Integrated Wide-Angle Seismic Imaging of the Moho 

and Lithospheric Structure Beneath Petit-Spot Volcanoes in the Japan Trench Outer-Rise Region 

Prof.  Andrzej Górszczyk

Institute of Geophysics of the Polish Academy of 

Sciences in Warsaw 

2026.06.24（星期三）13:30，理科二号楼 2821 

摘要： 

Imaging the Moho discontinuity and lithospheric structure in the trench-outer-rise region of the Japan Trench is particularly challenging due to bending-related faulting, intense fracturing, hydration, and petit-spot volcanism that modify the incoming Pacific Plate prior to subduction. These processes alter seismic velocities, disrupt sedimentary layering, and generate complex structures such as dikes, sills, and crack networks, which obscure the crust–mantle boundary and complicate conventional seismic imaging. High-resolution methods and dense wide-angle datasets are therefore required to accurately resolve the deep architecture of this tectonically active region. 

In this study, we investigate the structure of the oceanic plate using a 100-km-long 2D wide-angle seismic dataset acquired by JAMSTEC in 2017, consisting of 40 OBSs spaced at 2-km intervals. We integrate advanced seismic imaging approaches, combining full-waveform inversion (FWI) for velocity model building with reverse time migration (RTM) and kinematic migration (KM) for structural imaging. The velocity model is constructed using first-arrival traveltime tomography as a starting point and refined through FWI to achieve high-resolution reconstruction of sedimentary cover, oceanic crust, and uppermost mantle. RTM provides detailed reflectivity images, particularly around the Moho, while KM constrains reflector geometries by utilizing slope attributes, reducing uncertainties in structural interpretation. 

The integrated imaging results reveal a well-resolved Moho at approximately 12 km depth, along with velocity anomalies and fault-related structures extending from the seafloor to the uppermost mantle. Major bending-related faults can be traced continuously to near-Moho depths, supporting the hypothesis that lithosphere-scale fracturing associated with plate flexure creates vertical pathways for melt ascent. The strong agreement between velocity structure, reflectivity patterns, bathymetry, and magnetic lineations enhances confidence in the reconstructed architecture. 

Our results demonstrate that combining FWI, RTM, and KM applied to dense long-offset OBS data enables unprecedented resolution of deep crustal and lithospheric features beneath petit-spot volcanic fields. This integrated approach provides new insights into the structural controls on petit-spot formation and improves understanding of lithospheric modification processes preceding subduction, with broader implications for subduction dynamics and seismic hazard assessment.