Upper crustal structure of the Daiichi-Kashima Seamount at the junction between Japan and Izu-Ogasawara trenches, northwestern Pacific Ocean

Subducted seamounts are recognized as structures that influence seismicity in subduction zones. Understanding the detailed structures of seamounts, including before and during subduction, is essential for a comprehensive grasp of their influence. Of particular importance is their competency and deformation history during subduction. To better understand seamount subduction and related processes, we analyzed seismic reflection profiles around the Daiichi-Kashima Seamount, the Katori Seamount, and a knoll situated on the oceanward slope. These three features are located at different distances from the trench axis but all fall within the hinge-line of the subducting plate. The Daiichi-Kashima Seamount is currently subducting at the junction between the Japan and northern Izu-Ogasawara trenches, while the Katori Seamount and the knoll have not yet reached the trench axis. A thick limestone layer capping the Daiichi-Kashima Seamount indicates that the core of the Seamount is at least partially intact. On the other hand, our work reveals a large number of trench-parallel or sub-parallel faults within each of the edifices. The seismic profiles also reveal sediments draping the flanks of the edifices, interpreted as turbidite and/or debris flow deposits originating from slope failures. Consequently, our findings show that seamounts and other topographic features begin brittle deformation and failure due to flexural bending of the incoming oceanic plate long before actual subduction, with implications for the strength and competency of seamounts during subduction.