| Abstrakt: |
The complexity of the active tectonic system in East Indonesia has influenced various types of seismicity in the Seram Sea, resulting in several significant earthquakes. In the last decade, there were two devastating earthquakes, Mw 6.2 (October 31, 2017) occurred with the thrust mechanism as typical of active subduction while Mw 6.5 (September 26, 2019) occurred with a left-lateral mechanism due to the unknown active fault in the Seram Island and its surroundings. In the present study, we analyzed the current seismicity for 8-years (2015–2022) by using Bayesian moment tensor inversion for the two largest earthquakes and multivariate hypocenter clustering in the spatial-time-distance-magnitude (STDM) domain using an earthquake catalog. The Bayesian inversion was applied by using bootstrapping chain analysis with the regional seismic network with a distance of up to 4000 km to figure out the earthquake source mechanism. The inversion results show that both earthquakes have different mechanisms. The October 2017 event occurred in the actual plane with strike 262°, dip 21° and rake 68°, suggesting an active subduction with a steep dipping angle of 20°–25° towards the north direction. However, the September 2019 occurred with strike 346°, dip 85°, and rake −140°, indicating an offshore unidentified fault with rupture propagation towards the N–S direction and pure reverse slip. The grid search of the centroid location shows a converged solution with small shifting of 10–15 km to the east and 10 km to the south while the depth is consistent at 7–10 km. The obtained focal mechanism shows an uncertainty with 70–80% confidence intervals and a percentage of Double-Couple (DC) component > 90% that evidences the tectonic process. To support the earthquake mechanism results, earthquake clustering analysis was performed based on nearest-neighborhood distance with its components in STDM. The notable breaks in the STDM time series due to the Mw 6.2 (2017) and Mw 6.5 (2019) earthquakes are discussed, supported by a Multitaper spectral analysis for effective visualization of STDM spectral peaks. Finally, the relation between STDM time series is studied by magnitude-squared coherence with 95% confidence intervals. As a result, we found that although the magnitude of coherence was not high, they are weakly related to each other. This implies that the occurrence of future seismicity somehow depends upon these parameters. The comprehensive results of this study may support the existence of the South Seram subduction system that can be used to update the mitigation plan for different earthquake characteristics shortly. [ABSTRACT FROM AUTHOR] |
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