Different Faulting of the 2023 (M_w 5.7 and 5.9) South-Central Java Earthquakes in the Backthrust Fault System

Two moderate earthquakes struck the South-Central part of Java Island (in Indonesia’s archipelago) with M_w 5.7 and M_w 5.9 on June 06 and June 30, 2023, respectively. Both earthquakes were followed by ~100 aftershocks with widespread and strong impacts along Java Island where some regions suffered several damages. In this study, both earthquake mechanisms were derived from the Bayesian moment tensor inversion and configure a unique faulting with a thrusting mechanism that is striking perpendicularly with the trench in the N–S direction. The results of the hypocenter relocation, using an updated 1-D velocity model, show that the aftershocks of the M_w 5.9 occurred deeper than the interface zone, while the aftershocks of the M_w 5.7 were located shallower above the slab. Both earthquakes can robustly confirm possible evidence of different faulting vertically clustered above the intraslab zone. The M_w 5.9 can be assumed as the backstop system dipping to the east–west direction, while M_w 5.7 is the backthrust system with south-dipping. Furthermore, a broad impact of both earthquakes on the resilience of MMI VI intensity with PGA value > 100 gal can be used to update the mitigation plan for the intraslab earthquake in the near future.