Evidence of the West Java back-arc thrust from earthquake activity

In this study, a borehole seismic experiment was designed to investigate seismic activity associated with the West Java back-arc thrust. This passive source experiment was composed of seven borehole seismometers that extended across the Subang region and surrounding areas. The experiment recorded seismic data between December 2022 and September 2023 and was supplemented by data from the Agency for Meteorology, Climatology, and Geophysics of Indonesia (BMKG) permanent seismic station network. During the recording period of the temporary array, we identified and located 15 shallow crustal earthquakes and computed their focal mechanisms. The occurrence of thrust-type earthquakes in the back-arc region during this period offers evidence of continued tectonic activity along the West Java back-arc thrust. Our integrated analysis, which incorporates geological studies and previous research, reveals a complex fault network comprising the Citarum Front Fault, Citarum Fault, and Baribis Fault system, indicating significant horizontal and vertical changes in the regional stress field. Furthermore, we identify a previously unrecognized fault, which we call the Cirata Fault, which is characterised by earthquakes with thrust mechanisms. Intriguingly, we observe normal faulting at the eastern end of the Lembang Fault, suggesting reactivation of old fault structures associated with the ancient Sunda Caldera. These findings have significant implications for seismic hazard assessment and disaster preparedness in the region, and highlights the importance of continuous monitoring and collaborative efforts among geologists, seismologists, urban planners, and policymakers.