Shindy Rosalia

This study offers a new approach to probabilistic earthquake hazard assessment (PEHA) in the densely populated regions of Southern Sumatra and West Java, Indonesia. While much attention is given to powerful, offshore megathrust earthquakes, this research focuses on a different yet equally dangerous threat: shallow, moderate-magnitude earthquakes (4.5 ≤ M_w ≤ 6.5) that occur on land. These events, often caused by unmapped faults, pose a significant risk due to their proximity to major cities and infrastructure. To develop a more reliable model, a best-fit earthquake rate model was estimated using declustered shallow earthquake events as a reference. This model enhances existing methods by offering a more precise depiction of where these shallow, damaging earthquakes are likely to occur. We accomplished this by analyzing a comprehensive probability of exceedance (PoE) of earthquakes with magnitudes up to 6.5 and depths up to 50 km that occurred between 1963 and 2022, mapping and modeling both the known active faults and the historical seismic activity in the region, and using advanced statistical methods to create a highly reliable, integrated seismicity rate model. The final product, the Integrated Most Reliable Spatial Seismicity Rate Model (Model_IMRSSR), is proposed as a useful tool for government authorities and urban planners. It can be used to create detailed seismic hazard maps that highlight areas of highest risk, especially those with unmapped faults. By guiding development away from these high-risk zones and identifying specific locations for physical reinforcement, this research provides a framework for sustainable investment. The proactive use of these findings can lead to more resilient communities and a significant reduction in potential damage and loss of life from future earthquakes.

The Molucca Sea Plate, and Sangihe and Halmahera plates have a complex tectonic setting and interact to create the Molucca Sea Collision Zone. We re-picked 1647 events recorded from 2010 to 2017 from 32 of The Agency for Meteorology, Climatology, and Geophysics (BMKG) stations and obtained P- and S-arrivals of ~17,628 phases. Hypocenter locations were determined using the software NonLinLoc. Then, we performed a travel time tomography in order to image the subsurface and approximate the Molucca Sea Plate subduction angle beneath Sulawesi’s north arm, the relationship subduction zone and volcanic activity in Halmahera, and depth comparison of the Molucca Sea Plate. Our results show that (i) high Vp, high Vs, and low Vp/Vs are associated with the Molucca Sea Plate beneath Sulawesi’s north arm, and the approximate subduction angle is ~40°. (ii) Low Vp, low Vs, and high Vp/Vs beneath the northern and southern Halmahera Volcanic Arc are associated with a possible magma source. Volcanoes in the north have experienced eruptions and are dormant in the south. This group of volcanoes is connected by partial melting above the Molucca Sea Plate subducts to the east. (iii) High Vp, high Vs, and low Vp/Vs are interpreted as double subduction of the Molucca Sea Plate. It is submerged deeper in the north compared with the south, which is nearer to the surface.

This paper presents the depth inversion of Rayleigh wave group velocity to obtain an S-wave velocity model from seismic ambient noise cross-correlation in western Java, Indonesia. This study utilizes the vertical component data of a temporary seismograph network deployed in 2016, which was used in a previous study to estimate fundamental mode Rayleigh wave group velocity maps. In this study, the Neighborhood Algorithm was applied to invert the Rayleigh wave group velocities into 1D shear-wave velocity (Vs) profiles, which were then interpolated to produce a high-resolution, pseudo-3D Vs model. These tomographic images of Vs extend to ~ 20 km depth and show a pronounced NE-SW contrast of low and high Vs in the depth range 1–5 km that correlates well with the Bouguer anomaly map. We interpret the low Vs in the northeastern part of the study area as associated with alluvial and volcanic products from the Sunda Shelf and modern volcanic arc, whereas the high Vs in the southwestern part is associated with volcanic arc products from earlier episodes of subduction. We also obtained the depth of the northern Java Basin, which is in the range of 5–6 km, and the Garut Basin, which extends to 5 km depth. For greater depths, Vs gradually increases throughout western Java, which reflects the crystalline basement. This study provides estimates of the shallow crustal Vs structure underneath West Java with higher resolution than previous tomographic studies, which could be useful for supporting future earthquake studies in the region.

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.

The island of Ambon lies on complex tectonics, part of Banda Arc which is driven by the Australia – Eurasia collision. Historical earthquake data show that an earthquake resulting the greatest tsunami in Indonesia had occurred at Ambon Island. On 26 September 2019, Ambon was shaken by an M 6.5 earthquake at a depth of 10 km (BMKG). In this study, we use ambient noise data from 11 temporary stations deployed by ITB and 4 permanent stations owned BMKG which are recorded from October until December 2019. Here, we purely use the vertical component of seismogram to retrieve the Empirical Green’s Function of Rayleigh waves. Cross-correlations were obtained from the daily data series and stacked the day-by-day cross-correlation data into one inter-station cross-correlation. The Empirical Green’s Function is seen at the band period 1-15 s. As a part of our study, we analyze the Green’s Function with frequency-time analysis (FTAN) to get Rayleigh wave group velocity. The group velocity of Rayleigh waves varies from 1.04 km/s – 3.75 km/s. Low group velocity might be indicated the presence of sediment or volcanic deposits and high group velocity might be indicated metamorphic rocks. The result of this study might give a finer velocity model of the shallow crustal beneath Ambon Island and the surrounding area.

This paper presents the depth inversion of Rayleigh wave group velocity to obtain an S-wave velocity model from seismic ambient noise cross-correlation in the western part of Java, Indonesia. This study utilizes the vertical component data of a temporary seismograph network deployed in 2016, which was used in a previous study to estimate fundamental mode Rayleigh wave group velocity maps. In this study, the Neighbourhood Algorithm was applied to invert the Rayleigh wave group velocities into 1D shear-wave velocity (Vs) profiles, which were then interpolated to produce a high-resolution, pseudo-3D Vs model. These tomographic images of Vs extend to ~20 km depth and show a pronounced NE-SW contrast of low and high Vs in the depth range 1-5 km that correlates well with the Bouguer anomaly map. We interpret the low Vs in the northeastern part of the study area as associated with alluvial and volcanic products from the Sunda Shelf and modern volcanic arc, whereas the high Vs in the southwestern part is associated with volcanic arc products from earlier episodes of subduction. We also obtained the depth of the northern Java basin, which is in the range of 5-7 km, and the Garut Basin, which extends to 5 km depth. For greater depths, Vs gradually increases throughout western Java, which reflects the crystalline basement.

We develop and present a 3-D seismic velocity model of the source region of the 2018 Lombok, Indonesia earthquakes by using local earthquake tomography. The data consist of 28 728 P- and 20 713 S-wave arrival times from 3259 events which were recorded by 20 local seismic stations. The results show that most of the significant earthquakes occur to the edge of high-velocity regions. We interpret these to represent coherent blocks of the Flores Oceanic Crust underthrusting Lombok. At depths shallower than the nucleation area of the largest earthquake, many triggered aftershocks are located within a low-velocity, high-Vp/Vs region which is probably a highly fractured fault zone with a large amount of fluid. This fault zone is parallel to the dip of the Flores Back Arc Thrust and probably ruptured during this earthquake sequence. A prominent low-velocity, high-Vp/Vs region is colocated with the northwest and southern flank of the Rinjani volcanic complex. This large aseismic region is probably related to a wide area of the crust containing fluids due to ongoing magma intrusion beneath the volcano. To the east of Rinjani Volcano a cooled intrusive complex was imaged. It is characterized by high-velocity and low-Vp/Vs, supported by the presence of a high Bouguer anomaly. We confirm the existence of the Sumbawa Strait Strike-Slip Fault and find it is characterized by an elongated low-velocity, high-Vp/Vs zone.

Local seismic tomography is a well-known and commonly used method for obtaining detailed information about the internal structure of volcanoes. The eruption of Mt. Agung in 2017 was a vital opportunity scientifically because it is the first eruption that had sufficient seismic observation networks to carry out local seismic tomography at this volcano. In this study, we investigate the subsurface structure of Mt. Agung in Bali, which is one of the highest risk volcanoes in Indonesia. We conducted travel-time tomography using P- and S-wave arrival times of volcano-tectonic (VT) events to determine the three-dimensional (3D) Vp, Vs, and Vp/Vs ratio structure beneath Mt. Agung. We used 1,926 VT events, with corresponding 9482-P and 8683-S wave arrival times recorded by eight seismic stations over an observation time spanning from October 18 to December 31, 2017. We obtain the hypocenter solution for VT events using the maximum likelihood estimation algorithm and use an optimum 1D velocity model as input for the Joint 3-D seismic tomographic inversion. Local earthquake tomography revealed five anomalous regions that are useful to describe the overall seismic activity around Mt. Agung. We interpret these anomalous regions qualitatively due to limited data resolution in this study. We have successfully localized a high Vp/Vs ratio (∼1.82), low Vs (−1.9%) and high Vp (+3.8%), within a low seismicity zone at depths between 2 and 5 km below the Mt. Agung summit, which may be related to a shallow magma reservoir. There is also an anomalous region between Mt. Agung and Batur with moderate to high Vp/Vs ratios (1.76–1.79) where most of the earthquakes recorded before the 2017 eruption originated. We interpret this anomaly to be related to the existence of sub-vertical dyke complex at depths between 8 and 14 km. The results of our study provide new insights into the subsurface structure of the magma plumbing system beneath Mt. Agung, which can be used to improve the quality of determining the location of the hypocenter and source modeling for future eruption forecasting.

SUMMARY: High seismicity rates in eastern Indonesia occur due to the complex interaction of several tectonic plates which resulted in two deadly, destructive earthquake sequences that occurred in Lombok Island and the city of Palu, Sulawesi in 2018. The first sequence began in July with an Mw 6.4 event near Lombok, culminating in an Mw 7.0 event 8 d later. This was then followed by a nearby Mw 6.9 event 12 d later. Approximately 1000 km to the northeast, a separate sequence began several weeks later near Palu where an Mw 7.5 event occurred that triggered a tsunami. In this study, we present hypocentre relocations for both earthquake sequences as well as all other regional earthquakes in eastern Indonesia. The relocations were performed using a teleseismic double-difference relocation method and arrival times for P and S waves from stations at local, regional, and teleseismic distances. The catalogue and phase data were taken from the Agency for Meteorology, Climatology and Geophysics (BMKG) of Indonesia and the International Seismological Centre (ISC) for the period of April 2009 through November 2018. The relocated catalogue provides an improved view of seismicity in eastern Indonesia over the study period, sharpening locations and interpretations of seismogenic features throughout the region. In the Lombok area, the relocated earthquakes clearly show a backarc thrust to the north of the Sunda-Banda Arc transition zone. The relocated aftershocks show that the destructive Mw 7.0 and Mw 6.9 earthquakes of the Lombok sequence ruptured two different regions: The Mw 7.0 earthquake propagated westward, whereas the Mw 6.9 earthquake propagated eastward. The entire sequence of Lombok earthquakes was most likely started by the Mw 6.4 event as the initial event or foreshock, which then triggered backarc thrusts on both sides. Several weeks later and far to the northeast, the Mw 7.5 Palu earthquake occurred along the Palu-Koro Fault, filling a seismic gap that had not ruptured in an Mw 6.0 event or larger since at least 1900. The distribution of aftershocks indicates that the northern part of the Palu-Koro Fault has lower relative seismicity rates than the southern part at shallow depths, and that off fault aftershocks are mostly located to the east of the Palu-Koro Fault.