Abstract
Herein, we applied the fault instability criterion and integrated it with the static Coulomb stress change (ΔCFS) to infer the mechanism of the 2016 Mw 6.5 Pidie Jaya earthquake and its aftershock distribution. Several possible causative faults have been proposed; however, the existence of a nearby occurrence, the 1967 mb 6.1 event, created obscurity. Hence, we applied the fault instability analysis to the Pidie Jaya earthquake 1) to corroborate the Pidie Jaya causative fault analysis and 2) to analyze the correlation between ΔCFS distribution imparted by the mainshock and the fault instability of the reactivated fault planes derived from the focal solution of the Pidie Jaya aftershocks. We performed the fault instability analysis for two possible source faults: the Samalanga-Sipopok Fault and the newly inferred Panteraja Fault. Although the maximum instability value of the Samalanga-Sipopok Fault is higher, the dip value of the Panteraja Fault coincides with its optimum instability. Therefore, we concluded that Panteraja was the causative fault plane. Furthermore, a link between the 1967 mb 6.1 event and the 2016 Mw 6.5 earthquake is discussed. To analyze the correlation between the fault instability and the ΔCFS, we resolved the ΔCFS of the Pidie Jaya mainshock on its aftershock planes and compared the ΔCFS results with the fault instability calculation on each aftershock plane. We discussed the possibility of conjugate failure as shown by the aftershock fault instability. Related to the ΔCFS and fault instability comparison, we found that not all the aftershocks have positive ΔCFSs, but their instability value is high. Thus, we suggest that the fault plane instability plays a role in events that do not occur in positive ΔCFS areas. Apart from these, we also showed that the off-Great Sumatran Fault (Panteraja and Samalanga-Sipopok Faults) are unstable in the Sumatra regional stress setting, thereby making it more susceptible to slip movement.
Highlights
According to the Coulomb failure criterion (Sibson, 1985; Oppenheimer et al, 1988; Zoback, 2007), failure on a plane of rock can happen if shear stress acting on the plane exceeds the failure resistance
Based on the seismic history and the fault instability analysis, we suggest that Panteraja was the causative fault plane of the 2016 Pidie Jaya earthquake
By using the fault instability method for the Pidie Jaya causative fault analysis, we concluded that neither the fault responsible for the 1967 mb 6.1 event nor the Samalanga-Sipopok Fault is the causative fault; we strengthened the Panteraja Fault as the causative fault, due to its dip which coincides with the optimum dip range
Summary
According to the Coulomb failure criterion (Sibson, 1985; Oppenheimer et al, 1988; Zoback, 2007), failure on a plane of rock can happen if shear stress acting on the plane exceeds the failure resistance. Failure-resistance components include the rock cohesive strength and internal friction multiplied by normal stress acting on the plane. This basic concept is widely used by researchers to obtain two main methods, which relate to 1) fault reactivation potential or fault instability (Vavrycuk, 2011; Leclère and Fabbri, 2013) and 2) earthquake interaction (King et al, 1994). Fault reactivation potential estimates the ratio in which a fault plane is optimally oriented, based on the Mohr-Coulomb diagram (Sibson, 1985; Leclère and Fabbri, 2013). Based on calculating stress changes before and after an earthquake, an increase in ΔCFS is associated with subsequent events, while a decrease in ΔCFS is related to the stress shadow effect (King et al, 1994)
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