Seamount Subduction's Influence on Subduction Zone Dynamics: Seismological Insights from Northern Luzon

Seamount subduction plays a pivotal role in shaping subduction zone dynamics, significantly influencing deformation processes and seismicity. This study examines the crustal and upper mantle deformation associated with seamount subduction beneath northern Luzon, where the South China Sea Plate underthrusts the region. We employed local S-wave splitting techniques to characterize the deformation and present seismological evidence of seamount subduction’s role in modulating subduction dynamics.

Our findings reveal a dominant trench-normal fast-axis orientation, aligned with the P-axis from crustal earthquake focal mechanisms, across most forearc stations. This pattern differs from the trench-parallel fast-axis commonly observed in other forearc settings such as northeastern Japan, Cascadia, and Sumatra. The frequency-dependent delay times and trench-normal fast-axis orientation suggest seismic anisotropy associated with fluid-filled cracks aligned with the prevailing stress field, influenced by the seamount subduction.

Notably, delay times increase with focal depth, highlighting that the effects of seamount subduction extend from the overriding crust into the subducting slab. These results offer direct seismological evidence of seamount subduction shaping subduction zone dynamics, promoting aseismic creep and small earthquakes through fracture network formation. This study enhances to the understanding of the complex interactions within subduction zones and underscores the importance of seamount subduction in these processes.