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A destructive earthquake (Mw 5.6) struck Cianjur, West Java, Indonesia, on 21 November 2022, resulting in at least 321 deaths, damage to 47,000 buildings, and economic losses of up to 7.7 trillion Indonesian Rupiahs (∼US $546 million). The causative fault that generated this earthquake was not previously recognised, therefore making further analysis crucial for assessing future seismic hazard in the region. In this study, we undertake automated event detection and location using QuakeMigrate from 10 days before to 23 days after the mainshock, prior to relocation using a double-difference method. We also determine the source mechanism for selected aftershocks from waveform inversion. Our result show that the mainshock was preceded by three clear foreshocks, and intriguingly the aftershocks appear to reveal the presence of a conjugate fault pair trending northwest-southeast with a length of ~8 km and southwest-northeast with a length of ~5 km.

The Southeast Asia (SEA) region is tectonically very active as it accommodates the northward movement of the Indo-Australian plate in the south and the westward movement of the Philippine Sea plate in the east. Borneo and Sulawesi are located in the centre of SEA, which is our area of interest. Borneo has an intraplate setting, while Sulawesi is situated above several microplate boundaries. For that reason, Sulawesi is seismically and volcanically more active than Borneo. The tectonic link and evolution between the two islands are not well understood as we are missing some fundamental knowledge, such as the variations in their crustal thickness and structure. This includes the provenance of their respective lithosphere, which may have Eurasian and/or East Gondwana origin.Here, we show the results obtained from the receiver function (RF) study on seismic stations in the region to have a better understanding of the crust and mantle lithosphere beneath the two islands. The RF study includes H-k stacking, time-depth migration of the RF and inversion to estimate crustal thickness and the shear speed variation with depth. The finding from this study shows that the crust in Sulawesi is much more complex than that of Borneo. The crustal thickness gradually changes throughout Borneo, with northern Borneo having an overall thicker crust than other parts of the island. In Sulawesi, the crustal thickness is much more varied across small distances, especially along the northern and southern arms of the island.We also show some results from the Virtual Deep Seismic Sounding (VDSS) method, which we only applied to the seismic stations in northern Borneo. We used VDSS on Northern Borneo to learn more about its complex tectonic history, such as the two subduction episodes and a continent-continent collision in a recent geological time scale. Our finding reveals a band of alternating thick and thin crust striking NE-SW in this region, which we believed resulted from extensional tectonics related to the Sulu Sea basin opening in the Miocene.

On December 14, 2021, the Mw 7.3 Flores Sea earthquake occurred approximately 100 km to the north of Flores Island, one of the most complex tectonic settings in Indonesia. The existence of the causative fault that generated this earthquake was not been previously known, therefore making further analysis crucial for assessing future seismic hazard in the region. In this study, we relocated the hypocenter of the mainshock and aftershocks using a double-difference method, determine focal mechanisms using waveform inversion, and then analyse stress changes to estimate the fault type and stress transfer caused by this earthquake. 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 N288oW and dip 78o) that occurred on a West-East trending fault which we call the Kalaotoa Fault, while rupture of the other two segments located to the west and east of the mainshock (WSW-ESE directions, respectively) may have been triggered by this earthquake. The Coulomb stress change of the mainshock shows that areas to the northwest and southeast experienced an increase in stress, which is consistent with the observed aftershock pattern.

The Central and East Java region is part of the Sunda Arc which has an important role in producing destructive earthquakes and volcanic complexes as a result of the subduction of the Indo-Australian plate under the Eurasian plate. Seismic tomography is one geophysical tool that is adaptable to understanding the mechanism process related to tectonic activity, seismicity, and volcanism. We collected a series of waveforms from 1,519 events in the period January 2009 to September 2017 and re-picked 11,192 phases for P- and S-waves at 34 stations of the BMKG network. We determined the 3-D P- and S-wave velocity structure beneath this high-risk region down to a depth of 200 km. In this study, we compare the tomographic images and relocated seismicity in order to represent the subducted slab geometry and the features in the seismic zones, i.e. the 2006 Yogyakarta earthquake zone (Opak fault), south of the mainland, and the 1994 Banyuwangi earthquake zone. Low-velocity anomalies beneath the volcanoes, i.e. Merapi, Merbabu, Kelud, Semeru, Bromo, and Ijen also imply the existence of fluid material and possible partial melting of the upper mantle which migrated from the subducted slab.