TENSION VEINS AS EVIDENCES OF LOCALIZED FLUID DISCHARGE IN SUBDUCTION ZONES: AN EXAMPLE OF NOBEOKA THRUST, SOUTHWESTERN JAPAN

Mineral veins that we can observe in outcrops and core samples are the ancient fluid pass way (fossil of fluid pass way). The fluid loss by the formation of mode I cracks increases the fault strength and creates drainage asperities along plate interface (Sibson, 2013, Tectonophysics, 600, 142–152). We focus the pore fluid pressure loss and amount of fault strength recovery by the formation of mode I cracks in the post-seismic period around the fault zone of the Nobeoka Thrust. Then we discuss the relationships between fracturing and fluid migration during seismic cycles. The Nobeoka Thrust (Kyushu in southwestern Japan), an on-land example of an ancient megasplay fault, records the deformation and fluid flow at seismogenic depths (Kondo et al., 2005, Tectonics, 24, TC6008). Many quartz veins that filled mode I crack at the coastal outcrops suggested the normal faulting stress regime after faulting of the Nobeoka Thrust (Otsubo et al., 2016, Island Arc, 25, 421–432). These observations show that the pore fluid pressure is close to the lithostatic pressure around the Nobeoka Thrust (Normalized pore pressure ratio λ* > 0.9).

We estimated the decrease of the pore fluid pressure by formation of the mode I cracks around the Nobeoka Thrust in post-seismic period. Veins filling mode I cracks are constructed when the pore fluid pressure exceed σ₃ (Jolly and Sanderson, 1997, Jour. Struct. Geol., 19, 887–892). Here, we estimate the over over fluid pressure, ΔP_f (ΔP_f = P_f – σ₃, P_f: pore fluid pressure). The differential stress in post-seismic period and driving pore fluid pressure ratio P* (P* = (P_f – σ₃) / (σ₁ – σ₃)) are parameters to estimate the pore fluid over pressure. In the case of the Nobeoka Thrust (P* = ~0.4, Otsubo et al., 2016), the pore fluid over pressure is up to ~20 MPa (assuming Tensile strength = 10 MPa). ~20 MPa is equivalent to <10% of the total pore fluid pressure around the Nobeoka Thrust (depth = 10 km). In the case of the Nobeoka Thrust, the fault strength can increase by up to ~10 MPa (assuming frictional coefficient = 0.6). ~10 MPa is almost equivalent to an amount of stress drop in large trench type earthquakes. Hence, the fluid loss by formation of mode I cracks in the post-seismic period may play an important role by increasing frictional strength along the megasplay fault.