Andri Dian Nugraha

On 25 February 2022, a destructive Mw 6.1 earthquake struck western Sumatra in the early hours of the morning. The earthquake shaking was focused around the Pasaman Regency and resulted in 25 fatalities. Coseismic displacements were recorded by an existing network of Global Navigation Satellite System (GNSS) sensors managed by the Geospatial Information Agency of Indonesia. The GNSS data show a maximum of 2.0 cm of displacement with a right-lateral sense of motion across a northwest–southeast-trending fault. An investigation into the responsible fault producing the earthquake is performed through a series of coseismic slip inversions on a rectangular fault plane, comparing two fault models: (a) a fault segment that is an extension of the already identified Sianok segment of the Sumatran fault zone, and (b) the recently identified Talamau segment. The maximum slip of 14 cm occurred at 4 km depth, 13 km away from the epicenter. The mainshock is associated to a seismic moment of 1.58×1025 N·m, or equivalent to Mw 6.1. Our results indicate increased stress along the Sumpur segment and the northern Sianok segment. With no historical earthquakes reported along the Sumpur and northern Sianok segments since 1892, our analysis using a scaling relationship between fault length and earthquake magnitude indicates that a rupture in these areas could potentially generate an earthquake with a magnitude of 6.7.

We present a new 3‐D P‐wave velocity model of the crust and upper mantle beneath Sulawesi from regional earthquake body wave tomography. The arrival time data were sourced from the Agency for Meteorology, Climatology, and Geophysics (BMKG) Indonesian earthquake catalog for the period 2018 through to 2023, which is based on records from 126 seismic stations in Sulawesi and its neighborhood. Our results reveal clear evidence of high velocity subducting slabs beneath the Molucca Sea Collision Zone and northern Sulawesi, along with low velocity zones above the Sangihe and Halmahera slabs, that likely represent magma reservoirs. Furthermore, we found a diffuse low‐velocity anomaly in the mantle beneath Colo volcano, a solitary island volcano located in the distant back‐arc region of the North Sulawesi subduction zone, which is consistent with decompression melting induced by extension.

The Central and East Java region, which is part of the Sunda Arc, has relatively high seismic rates due to the convergence of two major tectonic plates in the Indonesian region; i.e., the Indo-Australian Plate subducting under the Eurasian Plate. Many devastating earthquakes have occurred in this area as a result of the interaction between these two plates. Two examples are the 1994 Banyuwangi earthquake (Mw 7.6) and the 2006 Yogyakarta earthquake (Mw 6.3). This study aims to determine precise earthquake locations and analyze the pattern of seismic distribution in Central and East Java, Indonesia. We manually re-picked P and S-wave arrival times that were recorded by the Agency for Meteorology, Climatology and Geophysics (BMKG) of the Indonesian earthquake network during the time period January 2009–September 2017. We then determined the earthquake locations using a non-linear method. To improve the accuracy of the earthquake locations, we relocated 1,127 out of 1,529 events, using a double-difference algorithm with waveform cross-correlation data. Overall, the seismicity in the Central and East Java region is predominantly distributed in the south of Java Island; e.g., the Kebumen, Yogyakarta, Pacitan, Malang, and Banyuwangi clusters. These clusters are probably related to the subduction activity in these regions. Meanwhile, there are clusters of earthquakes having shallow depths on the mainland that indicate the activity of inland faults in the region; e.g., the Opak Fault, the Kendeng Thrust, and the Rembang–Madura–Kangean–Sakala (RMKS) Fault Zone. Several other active inland faults have not shown any significant seismicity over the time period mentioned, i.e., the Pasuruan Fault, the Lasem Fault, the Muria Fault, the Semarang Thrust, and the Probolinggo Fault.

The Central and East Java region, which is part of the Sunda Arc, has an important role in producing destructive earthquakes and volcanic complexes in Indonesia as a result of the convergence between the Indo-Australian plate that subducts under the Eurasian plate. In this study, the 3-D seismic velocity structure (Vp, Vs, and Vp/Vs) of the crust and upper mantle was determined to reveal the presence of the subducted slab, volcanic sources, and seismogenic features in the Central and East Java region. We have manually re-picked P- and S-arrivals of 1,488 events from January 2009 to September 2017 recorded at 27 stations of the BMKG network. An iterative damped least-squares inversion method was applied to simultaneously calculate both hypocenter relocations and velocity structure beneath this high-risk region to a depth of 200 km. We then compare the tomographic results and seismicity to interpret structural features in the seismic zones. The subducted slab is dipping toward the north, imaged by the high-velocity regions with low Vp/Vs at depths of about 50–100 km. Low-velocity anomalies with high Vp/Vs above the slab at a depth of ∼100 km, imply the possible location of partial melting from slab dehydration. Fluids and melts are ascending to feed the volcanoes i. e., Merapi-Merbabu, Wilis, Pandan, Semeru, Bromo, and Ijen that also have similar low-velocity anomalies at 10–30 km depths, suggesting the presence of the sedimentary basin or magma reservoir. We also have redetermined the hypocenter location of the 2021 (Mw 6.1) Malang earthquake at 8.94^oS, 112.45^oE, with a depth of 59.7 km. The location error in the x, y, and z directions are 3.08, 6.39, and 11.91 km, respectively. This intraslab event with a thrusting mechanism is located in the high-velocity region and close to the intermediate-depth seismic clusters, which indicates the geometry of the oceanic slab. In the region of 1994 (Mw 7.8) Banyuwangi earthquake, we found a low-velocity anomaly at ∼ 50 km depth that might be associated with the presence of subducting seamount that is more hydrated than the surrounding slab. The slip over this subducting seamount caused the tsunamigenic earthquake.

Molucca Sea collision zone is a region which has very complex geology and tectonic setting, producing high seismicity and volcanoes activities. In this study, we have determined hypocenter location around the region using local & regional network of Agency of Meteorology, Climatology, and Geophysics, Indonesia (BMKG). We used 1,647 events that recorded by 32 seismic stations. We repicked the P-and S-phase manually and have been succesfully determined ~17,628 P and ~17,628 S arrival times. The P- and S-arrival times are used to determine the hypocenter location by applying NonLinLoc method which estimating the probability density function (PDF) using the oct-tree importance sampling algorithm. Our preliminary results show that the seismicity beneath the Molucca Sea collision zone forming a double subduction pattern which is dipping westward under the Sangihe Arc, reaching a depth of ~ 600 km and eastward under the Halmahera Arc, reaching a depth of ~ 250 km. The seismicity pattern under the Sangihe Arc deepens to the north and the deep earthquake events increase in number. The seismicity is related to the Molucca Sea Plate which is dipping into west and east direction beneath Sangihe-Halmahera Arc. To have a further understanding of the complex tectonic activity in this area, our future work will focus on conducting a seismic tomographic inversion to determine the 3D seismic velocities structure around the Molucca Sea collision zone.