The Indonesian region is very prone to earthquakes. Many destructive earthquakes occurred in this region. Quick determination of fault planes will be helpful to estimate damages in the areas. We conducted retrospective study by relocating six M ≥ 6.0 earthquakes since June 2013 until January 2014 using the Modified Joint Hypocenter Determination (MJHD) method to understand the characteristic of the earthquakes and to get good relocation parameters to be implemented in the future large earthquakes. We did relocation using P wave and both P and S waves. We compared focal mechanisms of Global CMT, BMKG, and SWIFT – NIED. We could determine the fault planes for two South of Java earthquakes (M 6.8 and M 6.2), and Northern Sumatra earthquake (M 6.2) using aftershocks which occurred within 24 hours. However, we could not determine the fault planes for two Banda Sea earthquakes (M 6.4 and M 6.4) due to limitations of their aftershocks and for the Southern Sumatra earthquake (M 6.1) because of the aftershocks did not distributed along one of the nodal planes.
Furthermore, we studied recent seismicity in the Aceh segment of the Sumatran Fault in northwestern Sumatra where a seismic gap was proposed and the seismicity increased in 2012. We conducted the relocation for the 1st January 2012 earthquake (M 5.2) and its aftershock, the group of earthquakes in the central part of the segment, and the 21st January 2013 earthquake (M 5.9) and its aftershocks. We confirmed the fault planes for the M 5.2 and M 5.9 earthquakes were in the same direction as the segment.
At last, we relocated all the earthquakes in Aceh segment since January to October 2012 together with the group of the 21st January 2013 earthquakes in order to know relative locations of all groups of the earthquakes. We found that earthquakes in the central Aceh segment were very shallow and correlated with the creeping zone in the upper crust, while earthquakes in the SE segment occurred at the mid crust and correlated with the locking zone in this area.