15:30 〜 15:45
[SCG45-27] Episodic tremor and slip via fault-fracture mesh development in forearc mantle wedge shear zones
★Invited Papers
キーワード：episodic tremor and slip、前弧マントルウェッジ、蛇紋岩、アンチゴライト、間隙流体圧、断層バルブ挙動
Episodic tremor and slip (ETS) occurs near the intersection of the tip of the forearc mantle wedge with the subducting oceanic crust at 25–45 km depth, downdip of the megathrust seismogenic zone (e.g., McCrory et al., 2014, J. Geophys. Res.). One might expect that the zone of ETS marks a thermally-controlled transition from unstable stick-slip to stable sliding. At Cascadia, however, a spatial gap exists between the seismogenic and ETS zones (Hyndman et al., 2015, J. Geophys. Res.), meaning that in the ETS zone, thermally activated viscous creep inhibits any types of earthquakes. This apparent contradiction can be reconciled by considering the presence of near lithostatic pore fluid pressure (Pf), which acts to reduce effective normal stress and thereby to promote brittle failure rather than viscous creep (Gao and Wang, 2017, Nature). However, it remains unclear which deformation mechanisms and processes operate in the ETS-generating fault zone.
Owing to its possible mechanical weakness (Hilairet et al., 2007, Science), antigorite, a high-temperature serpentine mineral, is expected to constitute a part of the ETS-generating plate-boundary zone. Recent high-pressure (P = 1 GPa) and high-temperature (T = 500 °C) deformation experiments have documented that as water content (i.e., Pf) increases, deformation behavior of intact antigorite samples transitions from stable sliding on a newly forming through-going fault (Hirauchi et al., 2020, J. Struct. Geol.) to a network of conjugate extensional (mode I) and extensional–shear (mode I–II) failures (i.e., fault-fracture meshes; Nagata, Hirauchi et al., in prep.). The fault-fracture mesh development has been observed in shallow (ca. 30 km) mantle wedge-derived serpentinite bodies in the Sanbagawa belt, central Shikoku, Japan (Hirauchi et al., 2021, Earth Planet. Sci. Lett.). The difference between nature and experiments is that antigorite was newly precipitated to fill pore spaces between the fractured blocks in nature. The newly precipitated antigorite grains on longer fractures that represent the coalescence of shorter mode I–II fractures have experienced dynamicre crystallization by subgrain rotation, suggesting the operation of localized viscous (dislocation) creep.
On the basis of field and experimental observations, a schematic model illustrating fracturing, viscous flow, and mineral dissolution/precipitation in mantle wedge serpentinite over ETS cycles, is depicted. Continuous fluid supply to the serpentinite-rich plate boundary fault zone increases the pore fluid pressure and thereby reduces the effective normal stress, which lead to the development of fault-fracture meshes throughout the fault zone, which may represent bursts of tectonic tremor and low frequency earthquakes (LFEs). Tectonic tremor and LFEs will occur again when Pf increases due to the closure of pore spaces until a failure condition is attained, suggesting that dissolution/precipitation rates of antigorite control the recurrence intervals (months to years) of ETS. At relatively lower Pf conditions after the fracturing, newly precipitated antigorite grains on longer fracture surfaces deform dominantly by dislocation creep. The viscous creep localized within the fault zone represent high strain rate deformation under excess shear stress conditions, and may correspond to short-term slow slip events.
Owing to its possible mechanical weakness (Hilairet et al., 2007, Science), antigorite, a high-temperature serpentine mineral, is expected to constitute a part of the ETS-generating plate-boundary zone. Recent high-pressure (P = 1 GPa) and high-temperature (T = 500 °C) deformation experiments have documented that as water content (i.e., Pf) increases, deformation behavior of intact antigorite samples transitions from stable sliding on a newly forming through-going fault (Hirauchi et al., 2020, J. Struct. Geol.) to a network of conjugate extensional (mode I) and extensional–shear (mode I–II) failures (i.e., fault-fracture meshes; Nagata, Hirauchi et al., in prep.). The fault-fracture mesh development has been observed in shallow (ca. 30 km) mantle wedge-derived serpentinite bodies in the Sanbagawa belt, central Shikoku, Japan (Hirauchi et al., 2021, Earth Planet. Sci. Lett.). The difference between nature and experiments is that antigorite was newly precipitated to fill pore spaces between the fractured blocks in nature. The newly precipitated antigorite grains on longer fractures that represent the coalescence of shorter mode I–II fractures have experienced dynamicre crystallization by subgrain rotation, suggesting the operation of localized viscous (dislocation) creep.
On the basis of field and experimental observations, a schematic model illustrating fracturing, viscous flow, and mineral dissolution/precipitation in mantle wedge serpentinite over ETS cycles, is depicted. Continuous fluid supply to the serpentinite-rich plate boundary fault zone increases the pore fluid pressure and thereby reduces the effective normal stress, which lead to the development of fault-fracture meshes throughout the fault zone, which may represent bursts of tectonic tremor and low frequency earthquakes (LFEs). Tectonic tremor and LFEs will occur again when Pf increases due to the closure of pore spaces until a failure condition is attained, suggesting that dissolution/precipitation rates of antigorite control the recurrence intervals (months to years) of ETS. At relatively lower Pf conditions after the fracturing, newly precipitated antigorite grains on longer fracture surfaces deform dominantly by dislocation creep. The viscous creep localized within the fault zone represent high strain rate deformation under excess shear stress conditions, and may correspond to short-term slow slip events.