[S-CG45] Science of slow-to-fast earthquakes

[SCG45-33] 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

*Dan Bassett 500203¹、Adrien Arnulf²、Shuichi Kodaira³、Ayako Nakanishi³、Alistair Harding⁴、Gregory F Moore⁵ (1.GNS Science、2.University of Texas at Austin、3.JAMSTEC、4.Scripps Institution of Oceanography、5.Department of Earth Sciences, University of Hawaii)

キーワード：Nankai Trough, Crustal Structure, Seismogenic Behaviour, Marine Geophysics, Seismic Hazard

Some subduction zones produce the largest earthquakes and tsunami on Earth, while others slip aseismically. To understand what factors impact subduction zone slip behavior, we have integrated two-decades of onshore-offshore, ocean-bottom seismometer, and passive-source data in SW Japan to construct the first high-resolution, 3-D image of an entire subduction zone. This image reveals large variability in the offshore extent of dense, rigid crustal rocks and suggests the position of this crustal backstop may influence the shallow transition from aseismic to seismic slip along the Nankai megathrust. The landward extent of seismic slip is also spatially variable and appears to be predominantly controlled by trajectory of the subducting plate. Collectively, along trench variability in the position of the crustal backstop (up-dip transition) and the geometry of the subducting slab (down-dip transition) combine to produce large variability in the width of the seismogenic zone. As earthquake magnitude is proportional to rupture area, pinching of the seismogenic due to the convergence of shallow and deep frictional transitions may explain the smaller magnitude of earthquakes offshore Kyushu. Our results suggest crustal-scale architecture is a key driver of profound along-strike transitions on the location of frictional transition zones, and the width and slip behavior of the intervening seismogenic zone at Nankai Trough.