the Samalanga-Sipopok Fault. The historical 1967 mb 6.1 earthquake took place between the Samalanga-Sipopok Fault and the Pidie Jaya earthquake. The Samalanga-Sipopok Fault has been well documented in prior studies, but Muzli et al. (2018) did not associate the 2016 Pidie Jaya earthquake with this fault due to the fact that neither of its focal mechanism strikes aligns with the strike of the Samalanga-Sipopok Fault. A fault instability analysis was performed to analyze the relation between the Pidie Jaya earthquake's possible causative faults and the 1967 mb 6.1 event. The analysis showed that the Panteraja Fault, a newly inferred fault in the west, is the most unstable fault plane. The Panteraja Fault dips 63° toward southeast and is located near the 1967 mb 6.1 event. The ΔCFS distribution imparted by the mainshock was compared with the fault instability calculation on each aftershock plane, and the results showed that not all aftershocks have positive ΔCFSs, but their instability value is high. The study suggests that the fault plane instability plays a role in events that do not occur in positive ΔCFS areas. The off-Great Sumatran Fault (Panteraja and Samalanga-Sipopok Faults) are unstable in the Sumatra regional stress setting, making them more susceptible to slip movement. The study also discusses the possibility of conjugate failure as shown by the aftershock fault instability.