Crustal structure, seismogenesis and volcanism in the Ryukyu subduction zone revealed by active-source seismic surveys

Slip behaviors along subduction megathrust faults range from aseismic creep to fast rupture leading to destructive earthquakes. However, physical and structural characteristics controlling the slip speed and rupture extent are still major subjects of debate. In this seminar, I summarize our recent seismic investigations in the Ryukyu subduction zone which provide new insights on this issue. Seismic reflection data in the southern Ryukyu Trench where a huge tsunami earthquake is thought to have occurred in 1771 reveal that it shares common subduction features with other tsunamigenic regions, such as a small-scale low-velocity sedimentary wedge at the toe of the overriding plate and landward-dipping branching faults that stem from the plate interface and breach the seafloor (Arai et al., 2016, Nature Com.). As evidenced by negative polarity of seismic reflections, the plate interface is enriched in fluids down to a depth of at least ~25 km, suggesting that the regionally extensional stress regime, as exemplified by the slab roll-back at the trench and the back-arc rifting, contributes to increasing the permeability along the plate interface. As such, slow earthquakes including low-frequency earthquake, very-low-frequency earthquake and slow slip event occur at these depths. Another important control on subduction earthquakes is the roughness of the plate interface. We find in northern Ryukyu that the plate interface is vertically displaced (> 1 km) at the downdip of the subducting seamount (Arai et al., 2017, GRL). These near-vertical faults are active, as demonstrated by large normal-fault earthquakes with a magnitude of 7.1 in 1995, and suggest that internal heterogeneity of the slab, especially significant buoyancy acting on the incoming plateau, can produce sufficient differential stress leading to high-angle normal-fault earthquakes within the slab.