Seismogenic Structures in Subduction Zones

Abstract Recent studies of high-resolution seismic tomography of source zones of large crustal earthquakes, megathrust earthquakes, and intraslab earthquakes are reviewed, which shed new light on seismogenic structures and fluids in subduction zones. Large crustal earthquakes generally occurred in high-velocity (high-V) zones in the brittle upper crust, whereas low-velocity and high Poisson’s ratio anomalies exist in the lower crust and upper (or uppermost) mantle, which may reflect fluids released from dehydration of the subducting slab. The fluids may trigger large crustal earthquakes. The interplate megathrust zone exhibits prominent structural heterogeneities. Large megathrust earthquakes generally occurred in high-V areas, reflecting strongly coupled patches (or asperities) in the megathrust zone due to the subduction of seamounts or topographic plateaus in the incoming oceanic plate. The megathrust seismogenesis may be affected or controlled by structural anomalies in both the upper and lower plates, as well as hot upwelling flows in the subslab mantle. Lower-velocity anomalies are revealed in source zones of large intraslab earthquakes, which are attributed to the process of dehydration embrittlement resulting from dehydration of hydrous minerals in the slab, which may trigger the mainshock and aftershock sequences by enhancing pore pressures along preexisting faults and fractures in the slab. All these results indicate that fluids play an important role in the generation of most earthquakes in subduction zones.