Active tectonics of the Yogyakarta area (Central Java, Indonesia): preliminary findings obtained from a tectonic-geomorphic evaluation

Throughout the island of Java, several shallow faults located in the vicinity of densely populated areas accommodate the tectonic strain generated by the subduction of the Indo-Australian Plate beneath the Sunda Plate. These faults are characterized by a poor surface geomorphological expression due to the tropical climate that masks paleoseismological evidence making the identification of active faults in this environment challenging. We present preliminary data of a tectonic-geomorphic study that aims to identify and characterize active faults in the province of Yogyakarta (Central Java) as a basis to improve seismic hazard assessment. Here we focus on the Opak fault by describing its geomorphology and structure, using field data and remote sensing-based observations to contribute to the understanding of its geometry, kinematics, and tectonic activity. We show preliminary data supporting the Opak Fault as an active SW-NE transtensional left-lateral strike-slip reactivated normal fault consisting of several parallel fault strands. Although geomorphological expression of active tectonics is poor and diffuse, we found numerous field evidence of active tectonics ranging from tilted Quaternary fluvial terrace risers, triangular facets, and linear valleys to peculiar drainage patterns that allow us to provide evidence of the Holocene activity of the Opak Fault.