Variations in stress, pore fluid pressure and rock strength using minor faults and mineral veins along megasplay fault in subduction zone

Makoto Otsubo

Geological Survey of Japan

Date & Time
Building 3, Rambo Auditorium

The crustal stresses and pore fluid pressures at depth are difficult to quantify directly, and the downhole measurements of in-situ stress are generally limited to a few km depth. The Nobeoka Thrust, southwestern Japan is an on-land example of an ancient megasplay fault and provides an excellent record of deformation and fluid flow at seismogenic depths. In this study, we present (1) temporal stress changes for the seismic period of the Nobeoka Thrust and (2) spatial heterogeneity of the pore fluid pressure and rock strength by using the minor faults and mineral veins around the fault zone in the Nobeoka Thrust.

Minor faults can be observed from drillcores that intersected the Nobeoka thrust. The orientation clusters detected by the stress tensor inversion are characterized by the temporal changes for the orientations of the sigma 1 and sigma 3 principal stress axes that involve alternation between horizontal and vertical. The findings are probably due to a change in stress state before and after earthquakes that occurred on the fault; similar changes have been observed in active tectonic settings, such as the 2011 Tohoku-Oki earthquake (Japan).

Many quartz veins that filled mode I cracks can be observed in the hanging wall and footwall of the Nobeoka 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 sigma 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 postseismic stress buildup after megathrust earthquakes. We propose two possible explanations for the variations of vein attitudes: (a) spatial variations in pore fluid pressure are directly responsible for these variations, or (b) these variations are controlled by differences in rock strength between the hanging wall and footwall.

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