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We present here an analysis of the destructive Mw 6.2 earthquake sequence that took place on 14 January 2021 in Mamuju–Majene, West Sulawesi, Indonesia. Our relocated foreshocks, mainshock, and aftershocks and their focal mechanisms show that they occurred on two different fault planes, in which the foreshock perturbed the stress state of a nearby fault segment, causing the fault plane to subsequently rupture. The mainshock had relatively few aftershocks, an observation that is likely related to the kinematics of the fault rupture, which is relatively small in size and of short duration, thus indicating a high stress-drop earthquake rupture. The Coulomb stress change shows that areas to the northwest and southeast of the mainshock have increased stress, consistent with the observation that most aftershocks are in the northwest.

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The 2020 Sumbawa earthquake of moderate magnitude (M 5.3) produced very significant aftershocks. Based on the computation of Utsu’s method, those aftershocks would be ended after the 20th day. Those earthquakes along 20 days were relocated using double-difference method. The relocation results show the southwest-northeast orientation and getting deeper into the northwest direction. Those two directions show the strike and the dip from the fault plane of the earthquake which was consistent with the focal mechanism released by the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG). Those results showed the majority of earthquakes occurred at a depth of shallower than 20 km. Those earthquake depths were fit with the previous study showing the crustal thickness beneath Sumbawa Island that was about 28 km. We also found that those earthquakes occurred at splay faults propagating to decollement structure. This study is beneficial for earthquake disaster mitigation especially in updating active faults on Sumbawa Island.

The earthquake that occurred in the Palu city and its surroundings on September 28, 2018, with a magnitude of 7.5 Mw caused a lot of casualties and infrastructure damages. Tsunami and liquefaction are secondary effects that contribute significantly to these two losses. The earthquake occurred at the Palu Koro Fault with a strike-slip mechanism. The BMKG seismic network successfully recorded aftershocks caused by the earthquake. Hypocenter relocation using the double-difference method is applied in this study to analyze the mainshock and its aftershocks so that the fault geometry of the earthquake can be known clearly. This study used the BMKG earthquake catalog at coordinates 40S-10N and 1180-1220E until the end of December 2018. The results show an improvement in the hypocenter position with loss of fix depth at a depth of 10 km so that the area can be interpreted tectonically. The results also show that the majority of the earthquakes beneath the fault are at a depth of less than 25 km. The relocated hypocenter parameters are very useful as a database for further tectonic studies that are useful for earthquake disaster mitigation in the area.