Variations in stress and driving pore fluid pressure ratio using vein orientations along megasplay faults : Example from the Nobeoka Thrust, Southwest Japan

Abstract

The Nobeoka Thrust of Southwest Japan is an on-land example of an ancient megasplay fault that provides an excellent record of deformation and fluid flow at seismogenic depths. The present study reports: (i) temporal stress changes for the seismogenic period of the Nobeoka Thrust; and (ii) spatial heterogeneities in driving pressure ratios P* obtained from mineral veins around the Nobeoka Thrust fault zone. Many quartz veins that filled mode I cracks can be observed in the hanging wall and footwall of the thrust. Inversion for stress orientation suggests that normal faulting dominated in both the hanging wall and footwall, with similar stress axis orientations in both. The orientation of σ 3 for the estimated stress regime is parallel to the slip direction of the Nobeoka Thrust. The detected normal-faulting-type stress regimes likely resulted from post-seismic stress buildup after megathrust earthquakes. The hanging wall of the Nobeoka Thrust has smaller P* values than the footwall. Two possible explanations are proposed for the observed spatial variations in the driving pore fluid pressure ratio, P*: spatial variations in pore fluid pressure P f are directly responsible for P* variations, or P* variations are controlled by differences in mechanical properties between the hanging wall and footwall.

Temporal stress variations along a seismogenic megasplay fault in the subduction zone: An example from the Nobeoka Thrust, southwestern Japan

“…The stress regime in the outer wedge ( OW ) was inferred from earthquake focal mechanism solutions (Hasegawa et al, , ). The pre‐seismic stresses at the tectonic phase I correspond to the stress solutions E and F in Group I in this study, whereas the post‐seismic stresses at the tectonic phase I were inferred from mineral veins (Otsubo et al, ). The pre‐seismic stresses at the tectonic phase II correspond to the stress solutions A and C in G roup II in this study, whereas the post‐seismic stresses are stress solutions B and D…”

“…Left panel shows the relatively long‐term stress state along the décollement ( DC ) (after Hashimoto et al, ) and on the megasplay fault (in this study, related to subduction during the pre‐seismic period). Right panel shows the post‐seismic stress regime after a large earthquake, due to a drop in shear stress along the megasplay fault (Otsubo et al, ). The post‐seismic stresses at the tectonic phase I were inferred from mineral veins (Otsubo et al, ).…”

“…Right panel shows the post‐seismic stress regime after a large earthquake, due to a drop in shear stress along the megasplay fault (Otsubo et al, ). The post‐seismic stresses at the tectonic phase I were inferred from mineral veins (Otsubo et al, ). The stress regime in the outer wedge ( OW ) was inferred from earthquake focal mechanism solutions (Hasegawa et al, , ).…”

Proceedings of the 11th United States-Japan natural resources panel for earthquake research, Napa Valley, California, November 16–18, 2016

“…In this study, we estimated the fluid volumes for the formation of quartz veins in the two cases: (1) the extension veins of mode I around Nobeoka Thrust, Kyushu, SW Japan (Otsubo and others, 2016) and (2) en echelon cracks (arrays) in the Kodiak accretionary complex, Alaska (Fisher and Brantley, 2014). In the coastal region of the Nobeoka Thrust (ancient megathrust as the boundary between northern and southern Shimanto Belts, depth: ~10 km, pore fluid pressure: 260 MPa, temperature: 250 °C, Kondo and others, 2005), the fluid volume for formation of a vein is equal to the fluid volume within radius 0.15 to 0.5 m of a crack (in case of porosity = 3 percent).…”

“…The results indicate that the dense veins are the cumulative during seismic cycles. In the Kodiak accretionary complex (Kodiak Formation, depth: ~10 km, pore fluid pressure: 260 MPa, temperature: 280 °C, porosity: 3 percent, Fisher and Brantley, 2014), the fluid volume for formation of veins on a array is equal to the fluid volume within radius 0.6 to 1.5 m of a array. The results indicate that the fluid volume within the distance (~0.5 m) estimated by Fisher and Brantley (2014) may be insufficient for precipitation of quartz in cracks.…”