The Role of Fluid‐Related Heterogeneous Structures in Controlling the Fault Slip Behavior in the Slow‐Earthquake Source Region Along the Nankai Subduction Zone, Southwest Japan

A seismic array observation across the slow‐earthquake source region in the eastern Kii Peninsula, southwest Japan, reveal the detailed structure and seismicity along the upper boundary of the subducted Philippine Sea Plate. Tomography analysis and earthquake reflection imaging clarify the geometry of the upper boundary of the Philippine Sea Plate with a subduction angle of ∼15° at a depth range of 22–30 km. We observe intraslab low‐frequency earthquakes (LFEs) in and around the low‐Vp and high‐Vp/Vs zones, which are located above the intraslab earthquakes that are controlled by the dehydration embrittlement of serpentine. The intraslab LFE activity may be related to fluid movement. Fluids, which are derived from both dehydration of the serpentinized oceanic mantle and crustal eclogitization, may control the fault slip behavior. Therefore, the fluid behavior around the subducting plate boundary has an important role in controlling the downdip limit of the seismogenic zone. Plain Language Summary The Nankai Trough region, where the Philippine Sea Plate subducts beneath southwest Japan, is a well‐known seismogenic zone of large interplate earthquakes. Slow earthquakes, which have various slip motions with different time‐scales, have been recognized in regions adjacent to the seismogenic zone. A low‐frequency earthquake (LFE), which is a type of slow earthquake, has been observed in the area downward of the seismogenic zone. Revealing the crustal structure in and around the slow‐earthquake source region would help resolve the controlling factors of megathrust events. Seismic imaging is an effective tool for investigating the crustal structure in detail. Here, we present the results of seismic array observation across the slow‐earthquake source region beneath the eastern Kii Peninsula, southwest Japan, which highlight fluid‐related heterogeneous structures along the Nankai subduction zone. The intraslab LFEs are located in and around the low‐Vp and high‐Vp/Vs zone. A broad depth distribution of intraslab LFEs, which are generated by fluid movement, is located above the intraslab earthquakes. Fluids near the plate boundary may control the fault slip behavior in the slow‐earthquake source region, downdip of the seismogenic zone. Our results indicate that fluids are one of the key factors controlling the downdip limit of the seismogenic zone. Key Points Bright northward‐dipping reflective zone that corresponds to the Philippine Sea Plate is observed in the slow‐e