Frequency-dependent anisotropy and upper plate deformation due to seamount subduction in northern Luzon

Seamount subduction influences subduction zone dynamics by altering stress fields, fracture patterns, and seismic anisotropy. This study utilizes local S-wave splitting analysis to investigate crustal and upper mantle deformation associated with seamount subduction beneath northern Luzon. Our observations reveal predominantly trench-normal fast-axis orientations and frequency-dependent delay times. These patterns suggest that anisotropy arises primarily from fluid-filled cracks and possible serpentinization, with effects extending from the overriding crust into the subducting slab, spanning multiple structural depths. In contrast, trench-parallel directions in southern non-seamount subduction regions indicate either ductile overriding lithosphere deformation or toroidal mantle flow around the slab edge. Event depth and raypath geometry further indicate that seamount subduction promotes stress heterogeneity and vertical anisotropic layering. These findings demonstrate that subducting features such as seamounts produce distinct anisotropy signatures, offering new insights into subduction dynamics and lithospheric deformation.