Geometry of the Nojima Fault at Nojima-Hirabayashi, Japan – I. A Simple Damage Structure Inferred from Borehole Core Permeability

Abstract

The 1995 Kobe (Hyogo-ken Nanbu) earthquake, M = 7.2, ruptured the Nojima fault in southwest Japan. We have studied core samples taken from two scientific drillholes that crossed the fault zone SW of the epicentral region on Awaji Island. The shallower hole, drilled by the Geological Survey of Japan (GSJ), was started 75 m to the SE of the surface trace of the Nojima fault and crossed the fault at a depth of 624 m. A deeper hole, drilled by the National Research Institute for Earth Science and Disaster Prevention (NIED) was started 302 m to the SE of the fault and crossed fault strands below a depth of 1140 m. We have measured strength and matrix permeability of core samples taken from these two drillholes. We find a strong correlation between permeability and proximity to the fault zone shear axes. The half-width of the high permeability zone (approximately 15 to 25 m) is in good agreement with the fault zone width inferred from trapped seismic wave analysis and other evidence. The fault zone core or shear axis contains clays with permeabilities of approximately 0.1 to 1 microdarcy at 50 MPa effective confining pressure (10 to 30 microdarcy at in situ pressures). Within a few meters of the fault zone core, the rock is highly fractured but has sustained little net shear. Matrix permeability of this zone is approximately 30 to 60 microdarcy at 50 MPa effective confining pressure (300 to 1000 microdarcy at in situ pressures). Outside this damage zone, matrix permeability drops below 0.01 microdarcy. The clay-rich core material has the lowest strength with a coefficient of friction of approximately 0.55. Shear strength increases with distance from the shear axis. These permeability and strength observations reveal a simple fault zone structure with a relatively weak fine-grained core surrounded by a damage zone of fractured rock. In this case, the damage zone will act as a highpermeability conduit for vertical and horizontal flow in the plane of the fault. The fine-grained core region, however, will impede fluid flow across the fault.

Changes in permeability of the Nojima fault damage zone inferred from repeated water injection experiments

“…All three components are likely present, parallel to the slip surface, along the Nojima branch fault (Mizoguchi et al 2008); the fault core comprises fault gouge zone and fault breccia, the damage zone consists of fractured granite, and the protolith is undeformed granite. The fractured granite is 20 m thick (Mizoguchi et al 2008) based on surface samples around this area, and the thickness of the permeable zone has been estimated to be 15-25 m (Lockner et al 2009) based on core samples from a borehole drilled 4 km northeast of our study site. On the basis of these estimated thicknesses, we consider both the injection point in the 1800-m borehole and the open interval in the 800-m borehole to be located in or near the damage zone of the Nojima branch fault (Fig.…”

“…Both seismic (Nishigami et al 2008) and self-potential observations were made at the surface around the 1800-m injection borehole (Murakami et al 2007) during the water injection experiments. Water injection experiments were conducted in 1997, 2000, 2003, 2004, 2006, 2009, and 2013. From 1997to 2009, water was injected into the top of the 1800-m borehole.…”

“…Water injection experiments were conducted in 1997, 2000, 2003, 2004, 2006, 2009, and 2013. From 1997to 2009, water was injected into the top of the 1800-m borehole. At first, the experiment scheme was to inject water from the screen at 1596 to 1671 m depth of the 1800-m borehole, though the vertical distribution of temperature in the borehole during these experiments showed that the injected water leaked out of the borehole at a depth of around 540 m, but not at greater depths (Yamano and Goto 2005).…”

ON THE STRUCTURE AND FORMATION OF EARTHQUAKE SOURCES IN THE FAULTS LOCATED IN THE SUBSURFACE AND DEEP LEVELS OF THE CRUST. Part I. SUBSURFACE LEVEL

“…В еще одной скважине ширина зоны влияния составила 70-130 м. Отмечается, что разломная зона стано-вится более широкой и сложной с глубиной, раз-ветвляясь на два разлома между скважинами GSJ и NEID. Она представляет собой тонкую малопроч-ную и низкопроницаемую зону, обрамленную с обеих сторон высокопроницаемой породой, кото-рая подверглась сдвиговым смещениям [Lockner et al, 2009]. Специальные исследования методом электронно-спинового резонанса позволили уста-новить, что имел место фрикционный нагрев в зоне шириной 6 мм.…”

“…Д.А. Локнер с соавторами предположили, что скважина пересекла две зоны скольжения на глу-бинах 1140 и 1312 м [Lockner et al, 2009]. Все наблюдения в скважинах, пробуренных через Nojima Fault, показывают, что процессы «залечивания» в виде восстановления уровня сдвигового сопро-тивления в местах плотного контактирования в виде пятен скольжения, вероятно, могут происхо-дить с высокой скоростью, что делает трудным распознавание недавних косейсмических разрывов всего лишь через один год после землетрясения в Кобе (вероятно, из-за явлений механического смы-кания стенок разрывов, что фиксировалось в ходе полевых наблюдений в зонах сейсмодислокаций в приповерхностных условиях).…”