The earthquake swarm events sequence occurred in west Halmahera, north Molucca, Indonesia for the period of October 2015 to February 2016 as reported by Meteorological, Climatological, and Geophysical Agency (BMKG) of Indonesia. There were tenths swarm events with Magnitude larger than four in the region during the period. In this study, we used the earthquake catalog data compiled by BMKG to improve the location of swarms event in west Halmahera, north Molucca, Indonesia. We relocated 86 swarm events by applying teleseismic double-difference method and 3D seismic velocity model. The focus depth of swarm events mainly concentrated at depth of 5 to 12 km at south-east of Jailolo volcano. Our preliminary interpretation the earthquake swarms may be related to the stress change around the deep magma region of the volcano.
Abstract This study’s research aim has been to clearly reveal the geometry of the Molucca Sea Plate subduction zone beneath Sulawesi’s north arm, the relationship between the subduction zone and volcanic activity in Halmahera, and analyze the depths of the upper arch of the double subduction of the Molucca Sea Plate. The use of travel time tomography made it possible to reconstruct subsurface images of the complex Molucca Sea Collision Zone at greater depths and in more detail. Local and regional arrival times of P- and S- wave data from the BMKG station network were used. The network consists of 32 stations; data was recorded from January 1, 2010 to December 31, 2017. Events with magnitude > 4 were used, comprising a total of 1,647 events. The same amount of P- and S-wave arrival times were repicked at each station; ~17,628 P-wave phases and ~17,628 S-wave phases were identified. Probabilistic non-linear location was used to determine the earthquake hypocenter along with the damped least square method with 3-D seismic velocity as the initial model beneath Indonesia for the travel time tomography inversion. The tomographic inversion results show that high Vp, high Vs, and low Vp/Vs beneath the Gorontalo Basin and Sulawesi’s north arm are associated with the Molucca Sea Plate, geometrically subducting to the north at a ~40° angle. Also observed were low-Vp, low-Vs, and high-Vp/Vs in the crust mantle beneath the northern and southern Halmahera Volcanic Arc (Bacan Island), which might be associated with a possible magma source. Volcanoes in the northern part have experienced eruptions, e.g., the Gamkonora and Ibu Volcanoes. Meanwhile, volcanos in the southern part are in a dormant state and their last eruptions are unknown. This latter group of volcanoes is clearly connected with partial melting (low-Vp, low-Vs, high-Vp/Vs) above the Molucca Sea Plate, which subducts to the east. The double subduction of the Molucca Sea Plate is shown by high-Vp, high-Vs, low-Vp/Vs which subduct westward beneath Sulawesi’s north arm and the Sangihe Islands, and subducts eastward beneath Halmahera Island. The tomographic image also shows that the Molucca Sea Plate in the north is sinking deeper (at a depth of ~150 km) compared to the south (at a depth of ~50 km). Anomalies of low-Vp, low-Vs, and high-Vp/Vs in the crust mantle beneath the Central Ridge of the Molucca Sea indicate highly fractured and water-saturated rock material; while in the west and east, beneath the Sangihe Trench (ST) and Halmahera Trench (HT), the anomaly of high-Vp, high-Vs and low-Vp/Vs indicate rock material that experienced micro-cracks that have since been closed due to high pressure. Further north, the highly fractured and water-saturated rock material shifted eastward, extending to the HT with a clear boundary in the Central Molucca Sea.
We have successfully conducted the first ambient noise tomography on the island of Lombok, Indonesia using local waveform data observed at 20 temporary stations. Ambient noise tomography was used to delineate the seismic velocity structure in the upper crust. The waveform data were recorded from August 3rd to September 9th, 2018, using short-period and broadband sensors. There are 185 Rayleigh waves retrieved from cross-correlating the vertical components of the seismograms. We used frequency-time analysis (FTAN) to acquire the interstation group velocity from the dispersion curves. Group velocity was obtained for the period range of 1 s to 6 s. The group velocity maps were generated using the subspace inversion method and Fast Marching Method (FMM) to trace ray-paths of the surface waves through a heterogeneous medium. To extract the shear wave velocity (Vs) from the Rayleigh wave group velocity maps, we utilize the Neighborhood Algorithm (NA) method. The 2-D tomographic maps provide good resolution in the center and eastern parts of Lombok. The tomograms show prominent features with a low shear velocity that appears up to 4 km depth beneath Rinjani Volcano, Northern Lombok, and Eastern Lombok. We suggest these low velocity anomalies are associated with Quaternary volcanic products, including the Holocene pyroclastic deposits of Samalas Volcano (the ancient Rinjani Volcano) which erupted in 1257. The northeast of Rinjani Volcano is characterized by higher Vs, and we suggest this may be due to the presence of igneous intrusive rock at depth.
An update version of 3-D Shear Wave Velocity (Vs) & Crustal Interface (Sediment Basement & Moho) Models of Borneo, Makassar Strait and Sulawesi region obtained from group velocity tomography. Group velocity is retrieved from dispersion analysis of Rayleigh waves extracted from the ambient noise field by cross-correlating long-term recordings from 108 seismic stations over a period of 8 months. A 3-D shear wave velocity model then are produced via a two-stage process in which group velocity maps are computed across a range of periods and then sampled over a dense grid of points to produce pseudo-dispersion curves; these dispersion curves are then separately inverted for 1-D shear wave velocity (Vs), with the resultant models combined and interpolated to form a 3-D model.
Abstract In 2006, the Special Region of Yogyakarta was shaken by a destructive earthquake. The United States Geological Survey (USGS) recorded that the earthquake had a magnitude of 6.3 M w at a depth of 12.5 km below the surface that was triggered by strike slip fault activity. The severe damage occurred in Bantul district and Klaten district, where the Klaten area location were far from the earthquake epicenter. This proves that the magnitude and the earthquake source distance are not the only parameters for seismic hazard potential, but rather the presence of local site effects and building conditions. The present study aims to determine the characteristics of the soil and estimate the seismic hazard potential by using HVSR method. Horizontal to Vertical Spectral Ratio (HVSR) is one method that can be used to obtain the subsurface information from single station measurements. Furthermore, the inversion of Rayleigh wave ellipticity curve is used to obtain 1-D of shear wave velocity (Vs). The parameters from HVSR calculation and Rayleigh wave inversion were mapped to understand the subsurface structure beneath the Yogyakarta area. Based on the soil classification to Vs, Yogyakarta area is categorized to SD (Stiff soil/soft soil) and SC (Very dense soil and soft rock). The results of the mapping analysis indicated that Bantul district is an area with the highest potential of seismic hazard.
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.
Abstract The Lembang Fault is located about eight km north of Bandung City, the capital city of the West Java province of Indonesia, with about 8.79 million people (dated 2020). Several studies state that the Lembang fault is an active fault with a maximum magnitude of 6.5 to 7. In terms of earthquake disaster mitigation, distinguishing characteristics of seismic velocity structures around active faults is critical. This manuscript presents our new research on ambient seismic noise tomography around the Lembang Fault by deploying 73 local temporary seismometer networks to develop a 3D high-resolution shear wave velocity model. We conducted a design acquisition analysis to obtain optimal results using a ray density analysis approach to image the geometry of the Lembang fault with good illumination and resolution, which is essential for identifying shallow geological features. Our result showed that variations in velocity values, both laterally and vertically, were related to variations in the volcanic rock on the near-surface. The geometry of the Lembang Fault and the existence of a low-velocity zone are well-identified from the seismic velocity structure, which low-velocity zone area has the potential for amplifying earthquake waves during an earthquake.
Summary In this paper, we compare two different methods for group velocity inversion: iterative, least-squares subspace optimization, and probabilistic sampling based on the Trans-dimensional Bayesian method with tree-based wavelet parameterization. The wavelet parameterization used a hierarchical prior for wavelet coefficients which could adapt to the data. We applied these inversion methods for ambient noise tomography of the western part of Java, Indonesia. This area is an area prone to multiple geological hazards due to its proximity to the subduction of the Australia Plate beneath Eurasia. It is therefore important to have a better understanding of upper crustal structure to support seismic hazard and disaster mitigation efforts in this area. We utilized a new waveform dataset collected from 85 temporary seismometers deployed during 2016–2018. Cross-correlation of the waveform data was applied to retrieve empirical Rayleigh wave Green's functions between station pairs, and the spatial distribution of group velocity was obtained by inverting dispersion curves. Our results show that, although computationally expensive, the Trans-dimensional Bayesian approach offered important advantages over optimization, including more effective explorative of the model space and more robust characterization of the spatial pattern of Rayleigh wave group velocity. Meanwhile, the iterative, least-square subspace optimization suffered from the subjectivity of choice for reference velocity model and regularization parameter values. Our Rayleigh wave group velocity results show that for short (1–10 s) periods group velocity correlates well with surface geology, and for longer periods (13–25 s) it correlates with centers of volcanic activity.
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