Back-arc thrusting in the Jakarta basin

The Java subduction megathrust is undoubtedly the source of high magnitude, extremely damaging earthquakes. In the back-arc of the subduction zone, severe earthquakes also affect the northern part of Java. The Jakarta basin lies at the western end of the Java back-arc thrust, which stems on the seismogenic Flores thrust in the east and propagates westward across Java. The tectonic activity of the Java Back-arc Thrust in the Jakarta basin has been overlooked because of its low recurrence time. Yet, historical records reveal that it was destructive, resulting in severe destruction in Bogor and Jakarta. Tracking fault activity in large cities is problematic because the original landscape is often profoundly anthropized and has little to do with its pre-industrial physiography. In the Jakarta basin, this is even more complex owing to the fast Plio-Quaternary sedimentation that conceals the morphotectonic features associated with the fault. We combine geomorphic observations and subsurface data using DEMs and optical imagery, seismic reflection and biostratigraphic well data. At depth, seismic data reveal a partitioned fault network of compressive fault-propagation folds and transpressive flower structures that deform the Plio-Quaternary sedimentary layers of the Jakarta basin and interplay with volcanoes. At the surface, morphological observations in the rims of the basin reveal that several river meanders were abandoned and uplifted hundreds of meters above the current riverbeds above the fault network. In the basin, multiple meter scale waterfalls that we interpret as knickpoints above active faults scar the flat surface of the basin. We conclude that the western end of the Java back-arc thrust fault bears a potentially high risk for the infrastructures of the densely populated province of Jakarta.