Abstract
The Nankai Trough in Southwest Japan exhibits a wide spectrum of fault slip, with long-term and short-term slow-slip events, slow and fast earthquakes, all associated with different segments down the plate interface. Frictional and viscous properties vary depending on rock type, temperature, and pressure. However, what controls the down-dip segmentation of the Nankai subduction zone megathrust and how the different domains of the subduction zone interact during the seismic cycle remains unclear. Here, we model a representative cross-section of the Nankai subduction zone offshore Shikoku Island where the frictional behavior is dictated by the structure and composition of the overriding plate. The intersections of the megathrust with the accretionary prism, arc crust, metamorphic belt, and upper mantle down to the asthenosphere constitute important domain boundaries that shape the characteristics of the seismic cycle. The mechanical interactions between neighboring fault segments and the impact from the long-term viscoelastic flow strongly modulate the recurrence pattern of earthquakes and slow-slip events. Afterslip penetrates down-dip and up-dip into slow-slip regions, leading to accelerated slow-slip cycles at depth and long-lasting creep waves in the accretionary prism. The trench-ward migrating locking boundary near the bottom of the seismogenic zone progressively increases the size of long-term slow-slip events during the interseismic period. Fault dynamics is complex and potentially tsunami-genic in the accretionary region due to low friction, off-fault deformation, and coupling with the seismogenic zone.
Highlights
Subduction megathrusts accumulate and release mechanical energy through a variety of slip modes, including earthquakes, low-frequency earthquakes, slow-slip events, and stable creep
The Nankai Trough offers a striking example of down-dip segmentation, with shallow low-frequency and very-low-frequency earthquakes, large megathrust earthquakes, long-term and short-term slow-slip events, the latter accompanied with deep low-frequency tremors (Fig. 1)
The down-dip segmentation of rupture styles at subduction zones indicates a strong control from the thermomechanical structure and composition of the upper plate
Summary
Subduction megathrusts accumulate and release mechanical energy through a variety of slip modes, including earthquakes, low-frequency earthquakes, slow-slip events, and stable creep. Using the down-dip width of the megathrust that intersects with the accretionary wedge, i.e., W = 41.75 km , and the frictional properties shown, the resulting non-dimensional parameters Ru = 1.82 and Rb = 0.025 are associated with a complex long-term slow-slip regime (Barbot 2019b). Using W = 148.75 km as the characteristic length scale of the seismogenic zone and with the combination of effective normal stress, characteristic weakening distance, and kinematic friction coefficients, we obtain Ru = 29.7 , a value high enough to generate complex seismic cycles with a variety of rupture sizes. System‐level dynamics of a subduction megathrust The two-dimensional subduction model simulates the dynamics of the megathrust and the surrounding asthenosphere during seismic cycles, including full and partial ruptures of the seismogenic zone, foreshocks and aftershocks, shallow slow-slip events, deep long-term and short-term slow-slip events, creep and afterslip, and viscoelastic relaxation in the ductile substrate The postseismic strain-rate components ε22 , ε23 and ε33 increase to 50 times larger than the steady-state values, while the locations of the peak strain rates and their directions remain the same except for the region at a depth of 100 to 200 km below the deeper portion of the seismogenic zone and the slow-slip segments (Fig. 7)
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