Seismological Structures on Bimodal Distribution of Deep Tectonic Tremor

Deep tectonic tremors occur at the downdip extent of the seismogenic zone due to fluid processes. Beneath the northeastern Kii Peninsula, southwestern Japan, there is an along-dip bimodal distribution of tremor. However, no constraint exists on the structures controlling that distribution. We extract detailed seismological structures from multi-band receiver functions and evaluate conditional differences in the distribution. To achieve high resolution images along the plate interface, we utilize records of regional deep-focus earthquakes from the Pacific slab. Cross-section images show the subducting oceanic plate with depth-dependent phases along the bimodal distribution, revealing a conspicuous plate interface at the updip portion and an inconspicuous interface below the mantle wedge at the downdip portion. This indicates that episodic tremors occur in the high pore-fluid plate interface below the impermeable forearc crust, and that continual tremors occur at the permeable mantle wedge corner, owing to continuous fluid supply from the oceanic crust. [Plain Language Summary] Deep slow earthquakes have mainly been detected at the deeper extent of estimated large-slip regions of large-scale regular earthquakes in the Nankai subduction zone, southwestern Japan. Epicenters of tectonic tremors are also downdip-aligned. However, some clusters of continual tremor with frequent small bursts were found at further downdip portions beneath the northeastern Kii Peninsula. The complexity of the bimodal tremor distribution poses a structural question regarding whether the tectonic tremor occurs below a mantle wedge or below the continental crust. We utilize a receiver function method that surveys subsurface velocity boundaries and evaluate detailed seismological structures around the plate interface using a multi-band analysis. Furthermore, regional deep-focus earthquake records are effectively utilized for receiver function mapping. The high-frequency cross section exhibits depth dependence of plate-interface phases, which demarcates active regions of updip events and downdip continual tremor, thus revealing that episodic tremor occurs below the continental crust and continual tremor occurs at the mantle wedge corner. The high-contrast updip interface reveals that a large amount of fluid is confined at the plate interface below the impermeable forearc crust, which may lead to active episodic slow earthquakes at updip portions.