Geodetic constraints on slip partitioning at a convergent plate boundary: Reconciling long-term structural deformation with earthquake cycle observations in West Java

Slip partitioning models have been used extensively to describe the broad zones of deformation in oblique convergence zones. Traditionally these models have exploited course spatial and temporal constraints from geomorphology, seismology and geodesy. However, the development of high resolution geodetic measurements in the recent years have allowed the imaging of multiple transient processes occurring during the seismic cycle ranging from post-seismic relaxation to slow slip events, which raise questions about our ability to understand the long-term patterns of relative motion in actively convergent deforming regions. Here we present the example of West Java, where continuous GPS observations show a time-dependent change in the linear rate of surface motion, which we interpret as an ongoing long-term post-seismic deformation following the 2006 Mw 7.8 Java earthquake. We use a combination of a regional tectonic block model and a viscoelastic relaxation model to consider the post-seismic transient and we propose a kinematic model of convergence of the Australian Plate and the Sunda Block, involving a slip partitioning between the Java Trench and a left-lateral structure extending E-W along Java, with most of the normal component is occurring onto the subduction Thrust and a much smaller parallel motion accommodated along the Barabis ( 5± 0.2 mm/yr) and Kendeng (∼ 0.8 ± 0.3 mm/yr) Thrusts. Our findings correlate with observations from historical seismicity and highlight a new potential seismic hazard in Java Island.