Seismic observations in the DPRI 1800 m borehole drilled into the Nojima Fault zone, south-west Japan

Abstract Seismometers were installed at three depths in the Disaster Prevention Research Institute, Kyoto University (DPRI) 1800 m borehole drilled into the Nojima Fault zone, southwest Japan. The waveforms recorded by these seismometers are rather simple compared with those recorded at the DPRI 800 m borehole or on the ground surface. These data should be well suited for detecting fault zone-trapped waves and estimating the fault zone structure and its temporal variation related to the healing process of the ruptured fault. Typical waveforms trapped in the fault zone were observed by a surface seismographic array across the Nojima Fault just after the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake). Among the wave data recorded in the DPRI 1800 m borehole, however, clear evidences of fault zone-trapped waves have not yet been found, and further studies are continuing. The present study outlines the observation system in the DPRI 1800 m borehole, which will make it easier to access and analyze the borehole data.     

ESR analysis of the Nojima fault gouge, Japan, from the DPRI 500 m borehole

Abstract Electron spin resonance (ESR) analyses of quartz grains in fault gouge were performed for a core sample taken from the Nojima Fault that moved during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake). Distribution of radiation-induced defects in the gouge at a depth of 389.4 m was measured by extracting quartz grains from seven discrete positions within 30 mm of the fault plane on the granite side. The decrease in E'₁ and Al centers was observed within 2 mm of the fault plane, suggesting partial annealing due to faulting. Partial annealing even at that depth suggested that conventional ESR dating, which is based on the hypothesis of complete annealing during faulting, was not applicable. Theoretical calculations of the temperature rise and of the thermal annealing of defects have been made by assuming a simple annealing model of heat generation on the fault plane. Thermal energy was calculated to have been approximately 8 MJ/m² to explain the profile of the heat-affected region. Thermal energy was much larger than the one estimated from hydrothermal solution, and corresponded to the frictional heat calculated for a normal stress of 20 MPa, a displacement of 2 m, and a frictional coefficient of 0.2.     

Quaternary vertical offset and average slip rate of the Nojima Fault on Awaji Island, Japan

Abstract Drilling was carried out to penetrate the Nojima Fault where the surface rupture occurred associated with the 1995 Hyogo-ken Nanbu earthquake. Two 500 m boreholes were successfully drilled through the fault zone at a depth of 389.4 m. The drilling data show that the relative uplift of the south-east side of the Nojima Fault (south-west segment) was approximately 230 m. The Nojima branch fault, which branches from the Nojima Fault, is inferred to extend to the Asano Fault. From the structural contour map of basal unconformity of the Kobe Group, the vertical component of displacement of the Nojima branch–Asano Fault is estimated to be 260–310 m. Because the vertical component of displacement on the Nojima Fault of the north-east segment is a total of those of the Nojima Fault of the south-west segment and of the Nojima branch–Asano Fault, it is estimated to total to 490–540 m. From this, the average vertical component of the slip rate on the Nojima Fault is estimated to be 0.4–0.45 m/10³ years for the past 1.2 million years.     

In situ stress measurements in a borehole close to the Nojima Fault

Abstract In situ stress was measured close to the fault associated with the 1995 Kobe Earthquake (Hyogo-ken Nanbu earthquake; January 1995; M 7.2) using the hydraulic fracturing method. The measurements were made approximately 2 years after the earthquake. The measured points were approximately 40 m from the fault plane at depths of about 1500 m. The maximum and the minimum horizontal compressive stresses were 45 MPa and 31 MPa, respectively. The maximum compressive stress and the maximum shear stress are very small in comparison with those of other seismically active areas. The azimuth of the maximum horizontal compressive stress was estimated from the observed azimuths of well bore breakouts at depths between 1400 m and 1600 m and was found to be N135° (clockwise). The maximum stress axis is perpendicular to the fault strike, N45°. These features are interpreted in terms of a small frictional coefficient of the fault. The shear stress on the fault was released and dropped almost to zero during the earthquake and it has not yet recovered. Zero shear stress on the fault plane resulted from the perpendicular orientation of one of the principal stress to the fault plane.     

Strain and tilt changes measured during a water injection experiment at the Nojima Fault zone, Japan

Abstract In order to make geophysical and geological investigations of the Nojima Fault on Awaji Island, Japan, three boreholes measuring 1800 m, 800 m and 500 m deep were drilled into the fault zone. The fault is one of the seismic source faults of the 1995 Hyogo-ken Nanbu earthquake of M 7.2. A new multicomponent borehole instrument was installed at the bottom of the 800 m borehole and continuous observations of crustal strain and tilt have been made using this instrument since May 1996. A high-pressure water injection experiment within the 1800 m borehole was done in February and March 1997 to study the geophysical response, behavior, permeability, and other aspects of the fault zone. The injection site was located approximately 140 m horizontally and 800 m vertically from the instrument. Associated with the water injection, contraction of approximately 0.7 × 10⁻⁷ str (almost parallel to the fault) and tilt of approximately 1 × 10^-7 rad in the sense of upheaval toward the injection site were observed. In addition to these controlled experiments, the strainmeter and tiltmeter also recorded daily variations. We interpret strain and tilt changes to be related to groundwater discharge and increased ultra-micro seismicity induced by the injected water.     

Changes in magnetic and fractal properties of fractured granites near the Nojima Fault, Japan

Abstract Anisotropy of magnetic susceptibility (AMS) has been used to infer finite strain fabrics in plastically deformed rocks, but there are few studies of magnetic properties in fractured fault rocks. Changes in magnetic and fractal properties of fractured granites from the Disaster Prevention Research Institute, Kyoto University (DPRI) 500 m drilling core towards the Nojima Fault and of the well-foliated fault gouge are described. Fractal analysis of fractured granites shows that the fractal dimension ( D ) increases linearly toward the gouge zone of the fault. In weakly fractured granites ( D = 1.05–1.24), it was found that the degree of AMS correlates positively with the fractal dimension, suggesting a fracture-related magnetic fabric due to fracturing. In strongly fractured granites ( D = 1.25–1.50), weaker, nearly isotropic AMS is found, suggesting erasure by the fragmentation of the magnetic minerals. Within the fault gouge zone, an isotropic AMS fabric was found, as well as twofold increases in magnetic intensity and susceptibility. These changes reflect the production of new magnetite grains, subsequently confirmed by hysteresis studies, which suggests that fault gouge might be regarded as the source of the regional geomagnetic field contrast along active faults. Thus, AMS is clearly a potentially useful tool for inferring the fracturing texture of magnetic minerals in fractured rocks and detecting active faults from the high susceptibility contrast of fault gouge.     

Alteration and mass transfer inferred from the Hirabayashi GSJ drill penetrating the Nojima Fault, Japan

Abstract Mineralogical and geochemical studies on the fault rocks from the Nojima–Hirabayashi borehole, south-west Japan, are performed to clarify the alteration and mass transfer in the Nojima Fault Zone at shallow depths. A complete sequence from the hornblende–biotite granodiorite protolith to the fault core can be observed without serious disorganization by surface weathering. The parts deeper than 426.2 m are in the fault zone where rocks have suffered fault-related deformation and alteration. Characteristic alteration minerals in the fault zone are smectite, zeolites (laumontite, stilbite), and carbonate minerals (calcite and siderite). It is inferred that laumontite veins formed at temperatures higher than approximately 100°C during the fault activity. A reverse component in the movement of the Nojima Fault influences the distribution of zeolites. Zeolite is the main sealing mineral in relatively deep parts, whereas carbonate is the main sealing mineral at shallower depths. Several shear zones are recognized in the fault zone. Intense alteration is localized in the gouge zones. Rock chemistry changes in a different manner between different shear zones in the fault zone. The main shear zone (MSZ), which corresponds to the core of the Nojima Fault, shows increased concentration of most elements except Si, Al, Na, and K. However, a lower shear zone (LSZ-2), which is characterized by intense alteration rather than cataclastic deformation, shows a decreased concentration of most elements including Ti and Zr. A simple volume change analysis based on Ti and Zr immobility, commonly used to examine the changes in fault rock chemistry, cannot account fully for the different behaviors of Ti and Zr among the two gouge zones.     

Estimating the permeability of the Nojima Fault Zone by a hydrophone vertical seismic profiling experiment

Abstract A multi-offset hydrophone vertical seismic profiling (VSP) experiment was done in a 747 m deep borehole at Nojima Hirabayashi, Hyogo prefecture, Japan. The borehole was drilled to penetrate the Nojima Fault, which was active in the 1995 Hyogo-ken Nanbu earthquake. The purpose of the hydrophone VSP is to detect subsurface permeable fractures and permeable zones and, in the present case, to estimate the permeability of the Nojima Fault. The analysis was based on a model by which tube waves are generated when incident P-waves compress the permeable fractures (or permeable zones) intersecting the borehole and a fluid in the fracture is injected into the borehole. Permeable fractures (or permeable zones) are detected at the depths of tube wave generation, and fracture permeability is calculated from the amplitude ratio of tube wave to incident P-wave. Several generations of tube waves were detected from the VSP sections. Distinct tube waves were generated at depths of the fault zone that are characterized by altered and deformed granodiorite with a fault gouge, suggesting that permeable fractures and permeable zones exist in the fault zone. Tube wave analysis shows that the permeability of the fault gouge from 624 m to 625 m is estimated to be approximately 2 × 10⁻¹² m².     

Crack-filling clays and weathered cracks in the DPRI 1800 m core near the Nojima Fault, Japan: Evidence for deep surface-water circulation near an active fault

Abstract Crack-filling clays and weathered cracks were observed in the Disaster Prevention Research Institute, Kyoto University (DPRI) 1800 m cores drilled from the Nojima Fault Zone, which was activated during the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake). The crack-filling clays consist mainly of unconsolidated fine-grained materials that fill opening cracks with no shear textures. Most of the cracks observed in the DPRI 1800 m cores are yellow-brown to brown in color due to weathering. Powder X-ray diffraction analyses show that the crack-filling clays are composed mainly of clay minerals and carbonates such as siderite and calcite. Given that the top of the borehole is approximately 45 m above sea level, most of the core is far below the stable groundwater table. Hence, it is suggested that the crack-filling clays and weathered cracks in the cores taken at depths of 1800 m were formed by the flow of surface water down to the deep fractured zone of the Nojima Fault Zone during seismic faulting.     

Paleostress orientation from 3-D orientation distribution of microcracks in quartz from the Cretaceous granodiorite core samples drilled through the Nojima Fault, south-west Japan

Abstract The orientations of both healed extension microcracks and microcracks in quartz grains sealed mostly by carbonate minerals were measured from Cretaceous granodiorite core samples drilled along the Nojima Fault, southwest Japan. The preferred orientations of both healed and sealed microcracks consist of approximately three orthogonal sets, (components) A, B and C, in which A strikes NS–NW-SE and dips vertically, B strikes EW–NE-SW and dips vertically, and C is subhorizontal. Both the healed and sealed microcracks were possibly formed by hydraulic fracturing, and the successive release of tensile stress due to pore fluid overpressure in the principal stress directions could have caused this microcracking in mutually orthogonal directions. The quartz grains are also very moderately plastically deformed, which is indicated by the occurrence of kink bands and undulose extinction. The association of healed microcracks and kink bands in the quartz suggests that these microstructures formed under subgreenschist facies conditions (≈300°C) during hydrothermal activity that could have occurred immediately after the emplacement of granodiorite during the Late Cretaceous period. Based on both the preferred orientation of microcracks, and c-axis fabrics of kinked and unkinked grains (so called kink method), it is inferred that the σ₁-and σ₃-axis were oriented horizontally in NS–NW-SE and EW–NE-SW directions, respectively. The inferred paleostress field does not conform to the east–west-trending compression during the Quaternary period, but to the activation of EW–NE-SW-trending, left–lateral strike–slip faults during the Late Cretaceous period in southwest Japan.     

Long-term monitoring of the temperature profile in a deep borehole: Temperature variations associated with water injection experiments and natural groundwater discharge

Long-term temperature monitoring was carried out in a borehole drilled for investigation of the Nojima fault, an active fault in SW Japan, using the distributed optical fiber temperature sensing (DTS) technique. Temperatures in the borehole had been measured every 1 m along an optical fiber cable with a resolution of about 0.1 K over a period of 6 years. Water injection experiments were conducted in this borehole in 1997, 2000 and 2003. Monitoring of the temperature profile was started after the first injection experiment, and the temperature profile remained very stable until the start of the second injection experiment. During the second and third experiments, the temperatures in the borehole dropped due to cooling by the injected water but no appreciable temperature change was observed below about 580 m. It clearly shows that the water leaked out of the hole around this point and the leaking depth is estimated to be about 540 m based on the shape of the temperature profile. After the injection was stopped, the recovery of the temperature to the undisturbed profile was exceptionally slow around the leaking point, resulting in a local temperature anomaly, probably because the water leaking out of the hole had cooled the surrounding formations extensively. A very similar temperature anomaly was observed at the beginning of temperature monitoring, which suggests that water leaked out at the same depth in the first injection experiment as well. Between the second and third injection experiments, the top of the borehole was kept open to allow groundwater discharge for about 1 month in 2000 and 2003. In both periods, groundwater flowed out continuously and the shapes of the observed temperature profiles indicate that the groundwater entered in the hole at the same depth as the leaking point during the injection experiments. The temperature records also show that the rate of discharge had been nearly constant through the two test periods. The water discharge appears to have been little affected by the water injection. These results demonstrate that the optical fiber temperature monitoring system is a very effective tool for hydrological experiments.     

Deformation mechanisms and fluid behavior in a shallow, brittle fault zone during coseismic and interseismic periods: Results from drill core penetrating the Nojima Fault, Japan

Abstract This paper describes the results of petrographical and meso- to microstructural observations of brittle fault rocks in cores obtained by drilling through the Nojima Fault at a drilling depth of 389.52 m. The zonation of deformation and alteration in the central zone of the fault is clearly seen in cores of granite from the hanging wall, in the following order: (i) host rock, which is characterized by some intragranular microcracks and in situ alteration of mafic minerals and feldspars; (ii) weakly deformed and altered rocks, which are characterized by transgranular cracks and the dissolution of mafic minerals, and by the precipitation of zeolites and iron hydroxide materials; (iii) random fabric fault breccia, which is characterized by fragmentation, by anastomosing networks of transgranular cracks, and by the precipitation of zeolites and iron hydroxide materials; and (iv) fault gouge, which is characterized by the precipitation of smectite and localized cataclastic flow. This zonation implies that the fault has been weakened gradually by fluid-related fracturing over time. In the footwall, a gouge layer measuring only 15 mm thick is present just below the surface of the Nojima Fault. These observations are the basis for a model of fluid behavior along the Nojima Fault. The model invokes the percolation of meteoric fluids through cracks in the hanging wall fault zone during interseismic periods, resulting in chemical reactions in the fault gouge layer to form smectite. The low permeability clay-rich gouge layer sealed the footwall. The fault gouge was brecciated during coseismic or postseismic periods, breaking the seal and allowing fluids to readily flow into the footwall, thus causing a slight alteration. Chemical reactions between fluids and the fault breccia and gouge generated new fault gouge, which resealed the footwall, resulting in a low fluid condition in the footwall during interseismic periods.     

Vertical groundwater flux estimation from borehole temperature profiles by a numerical model,RFLUX

We developed a numerical model, RFLUX, which uses the heat tracer method for vertical groundwater flux estimation, and applied it to the Leizhou Peninsula, South China, to provide information to inform local groundwater resource utilization and management. The temperature–depth (TD) profiles of 24 boreholes, along with the observed ground surface temperature (GST) and surface air temperature (SAT) series in recent decades, were collected in this area. Underground TD data demonstrated the capacity to identify groundwater flow patterns, and local GST and SAT data demonstrated a strong correlation with each other over monthly, seasonal, and annual scales. In the RFLUX model, the average GST and SAT series were applied as an upper boundary condition, and a nonlinear initial condition was set using an analytical solution from the literature. The model results of selected TD profiles demonstrated that the annual vertical groundwater flux was about 0.15 m a^?1, which tended to be overestimated if a linear initial condition was used. This model can be easily applied with minor modifications, considering its clear purpose and simplicity.     

Detection of Aseismic Slip on an Inland Fault by Crustal Movement and Groundwater Observations: A Case Study on the Yamasaki Fault, Japan

To understand the detailed process of fault activity, aseismic slip may play a crucial role. Aseismic slip of inland faults in Japan is not well known, except for that related to the Atotsugawa fault. To know whether aseismic slip does not occur, or is merely not detected, is an important question. The National Institute of Advanced Industrial Science and Technology constructed an observation site near Yasutomi fault, a part of the Yamasaki fault system, and has collected data on the crustal strain field, groundwater pressures, and crustal movement using GPS. In a departure from the long-term trend, a transient change of the crustal strain field lasting a few months was recorded. It indicated the possibility of an aseismic slip event. Furthermore, analyses of data from the extensometers at Yasutomi and Osawa observation vaults of Kyoto University, as well as GPS data from the Geographical Survey Institute (GEONET), revealed unsteady crustal strain changes. All data could be explained by local, left-lateral, aseismic slip of the order of 1 mm in the shallow part of the Yasutomi fault.     

High resolution temperature monitoring in a borehole,detection of the deterministic signals in noisy environment

Temperature was monitored as a function of time at several selected depth levels in a slim experimental borehole. The hole is 15 cm in diameter, 150 m deep, and effectively sealed from the influx of ground water by a plastic tube of 5 cm diameter. The mean temperature gradient is 19.2 mK/m. The borehole was drilled in 1993 and has been in equilibrium since then. The data obtained reveal that: (1) the temperature-time series showed a complex, apparently random oscillation pattern with amplitudes of up to 25 mK; (2) irregular temperature variations characterized by larger oscillations may alternate with relatively “quiet” intervals; and (3) the character of the oscillation may vary both in depth as well as in time and the transition between two distinct regimes may be sudden.The Fourier analysis detected “red noise” behavior of the signal but did not highlight any specific peak(s) corresponding to periodicity in the measured temperature series. We employed a variety of techniques (roughness coefficient, local growth of the second moment, recurrence and cross recurrence plots) to reveal the deterministic framework of the system behavior. All above methods were proven to be quite robust in the face of noise, and enabled the discovery of structures hidden in the signals produced by complex natural processes.Statistical analysis suggested the existence of a quasi-periodic intra-hole oscillatory convection. The temperature field in the hole has a dual-frequency structure, in which short period oscillations of about 10–30 minutes are superposed on longer variations of up to several hours. At certain conditions, so far not fully understood, the temperature oscillations may practically stop. The temperature remains within 1–2 mK for a period of several days when the oscillation pattern (convection ?) suddenly resumes.     

Resistivity structure of a seismic gap along the Atotsugawa Fault, Japan

Seismicity along the Atotsugawa Fault, located in central Japan, shows a clear heterogeneity. The central segment of the fault with low-seismicity is recognized as a seismic gap, although a lot of micro-earthquakes occur along this fault. In order to elucidate the cause of the heterogeneity in seismicity, the electrical resistivity structure was investigated around the Atotsugawa Fault by using the magnetotelluric (MT) method. The regional geoelectrical strikes are approximately parallel to the fault in a low-frequency range. We constructed two-dimensional resistivity models across the fault using TM-mode MT responses to minimize three-dimensional effects on the modeling process. A smooth inversion algorithm was used, and the static-shifts on the apparent resistivity were corrected in the inversion process.A shallow, low resistivity zone along the fault is found from the surface to a depth of 1-2 km in the best-fit model across the high-seismicity segment of the fault. On the other hand, the corresponding low resistivity zone along the low-seismicity segment is limited to a shallower depth less than 1 km. The low resistivity zone along the Atotsugawa Fault is possibly due to fluid in the fracture zone; the segment with higher levels of seismicity may have higher fluid content in the fault zone compared with the lower seismicity segment. On a view of the crustal structure, a lateral resistivity variation in a depth range of 3-12 km is found below the fault trace in the high-seismicity segment, while a resistive layer of wide extent is found at a depth of about 5 km below the fault trace in the low-seismicity segment. The resistive layer is explained by less fluid condition and possibly characterized as high rigidity. Differences in the resistivity structures between low and high-seismicity segments of the fault suggest that the seismic gap in the central part of the Atotsugawa Fault may be interpreted as a locked segment. Thus, MT is an effective method in evaluating a cause and future activity of seismic gaps along active faults.The lower crust appears as a conductive zone beneath the low-seismicity segment, less conductive beneath the high-seismicity segment. Fluid is inferred as a preferable cause of the conductive zone in this study. It is suggested that the conductive lower crust beneath the low-seismicity segment is recognized where fluid is trapped by an impermeable layer in the upper crust. On the other hand, fluid in the lower crust may upwell to the surface along the high-seismicity segment of the fault.     

内蒙古乌海断陷岗德尔山西麓断裂晚第四纪特征分析

岗德尔山西麓断裂是乌海断陷的一条控盆边界断裂。通过野外地质调查、构造剖面解析结合浅层物探、地球化学探测,综合对断裂的第四纪活动特征进行分析和研究。结果显示,岗德尔山西麓断裂整体走向NNE,倾角60°~80°,具有成束成带发育特点,是一条全新世活动断裂,表现出以拉张为主的活动特征。     

Distribution of fault rocks in the fracture zone of the Nojima Fault at a depth of 1140 m: Observations from the Hirabayashi NIED drill core

Abstract Characteristics of deformation and alteration of the 1140 m deep fracture zone of the Nojima Fault are described based on mesoscopic (to the naked eye) and microscopic (by both optical and scanning electron microscopes) observations of the Hirabayashi National Research Institute for Earth Science and Disaster Prevention (NIED) drill core. Three types of fault rocks; that is, fault breccia, fault gouge and cataclasite, appear in the central part of the fault zone and two types of weakly deformed and/or altered rocks; that is, weakly deformed and altered granodiorite and altered granodiorite, are located in the outside of the central part of the fault zone (damaged zone). Cataclasite appears occasionally in the damaged zone. Six distinct, thin foliated fault gouge zones, which dip to the south-east, appear clearly in the very central part of the fracture zone. Slickenlines plunging to the north-east are observed on the surface of the newest gouge. Based on the observations of XZ thin sections, these slickenlines and the newest gouge have the same kinematics as the 1995 Hyogo-ken Nanbu earthquake (Kobe earthquake), which was dextral-reverse slip. Scanning electron microscopy observations of the freeze-dried fault gouge show that a large amount of void space is maintained locally, which might play an important role as a path for fluid migration and the existence of either heterogeneity of pore fluid pressure or strain localization.