We reveal the existence of a previously unknown fault that generated the Mw 7.3 Flores Sea earthquake, which occurred on 14 December 2021, approximately 100 km to the north of Flores Island, in one of the most complex tectonic settings in Indonesia. We use a double-difference method to relocate the hypocenters of the mainshock and aftershocks, determine focal mechanisms using waveform inversion, and then analyze stress changes to estimate the fault type and stress transfer. Our relocated hypocenters show that this earthquake sequence ruptured on at least three segments: the source mechanism of the mainshock exhibits dextral strike-slip motion (strike N72°W and dip 78° NE) on a west–east-trending fault that we call the Kalaotoa fault, whereas rupture of the other two segments located to the west and east of the mainshock (striking west-northwest and southeast, respectively) may have been triggered by this earthquake. The Coulomb stress change imparted by the rupture of these segments on nearby faults is investigated, with a focus on regions that experience a stress increase with few associated aftershocks. Of particular interest are stress increases on the central back-arc thrust just north of Flores and the north–south-striking Selayar fault in the northwest of our study region, both of which may be at increased risk of failure as a result of this unusual earthquake sequence.

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The west part of Java sits at the transition from oblique subduction of the Australian plate under the Sunda block of the Eurasian plate along Sumatra to orthogonal convergence along central and eastern Java. This region has experienced several destructive earthquakes, the 17 July 2006 Mw 7.7 earthquake and tsunami off the coast of Pangandaran and the 2 September 2009 Mw 7 earthquake, located off the coast of Tasikmalaya. More recently, on 15 December 2017, an Mw 6.5 earthquake occurred off the coast near Pangandaran, and, on 23 January 2018, an Mw 5.9 earthquake occurred offshore Lebak, between Pelabuhan Ratu and Ujung Kulon. Ground shaking and damage occurred locally and in Jakarta on the northern coast of Java. In this study, we use the double-difference technique to relocate both mainshocks and 10 months of seismicity (228 events) following the earthquakes. The relocation result improved the mainshock locations and depth distribution of earthquakes. Moment tensor of the December 2017 event located the hypocenter at ∼108  km depth within the subducting slab. The best-fit relocation places the depth at 61 km, close to the slab interface. Aftershocks occur between 68 and 86 km depth and align along a steeper plane than slab geometry models. The January 2018 event is located at ∼46  km depth. Aftershocks form a near-vertical, pipe-like structure from the plate interface to ∼10  km depth. A burst of aftershocks immediately following the mainshock shows a shallowing upward trend at a rate of ∼2  km/hr, suggesting that a fluid pressure wave released from the oceanic crust is causing brittle failure in the overriding plate, followed by upward migration of fluids. Five months later, shallow (<25  km) seismicity collocates with background seismicity, suggesting the January 2018 event activated the Pelabuhan Ratu fault system close to the coast.

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On 18 November 2022, a strong earthquake occurred in the near-trench of Sunda Arc southwest of southern Sumatra, generating a small tsunami recorded at four tide gauge stations (KRUI, BINT, SBLT, and SIKA). Four seismological agencies (BMKG, GCMT, GFZ, and USGS) obtained nearly similar earthquake parameters and focal mechanisms from a seismic approach. It is situated near two major historical earthquakes that generated destructive tsunamis. One of those historical tsunamis, the 2010 Mentawai tsunami, was produced by a rare shallow and slow rupture earthquake with a higher tsunami impact than predicted from the seismic moment. It is related to the low rock rigidity of the source location. This study aims to understand the source characteristics of the 2022 event, which were probably influenced by the depth-varying rigidity. We examined those four source models using numerical tsunami modeling. We tested five distinct rigidity values, such as 10, 12.5, 15, 17.5, and 20 GPa, for each source model to obtain the best match of simulated and observed tsunami waveform. Waveform correlation coefficient and NRMSE are used as similarity indicators. The Mw 6.7 shallow source model with low rigidity (10 GPa) is the best model, as indicated by the correlation of ~0.74 and the lowest NRMSE. This solution is consistent with the long duration of the source time function of this event issued by IPGP. It is necessary to consider the appropriate rigidity characteristic in the tsunami hazard assessment since improper rigidity strongly affects the tsunami impact prediction in the coastal area.