[S-CG40] Science of slow-to-fast earthquakes

[SCG40-10] Frontal Thrust ramp-up and Slow Earthquakes due to Underthrusting of Basement Relief in the Nankai Trough

*Gaku Kimura¹, Kazuya Shiraishi¹, Yasuyuki Nakamura¹, Shuichi Kodaira¹, Gou Fujie¹, Ryuta Arai¹, Gregory F. Moore² (1.Japan Agency for Marine-Earth Science and Technology, 2.Department of Earth Sciences, University of Hawaii)

Keywords:Slow earthquake, frontal thrust, Nankai Trough, seamount collision, accretionary prism

Recently, integrated geophysical-geological surveys in subduction zones have revealed that slow earthquakes repeatedly occur beneath the outer wedge of the forearc. During December 2020 to February 2021, clustered slow earthquakes were propagated around the frontal thrust of accretionary wedge in the Kumano region of the Nankai Trough, Japan. The frontal thrust ramps up from the basal décollement beneath the accretionary wedge and slips over trench-filling sediment along the landward edge of the Nankai trough floor. Ocean floor topography and geologic structure revealed by seismic reflection surveys by 2022 document that the basement of the Philippine Sea Plate beneath the frontal thrust presents seamount and horst-like high relief. The northern edge of the relief is located just at the ramp-up position of the frontal thrust. The 2020-21 clustered slow earthquakes started at the Paleo-Zenisu ridge beneath the inner-outer slope border and propagated to the topographic highs beneath the deformation front. Considering that the relative plate convergence between the upper Amurian Plate of the Nankai forearc and the subducting Philippine Sea Plate is ~6.0 cm/y, the basement high relief at the deformation front has kept jacking up the frontal crest of the wedge by an uplift rate of 2.7~5.7 mm/y for several tens to hundred thousand years in average, which is the same scale of Himalayan foreland. The slow earthquakes in the off-Kumano Nankai Trough in 2020-2021 appear to be the first snapshots of a “living” Nankai frontal thrust during the inter-large seismic period.