CHARACTERIZATION OF THE MAMASA EARTHQUAKE SOURCE BASED ON HYPOCENTER RELOCATION AND GRAVITY DERIVATIVE DATA ANALYSIS

Mamasa, located in West Sulawesi, Indonesia, is a populous dense area with a low seismic hazard. In this area, the question of fault reactivation was raised due to the 804 reported earthquakes from November 2018 to February 2019. This series does not show an apparently major earthquake or behaviors, such as swarms. One month earlier, on 28 September 2018, the 7.6 Mw Palu earthquake occurred at a distance of approximately 230 km north of Mamasa. It affected the occurrence of this swarm since Mamasa has many hydrothermal manifestations, which have been linked with seismic activity. To investigate the roles of these two factors, the series of earthquakes and their possible linked faults must be characterized. Specifically, the double-difference hypocenter relocation method was used to relocate the earthquakes and conduct a spatiotemporal analysis of this swarm. Furthermore, the FHD (first horizontal derivative) gravity data were used to illuminate the possible linked fault. To assess the impact of the Palu earthquake, the Coulomb stress was calculated, and it was reported that it was very small. Therefore, it played an insignificant role in triggering the swarm. In contrast, the relocated earthquakes are concentrated in an area surrounded by surface hydrothermal manifestations, and the swarm feature is consistent with the interpretation of associated seismicity. Therefore, hydrothermal activities play a critical role in triggering the swarm.