Source characterization of Intermediate-Depth earthquakes in southern Java, Indonesia

The rupture behavior of intermediate-depth earthquakes in southern Java remains poorly understood despite their potential seismic hazard. In this study, we performed finite-fault inversions to investigate the rupture processes and source characteristics of five intermediate-depth earthquakes (60–300 km depth) with moment magnitudes (M_W) ≥ 6.1 from 1998 to 2017 in the southern region of Java and its surrounding areas. Utilizing teleseismic body waves and surface waves, we employed a wavelet-based seismic inversion technique. Initially, we conducted preliminary inversions of the focal mechanisms (strike and dip) from the Global Centroid Moment Tensor (GCMT) database to determine the optimal fault plane orientation for slip distributions and source time functions (STFs). Our findings reveal that most of the earthquakes exhibited a simple rupture process characterized by a single and compact asperity with a single triangular STF, except for the 1998 earthquake. The results indicate that the ruptures primarily propagated unilaterally along the down-dip direction, except for the 2014 earthquake. We further analyzed the data incorporating directivity, which confirmed the rupture behavior. Three events suggested that the preferred rupture planes were near-vertical (down-dip), while two events exhibited subhorizontal orientations. Considering the challenges in determining the rupture plane associated with the subducting slab, the densely deployed national seismic networks in Java are expected to provide valuable insights into the dynamics of the subduction zone. By elucidating the source characteristics and rupture behavior of these intermediate-depth events, our study offers valuable insights for future seismic hazard assessments, particularly for densely populated regions of Java, Indonesia.