Subduction Zones Beneath Indonesia Imaged by Phase Velocity Tomography

The Indonesian archipelago represents one of the most tectonically complex regions on Earth, where the convergence and interaction of multiple plates drive ongoing subduction, arc-continent collision, and lithospheric accretion. To unravel the detailed structure and dynamics of this convergent margin, we develop a novel, high-resolution 3-D shear-wave velocity model of the lithosphere and upper mantle. This model is derived from a weighted joint inversion of complementary surface-wave datasets: teleseismic Rayleigh waves from 387 shallow earthquakes (MS ≥ 5.5) recorded across 31 stations, analyzed using a modified two-plane-wave tomography method, and ambient-noise correlations from two years of continuous data at 30 stations, processed with far-field approximation and image-transformation techniques. This integrated approach significantly enhances the resolution of shallow structures compared to previous body-wave tomographic models. Our model provides new insights into the four primary subduction systems. Along the Sunda-Java trench, we document a systematic along-strike transition in slab geometry: a continuous, well-defined slab in the west progressively gives way to increasingly disrupted and thickened structures eastward. This morphological evolution correlates with the subduction of progressively older oceanic lithosphere and is influenced by variations in slab age, dip, and the presence of deep slab tearing. Beneath the Banda Arc, we image an approximately 200 km-thick slab and attribute its dramatic 180° curvature to the mechanical interaction between the northward-subducting Australian plate and a distinct south-directed subduction system beneath the Seram region. In the Molucca Sea, our high-resolution tomography reveals a shallow (~50 km depth) low-velocity zone and details the complex geometry of an active double-sided subduction zone, characterized by asymmetric dips and intense seismicity, which illuminates the dynamics of ongoing arc-arc collision. Finally, beneath the Celebes Sea, a south-dipping slab is clearly resolved under North Sulawesi, while no substantial subduction signature is associated with the Sangihe Arc. Collectively, these findings provide unprecedented structural constraints on the segmentation, deformation, and interaction of subducting slabs in Indonesia. They underscore the control of lithospheric age and complex plate interactions on slab morphology and regional tectonics, offering a refined framework for understanding the geodynamic evolution of this exceptionally complex convergent boundary.