Java Island is highly active tectonic zone and also has a complex geological system as a result of Indo-Australian plate that subducted under the Eurasian plate. This complexity produces many large and destructive earthquakes. Moreover, Java is a most densely populated region in Indonesia. The precise earthquake location can define and evaluate the seismic hazard in the area. In this study, we determined hypocenter location around the central and east Java region by using a non-linear method. We manually picked P-and S-wave arrival time recorded by BMKG network for the time period of 2009 to 2017. We used the minimum 1D seismic velocity model of ak135 to locate the events. We used some criteria for the determination of event location including (i) at least recorded by 4 station which has clear onset P and S arrival, and (ii) has magnitude (MW) > 3. The seismicity in the study area is controlled by subduction and many active faults distributed along the Java Island, also clustered in several areas such as Kendeng thrust, Opak river fault, Kebumen, Banyuwangi, and others. The result of this study indicates the accurately main shallow seismicity zones in central and eastern part of Java region, and also confirm the presence of active inland faults. As further studies, we will conduct 1D velocity modeling and relocate the hypocenters using an updated local 1D velocity model beneath the study area.
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.
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.
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