Examination of mineral assemblage and chemical composition in the fracture zone of the Nojima Fault at a depth of 1140 m: Analyses of the Hirabayashi NIED drill cores

Abstract

A 1800 m deep borehole was drilled at Nojima Hirabayashi, Japan to penetrate through the Nojima Fault, which was activated at the time of the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake). Three fracture zones were recognized in cores at approximate depths of 1140 m, 1300 m and 1800 m. The mode of distribution of fault rocks, minerals and chemical elements were analyzed in an interval between depths of 1108 m and 1161 m, focusing on the fracture zone at the depth of 1140 m. Foliated blue-gray fault gouge constituted the central part of the fracture zone. The degree of fracturing appeared to be greater in the hanging wall than in the footwall. The relative amounts of minerals were estimated qualitatively. In the analyzed interval, not only were quartz, orthoclase, plagioclase, biotite and hornblende detected in the parent rock (granodiorite), but also kaolinite, smectite, laumontite, stilbite, calcite, ankerite and siderite, which are related to hydrothermal alteration. In particular, biotite disappeared both in the hanging wall and footwall across the central fault zone; it disappeared over a wider range in the hanging wall than in the footwall. The amounts of major chemical elements were analyzed quantitatively. Concentrations of Al 2 O 3 , Fe 2 O 3 , MnO, TiO 2 , and P 2 O 5 all decreased throughout the interval except at some points. H 2 O + and CO 2 increased throughout the interval. Na 2 O increased in the region outside the central plane, whereas MgO and CaO increased in the hanging wall and decreased in the footwall. SiO 2 and K 2 O decreased in the hanging wall and increased in the footwall. These results elucidate the higher degree of fracturing and chemical changes present in the hanging wall of the 1140 m fracture zone than in the footwall.

Mineralogic and textural analyses of drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes: Initial interpretations of fault zone composition and constraints on geologic models

“…There are at least two caveats in using exhumed fault zones as a proxy for the analysis of in situ processes in seismically active faults: (1) With the exception of pseudotachylytes, no fault-related rock retrieved from exhumed faults can defi nitively be shown to be the product of seismic slip, and (2) post-slip alteration, during uplift or while faults are inactive at depth, may alter the textures of fault-related rocks. Observations via drilling into active faults can help to overcome these issues and can tell us much about the deformation processes within faults (Moore et al, 1995;Ohtani et al, 2000;Ikeda, 2001;Matsuda et al, 2001;Hung et al, 2005). Drill hole-based studies provide opportunities to clarify the nature of fault slip at depth and reduce the impact of overprinting, associated with surfi cial processes, which may obscure the primary textures and geochemical signatures in faults.…”

In situ stress measurements in NIED boreholes in and around the fault zone near the 1995 Hyogo‐ken Nanbu earthquake, Japan

“…From these results, the dip of the Nojima Fault is estimated to be approximately 70° to 85°. Although further analysis is necessary to specify which part moved in the Kobe event, it is suggested that the fault activity has recurred in the fractured zones and that deformation and alteration have been significant throughout the long term of fault activity (Kobayashi et al 2001; Matsuda et al 2001). The logging data of density, resistivity and P‐wave velocity are comparatively shown in Fig.…”

Outline of the fault zone drilling project by NIED in the vicinity of the 1995 Hyogo‐ken Nanbu earthquake, Japan

“…From these results, the dip of the Nojima Fault is estimated to be approximately 70° to 85°. Although further analysis is necessary in order to specify which part moved in the Kobe event, it is suggested that the fault activity has recurred in the fractured zones and that deformation and alteration have been significant throughout the long term of fault activity (Ikeda et al 2001; Kobayashi et al 2001; Matsuda et al 2001f).…”

“…The value of these components are shown in relation to the value of fresh tonalite. These data are compiled and modified from papers by Ikeda et al (1998, 2001), Kobayashi et al (1998, 2001), and Matsuda et al (2001).…”

“…Core samples were classified into five types of fault rocks (Kobayashi et al 1998, 2001). Also, an asymmetric distribution pattern of the fault rocks found in the fracture zones was identified, and precise observations and chemical analyses of fault gouge by using an X‐ray defraction meter, X‐ray fluorescent meter and scanning electron microscopy (SEM) were performed (Matsuda et al 2001), examples of which are shown in Fig. 3.…”