Audio-frequency magnetotelluric imaging of the Hijima fault, Yamasaki fault system, southwest Japan
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB 2010
Received: 12 September 2007
Accepted: 22 December 2009
Published: 17 June 2010
An audio-frequency magnetotelluric (AMT) survey was undertaken at ten sites along a transect across the Hijima fault, a major segment of the Yamasaki fault system, Japan. The data were subjected to dimensionality analysis, following which two-dimensional inversions for the TE and TM modes were carried out. This model is characterized by (1) a clear resistivity boundary that coincides with the downward projection of the surface trace of the Hijima fault, (2) a resistive zone (>500 Ω m) that corresponds to Mesozoic sediment, and (3) shallow and deep two highly conductive zones (30–40 Ω m) along the fault. The shallow conductive zone is a common feature of the Yamasaki fault system, whereas the deep conductor is a newly discovered feature at depths of 800–1,800 m to the southwest of the fault. The conductor is truncated by the Hijima fault to the northeast, and its upper boundary is the resistive zone. Both conductors are interpreted to represent a combination of clay minerals and a fluid network within a fault-related fracture zone. In terms of the development of the fluid networks, the fault core of the Hijima fault and the highly resistive zone may play important roles as barriers to fluid flow on the northeast and upper sides of the conductive zones, respectively.
A common feature of magnetotelluric surveys is the appearance of changes in the apparent resistivity and/or phase value across the surface trace of an active fault (e.g., Ogawa and Honkura, 1997; Unsworth et al., 1997; Yamaguchi et al., 2001; Ritter et al., 2005). A prominent feature of such changes, and an important factor in characterizing faults, is a distinctive conductive zone termed the fault zone conductor (FZC). For example, Unsworth et al. (1999) reported different imaging characteristics for FZCs located near the ground surface within locked and actively creeping segments of the San Andreas fault, USA. Yamaguchi et al. (2002) imaged contrasting near-surface FZCs within segments of both the large displacement and the little displacement along the Nojima fault, Japan, which were initiated at the time of the 1995 Hyogo-ken Nanbu earthquake.
A second type of FZC has been found in deep regions near active faults. Ogawa and Honkura (1997) performed an audio-frequency magnetotelluric (AMT) survey across the Atera fault, a 66-km-long active fault in Central Japan, and found conductors located near the fault at a depth of 0.5–2.0 km. In a later publication, Ogawa et al. (2002) reported the presence of a deep conductor at mid-crustal depths (15–20 km) near the Itoigawa-Shizuoka Tectonic line, a structure that is much longer than the Atera fault and which crosses Central Japan. The authors of these earlier studies proposed that the conductors may represent localized zones of fluids that occupy areas of enhanced porosity in fracture zones associated with active faults. A similar conductor was found in the lower crust along a major tectonic fault in India (Normada-Son Lineament; Patro et al., 2005). It is interesting that conductors of varying sizes but with the same mechanism of formation have been found near faults.
The Earthquake Research Committee of Japan evaluates the probability of earthquake occurrence at major active faults in Japan, estimating the probability of earthquakes of a given magnitude occurring in the following 30 years. For the southeastern part of the Yamasaki fault system (Biwako and Miki faults), the committee has estimated a maximum probability of 5% for an earthquake with a magnitude of ∼7.3; this places the system in the high group of earthquake occurrence probability. For the northwestern part of the system (the Ohara, Hijima, Yasutomi, and Kuresakatouge faults), the maximum probability of a magnitude ∼7.7 event is 1%, corresponding to the slightly high group of earthquake occurrence probability (Earthquake Research Committee, 2003, 2007).
An FZC that is 1,000 m wide and with an apparent resistivity of 100–400 Ω m has been detected along the Yasutomi fault of the Yamasaki fault system (Electromagnetic Research Group for the Active Fault (ERGAF), 1982); however, the base of the zone has yet to be determined. Handa and Sumitomo (1985) conducted an ELF-MT (extremely low frequency magnetotelluric) survey around the Yasutomi fault with the aim of determining the resistivity structure beneath the fault. Their model is characterized by a large conductive zone (< 1,000 Ω m) that is 6 km wide and 3 km deep, including the surface fault trace; however, the precise nature of the resistivity structure beneath the Yamasaki fault system has yet to be established.
In this paper, we report a detailed two-dimensional (2-D) geoelectrical model across the Hijima fault of the Yamasaki fault system. The model was developed based on data collected during an AMT survey that was carried out with the aim of placing geophysical constrains on the nature of the Yamasaki fault system. This fault system is located close to large cities and is considered to be a high earthquake risk.
3. Data Analysis
4. Model Analysis and Results
A clear resistivity boundary between Site 5 and Site 6 below a depth of 600 m that coincides with the downward projection of the surface trace of the Hijima fault.
A conductive zone (C1; <100 Ω m and minimum resistivity of 40 Ω m) that is 350 m wide and 50–500 m deep beneath the surface trace of the Hijima fault.
A conductive zone (C2; <100 Ω m and minimum resistivity of 30 Ω m) that is 1,700 m wide and 800–1,800 m deep, truncated to the northeast by the Hijima fault.
A highly conductive zone (C3) near the surface between Site 1 and Site 2, with a thickness of 200 m and minimum resistivity of 30 Ω m. However, this zone is poorly resolved because of the large distance between Site 1 and Site 2.
A resistive zone (R; >500 Ω m and maximum resistivity of 1,000 Ω m) at a depth of 100–600 m beneath the area between Site 7 and Site 9.
5.1 Appropriate ranges of resistivity within the characteristic conductive and resistive zones
The optimum model is characterized by two large conductive zones (C1 and C2) and one resistive zone (R). To determine the appropriate range of resistivity in each zone, we constructed representative resistivity models in which the resistivity in each zone was replaced with a number of resistivity values. The forward responses were computed and then compared with the optimum model response and observed data.
For the cases of 80, 100, and 120 Ω m, the responses are similar to the observed data, with only negligible differences. Therefore, these models can be considered to be appropriate in explaining the observed responses. In contrast, for the case of 140 Ω m, the phase values at Site 5 and Site 6 in TE mode are markedly smaller than the observed values in the frequency range between 500 and 5,000 Hz. This model is therefore also considered to be unacceptable. Likewise, the models with resistivities of 180 Ω m and 220 Ω m are considered to be unacceptable because they show greater differences than those recognized in the model with a value of 140 Ω m. Based on this analysis, we conclude that an appropriate resistivity for zone C1 is 80–120 Ω m.
In contrast, in the case of 100 Ω m, the phase value of the TE mode at Site 6–Site 8 is unable to explain the large phase value in the frequency band <200 Hz. This same feature is also recognized in the phase value of the TM mode at Site 7. We therefore conclude that the model with a resistivity of 100 Ω m is unable to explain the observed values. The models with values of 200 and 300 Ω m show larger differences than that of the model in the case of 100 Ω m. We therefore conclude that only the 50 Ω m model yields appropriate resistivity in zone C2.
5.2 Appropriate range of the lower boundary of the conductive zone C2
Separate tests were carried out for those cases in which the boundary is shallower and deeper than their counterparts in the optimum model.
(a) Deeper case We constructed representative models in which we placed the base of the zone at depths of 2.0 and 2.3 km, respectively. We chose 2.3 km as the maximum depth because areas deeper than this are poorly resolved, as stated above. We assigned 50 Ω m to an extended area of zone C2 by deepening the base of the zone. The test results indicate that we cannot resolve the appropriate depth of the lower boundary of zone C2 at depths greater than that of the optimum model because a small difference in response between the representative models and the optimum model is only observed near the lowest range of our measurements for both representative models.
5.3 Sharp resistivity boundary coincident with the downward projection of the surface trace of the Hijima fault
A clear resistivity boundary occurs below a depth of 600 m, coinciding with the downward projection of the surface trace of the Hijima fault; this boundary is interpreted to represent the subsurface fault plane (Fig. 7(c)). Takemura and Suzuki (1996) presented a structural section (oriented N15°E–S15°W) across the Hijima fault, showing a vertical fault plane extending to a depth of ∼500 m. Our result indicates that the fault plane extends to a depth of 1.4 km along the resistivity boundary, possibly deeper.
5.4 Shallow conductive zone
A conductive zone similar to zone C1 described herein has been reported along the Yasutomi fault (ERGAF, 1982). This conductive zone comprises a three-part structure: an inner conductive belt (1,000 m wide and 100–400 Ω m) that contains highly conductive streaks (∼20 Ω m) near the surface trace of the fault, and an outer area that is somewhat resistive but still low (∼ 1,000 Ω m) compared with the surrounding area (>10,000 Ω m). The inner conductive zone along the Yasutomi fault is comparable with zone C1 along the Hijima fault in terms of its location relative to the fault, width, and resistivity. These observations suggest that highly conductive zones are a common occurrence along the Yamasaki fault system.
In order to assess porosity in zone C1, we used the resistivity of groundwater in a well at Yasutomi along the Yasutomi Fault as the reference value; this well is located very close to the study area (Fig. 2(b)). The National Institute of Advanced Industrial Science and Technology (AIST) of Japan have been monitoring the crustal strain field and groundwater pressure in the well since 1991 (Koizumi et al., 2000). The resistivity of groundwater sampled within the well at depths of 254–265 m is reported to be 43.1 Ω m (N. Koizumi, 2007, personal communication). Assuming that these fluids are representative of those along the fault zone, the ρw would then be 43.1 Ω m. To obtain ρ0= 100 Ω m in the shallow conductive zone would require a porosity of 57% for m = 1.5, which is the value for slightly cemented sandstone (Schön, 1998). It is worth noting that this estimation is the maximum porosity value.
5.5 Deep conductive zone
As with zone C1, the high conductivity measured in zone C2 may reflect the combined influences of a fluid network and the presence of clay minerals. Two other mechanisms are generally cited in explaining high conductivity in the upper crust: high temperatures and the presence of highly conductive materials (e.g., graphite and metallic minerals). The former mechanism is unlikely to play a significant role in the case described here because heat flow in the study area is <60 mW/m2 (Furukawa et al., 1998), which is among the lowest values in the area shown in Fig. 1(a). The significance of the latter mechanism cannot be assessed due to a lack of data.
For a fluid network to form in zone C2, it is important that the fault core of the Hijima fault and the resistive zone R act as barriers to fluid flow, which permeates from the surface along north-dipping strata (Takemura and Suzuki, 1996) or is derived from depth via the highly permeable damage zone that exists along the fault. The northeastern end of zone C2 is truncated along a plane that coincides in location with the downward projection of the surface trace of the Hijima fault; this sharp boundary suggests the presence of an impermeable zone along the Hijima fault. Ritter et al. (2003) described a similar conductor along the Araba fault of the Dead Sea Transform, Jordan. These authors proposed a model entailing a strong lateral contrast in conductivity where a highly conductive layer at a depth of 1.5–3 km is truncated at a position coinciding with the downward projection of the surface trace of the fault, which then acts as an impermeable barrier at depth. Caine et al. (1996) proposed a conceptual model of fault zone architecture and related permeability structure that consists of three components: the fault core, a damaged zone, and the protolith. In this model, most of the displacement is accommodated in the fault core, which acts as a barrier to fluid movement. The broad and highly permeable damaged zone represents a network of subsidiary structures that bound the fault core, while the protolith consists of undeformed country rocks.
In our study area, the resistive zone R above zone C2 may play an important role as a cap rock that defines the upper boundary of C2. If such a cap rock were not to exist, fluid in C2 (meteorological water and/or groundwater) would not be confined to the zone; rather, it would be dispersed throughout a wide area, with part of it reaching the surface.
5.6 Possibility of along-strike variation in the resistivity structure
In our explanation of the enhanced conductivity observed in zone C2, the resistive zone R plays an important role as a cap rock; as such, the immediate geological setting of the Hijima fault is an important factor in the development of a resistivity structure along the fault. The Mesozoic sediments (chert and mudstone of the Tamba zone) that correspond to the resistive zone are only exposed on the southern side of the Hijima fault; Paleozoic sediment (sandstone and slate of the Ultra-Tamba zone) occur around the Mesozoic sediments. In contrast, Paleozoic sediments occur on both sides of the Yasutomi fault; rhyolite and gabbro are distributed along the Ohara fault; rhyolite and Paleozoic sediment are distributed along the Kuresakatouge fault (Fig. 2(b)). These variations in geological setting along the strike of the Yamasaki fault system are expected to cause variations in resistivity structure. To clarify the factors that determine the resistivity structure of a fault system, it is important to identify both similar and contrasting features of resistivity structure along different fault systems. In a previous seismological study, Shibutani (2004) showed that both seismic activity and the b-value vary along the Yamasaki fault system. We have also proposed the existence of along-strike variations in structure and/or condition (e.g., distribution of fluid) beneath the fault system. Despite these differences, however, both the Hijima fault and Yasutomi fault are characterized by a shallow conductive zone along the surface fault trace.
Follow-up MT and resistivity surveys of those that have previously been undertaken along many profiles across the fault system would clarify the nature of along-strike variations in the subsurface structure along the Yamasaki fault system.
An audio-frequency magnetotelluric survey was undertaken along a profile across the Hijima fault, Yamasaki fault system, to image the subsurface structure. The detailed 2D inversion model for both TE and TM modes shows an electrical model of the Hijima fault. First, a clear resistivity boundary is recognized at a depth greater than 600 m at a position coinciding with the downward projection of the surface trace of the Hijima fault. This boundary represents the subsurface fault plane, indicating that the Hijima fault is near-vertical in orientation to at least a depth of 1.4 km, possibly deeper. Second, we imaged a highly conductive zone (<100 Ω m) that is 350 m wide and 50–500 m deep, including the surface trace of the Hijima fault. We interpreted this zone as reflecting the presence of clay minerals and a fluid network within a fracture zone generated by fault movement. A similar conductive zone occurs along the Yasutomi fault, suggesting that shallow conductive zones are common along the Yamasaki fault system. Finally, we newly identified a deeper conductive zone (<100 Ω m) that is 1,700 m wide and 800–1,800 m deep. The fault-core of the Hijima fault and a highly resistive zone form impermeable boundaries on the northeast side and upper surface of the zone, respectively. The fluid in this deep zone, which permeates downward from the surface or is derived from deeper levels via the damaged zone of the fault, occurs as a fluid network, enhancing conductivity along with the presence of clay minerals.
We thank the many private landowners in the area around the Hijima fault for allowing us to take measurements upon their land. This study was supported by the Joint Research Project (Exploratory Subject: 17H-1) of the Disaster Prevention Research Institute, Kyoto University. Financial support was also provided by the Research Center for Urban Safety and Security, Kobe University. We gratefully acknowledge Dr. T. Shibutani (Kyoto University) for providing hypocentral data, and Dr. N. Koizumi (AIST) for providing electrical conductivity data for groundwater in the well at Yasutomi station. We thank Mr. S. Kato (The Museum of Nature and Human Activities, Hyogo) for providing much useful geological and geomorphological advice that proved helpful in selecting the observation sites. The constructive comments from Dr. S. Takakura and Dr. P. K. Patro are deeply appreciated. Most of the figures were drawn using Generic Mapping Tools (Wessel and Smith, 1998).
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