Structural control on the shallow tremor distribution linked to seamount subduction: insights from high-resolution seismic imaging in Hyuga-nada

Tectonic tremors occur around the subducting seamount of the Kyushu Palau Ridge (KPR) in Hyuga-nada, Japan. We investigate the relationship between tremor activity and structural and physical characteristics using high-resolution reflection seismic imaging of the KR0114-8 line, encompassing areas with both high and low occurrences of tremors. The reflection data are reprocessed using broadband processing, reflection tomography and Kirchhoff pre-stack depth migration. The resulting image delineates complex deformation and lithological boundaries, such as the accretionary prism, underthrust sediments, décollement, and top of the seamount. The observed splay faults and seafloor uplift to the west of the KPR, coupled with frequent tremors, confirm the compressional stress regime on the leading side of the subducting seamount. A stress shadow over the seamount effectively suppresses tremors. Numerous faults indicate significant deformation of the overburden when positioned on the leading side of the seamount. The trailing side exhibits a compressional stress regime, rather than an extensional one, as evidenced by the development of in-sequence thrusts and frequent tremors. Local physical, mechanical, and structural factors critically influence the tremor activity. The increased frequency of tremors is correlated with the thickness of the underthrust sediments and presence of in-sequence thrusts, whereas it is inversely correlated with the reflectivity of the décollement. Several potential mechanisms for this phenomenon include elevated pore pressure above the décollement and/or within the underthrust sediments, as well as structural effects. High-resolution velocity imaging, scientific drilling, and precise tremor-depth estimation are essential for advancing our understanding of these mechanisms. Graphical abstract