Tracking the Transition from Postseismic to Interseismic Deformation in Java, Indonesia, with Global Navigation Satellite System Data

The southern coast of Java, Indonesia, is a high‐risk region characterized by intense seismic activity where the 17 July 2006 Mw 7.7 Java tsunami earthquake occurred and generated devastating tsunami. This study investigates the prolonged crustal deformation following this event by analyzing over a decade (2010–2022) of Global Navigation Satellite System (GNSS) data from stations along the southern coast of West Java. We quantify the contributions of two primary postseismic mechanisms—viscoelastic relaxation and afterslip—alongside ongoing interseismic coupling on the megathrust. Our results indicate that surface deformation was dominated by postseismic processes in the immediate years after the earthquake, with velocities of 2–3 mm/yr directed southward. Over time, the influence of postseismic deformation decayed significantly; viscoelastic relaxation diminished by 79% by 2019–2022. Afterslip distribution and moment release increased then decreased over the studied periods, releasing seismic moments equivalent to Mw 6.4–6.6. Concurrently, the deformation pattern gradually transitioned, revealing the increasing dominance of interseismic coupling as postseismic deformation progressively diminishes. The study highlights a transitional phase between postseismic and interseismic deformation along the Java subduction zone. These findings underscore the critical importance of long‐term geodetic monitoring for refining seismic hazard assessments in subduction zones.