Semarang Fault is an active thrust fault with documented historical earthquake (e.g. 1867, 1856). The NE-SW oriented fault zone is located at the northern coast of Central Java, near the city of Semarang. While posing direct seismic hazard to the ~1.5 million population of Semarang City, the geometry and plane configuration of this fault is not well-constrained. This study presents the first attempt to resolve the fault geometry and estimate the location of the active fault trace by examining their late Quaternary deformation. We combined remotely sensed- and field-based surface geology and geomorphological mapping methods using various dataset such as the 9-m resolution DEMNAS, cm-resolution UAV-generated DEM and orthophoto mosaic as our base maps. Our study indicates that the Semarang fault is dominated by thrusting with minor horizontal right lateral movement with varying dip from shallow (20-30°) to moderate (30-40°) along the strike. Based on our study, we estimate the entire length of active trace of the fault zone to be ~60 km, which segmented into smaller (25-30 km length) faults. Applying empirical relation for surface rupture length and magnitude (Wesnousky, 2008), we estimate a potential for an earthquake magnitude of ~M 6.8 along the maximum length of the identified fault segment (i.e. 30 km). This new understanding about the Semarang fault geometry and its earthquake potential will contribute toward the refinement of the seismic hazard assessment of Semarang and the surrounding region.
