Crustal Structure of the Nankai Subduction Zone Revealed by Two Decades of Onshore-Offshore and Ocean-Bottom Seismic Data: Implications for the Dimensions and Slip Behavior of the Seismogenic Zone

Abstract

Two-decades of onshore-offshore, ocean-bottom seismometer, and passive-source data are integrated to obtain high-resolution 3-D constraints on the architecture of the Nankai subduction zone. Our model reveals large along-strike variability in the width of the outer-forearc, with the crustal backstop (VP ≥ 5 km/s) extending within 50 km of Nankai Trough offshore Tokai and Kii Peninsula, but is a factor of 2 further landward (90–115 km) offshore Cape Muroto and Kyushu. The crustal backstop broadly coincides with the down-dip extent of shallow slow-earthquakes and the up-dip extent of co-seismic slip and geodetic locking, suggesting a link with the shallow frictional transition. In the inner-forearc, high seismic-velocities (VP > 6.6 km/s) in Kii Peninsula reflect Miocene plutons, impacting the nucleation and segmentation of large megathrust earthquakes. The complex geometry of the subducting slab drives spatial variability in the location of the down-dip frictional transition. Variability in the width of the outer-forearc, the dip of the subducting slab, and the depth distribution of deep slow-slip events combine to produce large changes in the width of the seismogenic zone. This is most pronounced across Bungo channel, where the seismogenic zone narrows from 90 to 120 km in Shikoku to <60 km offshore Kyushu. Pinching of the seismogenic zone may explain the smaller area, and thus magnitude, of earthquakes offshore Kyushu and the reduction in slip-deficit rate. Our results suggest crustal-scale architecture is a key driver of profound along-strike transitions in the location of frictional transition zones, and the width and slip behavior of the intervening seismogenic zone at Nankai Trough.