FOCAL MECHANISM ANALYSIS OF THE 2019 AMBON (MW 6.5) AFTERSHOCK EARTHQUAKE USING MOMENT TENSOR INVERSION

• Main Author: Widhi Baskara, Aria
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/69629
• Institution: Institut Teknologi Bandung

The 6.5 Mw Ambon Earthquake on September 26th, 2019, had a significant impact on increasing seismicity around Ambon Island, Seram Island, and its surroundings. The Aftershock monitoring conducted by ITB, BMKG, and BNPB using 11 local broadband temporary seismograph and 4 BMKG regional stations between October 18th until December 15th, 2019 managed to record 1778 aftershocks with magnitudes between 1.9-5.1 Mw and distributed in three separate seismic clusters. The tree seismic clusters formed in the distribution pattern of the aftershock sequence has led to speculation about the reactivation of preexisting faults caused by increased stress triggered from the mainshock. Aftershocks analysis, including hypocenter distribution and source mechanism estimation, are important to help reveal the rupture process of the mainshock. In addition, both pieces of information can be used to analyze the relationship between the mainshock and the aftershocks sequence, as well as its effect on the reactivation of faults in the vicinity. The distribution of the focal mechanism from the aftershock provides us a better understanding of possible active faults in the study area, thus the seismotectonic pattern can be revealed properly. In this study, We estimated the solution of the focal mechanism for significant aftershock events from the 2019 Ambon earthquake using the moment tensor inversion method. Mathematically, the moment tensor is suitable for describing the equivalent force and moment of a point source. Our MT results depict that the 2019 Ambon mainshock is followed by a geometrically complex rupture process. The three rupture area consists of: (i) The 35 km long N-S main fault characterized by dextral strike-slip, (ii) The NNE-SSW thrust segments with a ~45o dip northeastward located on the Southwest of Seram, ~10 km from Kairatu, and (iii) The 5 km NNW-SSE Strike-slips and E-W normal faults in Ambon Islands. We interpret The strike-slip segments in Ambon Islands as antithetic faults that reactivated in a Riedel shear structure system in this area. To conclude, 20 MTs solutions in this study have confirmed the presence of the reactivated faults in Southwest of Seram and Ambon islands during the aftershock sequences in which we successfully address the source characteristic for each reactivated fault.Given that both reactivated faults are located only a few kilometers from a densely populated urban region, the seismic hazard assessment should account for such complicated faulting settings