Faiz Muttaqy , Andri Dian Nugraha, Nanang T. Puspito, David P. Sahara, Zulfakriza Zulfakriza, Supriyanto Rohadi, Pepen Supendi

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 to 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.