Frontal Thrust Ramp-Up and Slow Earthquakes Due To Underthrusting of Basement High in the Nankai Trough

Recently, integrated geophysical-geological surveys in the Nankai subduction zone in Japan have revealed that slow earthquakes repeatedly occur beneath the outer wedge of the forearc. During December 2020 to February 2021, clustered slow earthquakes propagated around the frontal thrust of the accretionary wedge. The frontal thrust ramps up from the basal décollement and slips over trench-filling sediment along the landward edge of the Nankai trough floor. Here, the Paleo-Zenisu ridge has been subducted beneath the inner-outer slope border. In addition, ocean floor topography and geologic structure revealed by seismic reflection surveys completed before 2022 document that the basement of the Philippine Sea Plate beneath the frontal thrust has a seamount and a horst-like basement high. The northern edge of the basement high is located at the ramp-up position of the frontal thrust. The 2020–2021 clustered slow earthquakes started at the Paleo-Zenisu ridge 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/year, the basement high at the deformation front has uplifted the frontal crest of the wedge at an average rate of 2.7–5.7 mm/year for several tens to hundred thousand years. These rates are among some of the highest rock uplift rates measured in the world. The slow earthquakes in the off-Kumano Nankai Trough in 2020–2021 are a snapshot of a “living” Nankai frontal thrust during the megathrust interseismic period.