Magnetotelluric transect of Unzen graben, Japan: conductors associated with normal faults
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KeywordsMagnetotellurics Resistivity structure Unzen volcano Normal fault Chijiwa Fault
This magmatic pathway was investigated by conducting a seismic survey. A north–south seismic reflection survey across Unzen graben revealed strong reflectors at a depth of 3 km below sea level, which corresponds to 1 km above pressure source B (Fig. 1a and Fig. 8a; Matsumoto et al. 2012). The reflectors are interpreted as the top of the magmatic chamber. However, this interpretation is based solely on reflector geometry. To better understand and estimate the physical properties of the subsurface structure, other types of geophysical analysis are required.
Electrical resistivity structure provides useful information on volcanoes, because it is highly sensitive to magma, hydrothermal fluids, and hydrothermally altered clay minerals (e.g., Nurhasan et al. 2006; Aizawa et al. 2009a, b; Bertrand et al. 2012; Hata et al. 2016; Seki et al. 2016). The resistivity structure of Unzen volcano has been previously estimated using magnetotelluric (MT) analysis and time-domain electromagnetic (TDEM) methods (Kagiyama et al. 1999; Komori et al. 2013; Srigutomo et al. 2008). Using 1-D analysis, Kagiyama et al. (1999) suggested the presence of shallow groundwater and deep magma and assessed magma–groundwater interaction during the 1990–1995 eruptions. Srigutomo et al. (2008) used 1-D analysis to suggest that volcanic gases are being supplied to a water-rich layer through a system of faults. Using 2-D modeling, Komori et al. (2013) imaged a relatively resistive region beneath the younger lava domes and surrounding conductive zones. By comparing the model with scientific drilling data, they concluded that the conductive hydrothermal zone played an important role in heat and mass transfer within the Unzen magmatic system. These previous studies highlighted the importance of the shallow conductive hydrothermal zone in terms of volcanic activity. However, data from these surveys were processed using 1-D and 2-D inversions and were not sensitive to deeper structure beneath the hydrothermal zone. Furthermore, the site distribution in these studies was too sparse to investigate finer structures such as faults and conduits. To address these shortcomings, we performed a broadband MT survey along the same line as the seismic reflection survey by Matsumoto et al. (2012). Although the MT data were obtained along a single profile, we performed 3-D inversion and considered topography because of strong 3-D features in the MT data and steep topography. Then, we compared the resistivity profile with the seismic reflection profile of Matsumoto et al. (2012). Although MT and seismic reflections have been jointly interpreted in tectonically active or volcanic regions (e.g., Unsworth et al. 1997; Brasse et al. 2002; Comeau et al. 2016), correlating resistivity structure with a seismic reflection profile at fine spatial resolution is rare, especially at shallow levels beneath a volcano. We show how these two physical properties correlate with each other and discuss the relationships between magma, faults, and hydrothermal systems.
In Unzen graben, many east–west normal faults have developed under north–south tensile stress (Fig. 1). Our MT survey line intersects the Chijiwa Fault (CF), the Kusenbu Fault (KF), and the Oshidorino-Ike Fault (OF), as well as three minor unnamed faults (Matsuoka et al. 2005; Tsutsumi 1987, 2015). The CF, located near the northern boundary of the graben, is the longest (19 km at the surface) and the most active fault in this region. The average displacement rate at the surface is estimated to range from 2.8 m/ka in the east to 8.8 m/ka in the west, with total surface displacement ranging from 38 m (east) to 180 m (west). Two other major faults (KF and OF) are located in the graben, with respective surface lengths of 5.0–8.5 km and average surface displacement rates of 0.5–4.0 m/ka, respectively (Matsuoka et al. 2005; Tsutsumi 1987). The graben is thought to extend to a depth of 1 km b.s.l. and is filled with volcanic products (Hoshizumi et al. 1999). The basement of the graben is mainly high-density rock, interpreted as Paleogene sediments. Approximately 2 km to the east of the profile, lava domes associated with the younger stages of the Unzen complex (Nodake and Myokendake) and currently active regions (Fugendake and Heisei Shinzan) are exposed (Fig. 1b). The line of the MT profile spans relatively old (300–150 ka) volcanic products.
Broadband MT survey
Magnetotellurics is a passive electromagnetic geophysical method that measures temporal variations in induced natural electromagnetic fields on the Earth’s surface to derive the subsurface resistivity structure. We conducted an MT survey along an approximately 9-km-long north–south profile (Fig. 1b, Additional file 1: Figure S1). Measurement sites were located along the same profile as the seismic reflection survey by Matsumoto et al. (2012). To image detailed structures, our stations were closely spaced at 300–400-m intervals. The survey consisted of 23 telluric sites recording only electric field variations and 4 MT sites recording both electric and magnetic field variations. At each MT site, a Metronix ADU07e logger was connected to three induction coils and five electrodes (one of which was used as a common ground) to measure the magnetic and electric fields, respectively. At telluric sites, electric fields (Ex and Ey) were recorded using ELOG1 K loggers (NT System Design). Observations were conducted between April 25 and May 8, 2018. The typical recording duration at a given site was 7–10 days. The sampling frequencies were 32 Hz (00:00–23:50 UT) and 1024 Hz (17:00–18:00 UT).
2-D and 3-D inversions
Before 3-D inversion, we conducted 2-D inversion using the code of Ogawa and Uchida (1996), employing only the Zxy impedances. The regional strike of the area has roughly E–W orientation according to geological evidence (i.e., graben and surface fault traces, such as Chijiwa Fault) (i.e., Hoshizumi et al. 1999). On the assumption that the 2-D strike is the same as the surface traces of faults, Zxy corresponds to the transverse magnetic (TM) mode (electric current crosses the strike), while Zyx corresponds to the transverse electric (TE) mode (electric current flows along the strike). In this study, we used TM mode data, which is sensitive to lateral discontinuities of the structure and is relatively insensitive to the 3-D structure (e.g., Siripunvaraporn et al. 2005a).
We selected TM mode data of 20 periods (0.004–204.8 s) and manually eliminated several outliers, particularly at periods > 12.8 s. In the 2-D inversion, we take into account that the Zxy at telluric sites are calculated using magnetic fields at different sites. The apparent resistivity of the initial model is 100 Ωm with an error floor of 10%. Then, the final model was arrived at after 17 iterations. The RMS was 0.75. Figure 5a shows the best-fit 2-D structure.
In the case of strong 3-D features, 3-D inversion usually produces more reliable resistivity structures than 2-D inversion. The merits of applying 3-D inversion to single MT profile were first demonstrated by Siripunvaraporn et al. (2005a) using synthetic data. This procedure was consequently applied to real field data by Patro and Egbert (2011), Bertrand et al. (2013), Brasse et al. (2015), and Beka et al. (2016), and artifact structure produced in 2-D inversion can be eliminated in 3-D scheme. We performed 3-D inversion using the WSINV3DMT code, which is based on the data-space variant of Occam’s 3-D inversion scheme (Siripunvaraporn et al. 2005a, b; Siripunvaraporn and Egbert 2009).
The 2-D and 3-D models along the survey line are moderately similar (Fig. 5a, b). Both models show the first resistive layer is on top of the conductive layer, and the third layer displays high resistivity. However, significant differences are found at a depth of the third layer. Two conductive bodies in the 2-D model (black dashed lines in Fig. 5a) are not apparent in the 3-D model. The discrepancy between the models probably arose because of the 2-D modeling that was applied to a natural 3-D structure (Siripunvaraporn et al. 2005a; Brasse et al. 2015), especially for longer-period data. Thus, in this study, we focus on the 3-D structure.
Upper resistive layer (RL1)
Middle conductive layer (CL1)
Beneath RL1, at ~ 0–2 km beneath sea level, a conductive layer (< 50 Ωm CL1) is identified. We interpret CL1 as a hot, groundwater-rich layer characterized by hydrothermal alteration, as also indicated by previous studies. Density and magnetization distributions show that CL1 is correlated with hydrothermally altered Neogene rock characterized by low magnetization (1 A/m) and high density (Inoue 2013; Okubo et al. 2006).
CL1 contains a relatively low-resistivity zone; in particular, beneath the CF, a zone of low resistivity (< 10 Ωm) extends to greater depth. This conductor is separately referred to as C1 and discussed below. Conductive zones underlying faults have been reported previously and are interpreted as fractured regions that are rich in crustal fluids (Becken and Ritter 2012; Karas et al. 2017; Kaya et al. 2013; Xiao et al. 2017; Azeez et al. 2018). Considering the relatively conductive zone found beneath the faults at the same depth as RL1, as well as identification of fault planes in the seismic reflection profile (Fig. 8b), we interpret this region as a deeper extension of the fracture zone, given that fault zones are relatively conductive. In Figs. 5b and 8b, the white dashed line indicates the fractured zones associated with the fault inferred from the location of relatively conductive zones at the depth of RL1 and CL1.
Broad, highly conductive body (C1)
C1 is the dominant feature of the resistivity structure. The upper part of C1, approximately at the depth 500–700 m below sea level, corresponds to CL1. Although we could not examine the off-profile extension of C1, it is located near the CF (Chijiwa Fault) and pressure source B by Kohno et al. (2008). Some MT studies have suggested that a dominant conductive body can be interpreted as a magmatic body (e.g., Diaz et al. 2015; Hill et al. 2015; Hata et al. 2016; Brasse et al. 2015; Comeau et al. 2016; Cordell et al. 2018), but C1 is too large and shallow to represent a magma reservoir. C1 occupies a volume of ~ 10 km3, compared with 0.2 km3 of magma erupted during the period 1991–1995 (Nakada and Motomura 1999). The lack of geothermal activity above C1 is also inconsistent with the interpretation of C1 as a magma reservoir. According to Kohno et al. (2008), pressure source B is located ~ 4 km beneath sea level. If C1 represents a volume of interconnected melt, the pressure source would be observed at the top of C1, as a result of its buoyancy, rather than far beneath the top of C1. In this study, we interpret C1 as an interconnected fracture network around the CF, to which volatiles are supplied from pressure source B.
It is commonly thought that faults act as pathways for volatiles (e.g., Becken et al. 2011; Becken and Ritter 2012; Ichihara et al. 2011; Aizawa et al. 2016). The eruptions of Unzen volcano during 1991–1995 were effusive (i.e., mild) and were considered to have resulted from effective degassing during magma ascent. The degassing might have been maintained by volcanic gas discharge along many E–W trending faults (e.g., Komori et al. 2014), especially along the Chijiwa Fault, which is the longest and most active fault in Unzen graben. Indeed, groundwater gas analysis around Unzen volcano suggests that the CF provides a surface escape route for magmatic CO2 (Ohsawa et al. 2002).
C1 extends to a greater depth than the conductive zones beneath other faults, and the resistivity of C1 is lower than that of the zones beneath other faults. Given that C1 is interpreted to have originated by fracturing related to fault activity, it is possible that earthquakes have occurred repeatedly on the Chijiwa Fault.
Vertical resistive body (R1)
A comparison of the seismic reflection profile and the resistivity structure reveals a correlation between the sub-vertical high-resistivity zone (R1) and a narrow vertical zone of low reflectivity in the center of the survey line (Fig. 8). We interpret R1 as a cooled conduit through which dykes intruded to approach the summit. This interpretation is consistent with that of Matsumoto et al. (2012). The scientific drilling at site USDP4 (1.5 km east of the MT profile) revealed relatively high resistivity (~ 100 Ωm), low porosity (< 0.25), and high density (2.5 g/cm3) within the conduit due to the presence of solidified dykes related to eruptions of various ages (Ikeda et al. 2008; Sakuma et al. 2008). The presence of consolidated magma, low porosity, and/or high-density rocks are all plausible explanations of the high resistivity. Many MT studies have made similar interpretations of high-resistivity zones (e.g., Aizawa et al. 2008, 2009b, 2014 Diaz et al. 2015 Bedrosian et al. 2018). The result of the scientific drilling supports our interpretation of R1.
Although it is not confirmed that R1 is related to the latest (1990–1995) eruptions, the 500-m horizontal width of dykes confirmed by the drilling study is consistent with the horizontal extent of R1. The drilled conduit consists of multiple dykes and veins from various eruption stages, and the widths of individual dikes are up to 40 m. The dikes in the deepest part of the drill hole (~ 100 m below sea level) are believed to represent the most recent conduit, as inferred from petrological analysis, and these rocks cooled to < 200 °C within nine years after the eruptions (Nakada et al. 2005; Ikeda et al. 2008). Hydrothermal fluid circulation was determined to be the most likely cause of cooling (Fujimitsu et al. 2008). The moderately conductive layer CL1 and highly conductive body C1 in this study, interpreted as hydrothermal-water-rich layers, may be responsible for cooling the conduit (R1).
Relationship between resistivity and seismic reflection
A comparison of the resistivity structure with the seismic reflection profile of Matsumoto et al. (2012) revealed that RL1, CL1, and C1 correlate with areas of strong seismic reflectivity (Fig. 8). On the other hand, the resistive zone beneath CL1 is imaged as zones of weak reflectivity. On the basis of these spatial correlations and the interpretations in Sects. Upper resistive layer (RL1)–Vertical resistive body (R1), we suggest that an interconnected pore network controls the relationship between electric resistivity and seismic reflectivity. Several qualitative interpretations have been made. Joint interpretation of MT and seismic data suggests that fluid-rich zones are indicative of high porosity with good connectivity and are electrically conductive (Hyndman and Shearer 1989; Jones 1987; Unsworth et al. 1997; Yan et al. 2016). Differences in porosity may create large contrasts in elastic properties, thus enhancing reflectivity in areas of high porosity. The multilayered structures characteristics of volcanic deposits may also contribute to high seismic reflectivity since it has high porosity, such as found in RL1. CL1 and C1 are also interpreted as hydrothermal-water-rich zones marked by high porosity and maintained interconnectivity. Consequently, both of these areas show strong reflectivity. In contrast, the resistive zone beneath CL1, which might correspond to the basement of Unzen volcano, is interpreted as a zone of low porosity and consequently shows weak reflectivity. Similar relationships between resistivity data and seismic profiles, and comparable interpretations, have been reported for paleo-magmatism in Sweden (Hubert et al. 2013; Yan et al. 2016).
The estimated dip, depth extent, and the number of faults detected in this study differ slightly from those estimated by Matsumoto et al. (2012). For example, the KF is imaged as sub-vertical in the seismic reflection profile, whereas the MT model suggests it dips toward the south. Seismic profiles show offsets in reflection patterns, allowing fault planes to be identified by their displacement (e.g., Matsumoto et al. 2012). In contrast, MT cannot image fault planes but can identify voluminous fracture zones (e.g., Unsworth and Bedrosian 2004; Yamaguchi et al. 2010; Karas et al. 2017). It is possible that fault planes and their associated fractured zones have different dip angles.
The resistivity structure beneath our survey line basically shows three-layer structure. The first layer is near-surface high-resistivity zone that corresponds to a zone of cold groundwater with old Unzen deposits. This layer is cut by four faults. The second layer is the mid-conductive layer associated with hydrothermal zone which shows relatively conductive values near the faults. The third layer is a bottom resistive zone related to the basement of the Unzen graben. At the same depth as the third layer, we found two distinctive anomalies. One is a highly conductive zone (C1) beneath Chijiwa Fault. Another is highly resistive zone R1 in the center of profile. C1 is interpreted as zone of interconnected fracture network around Chijiwa Fault, to which volcanic fluids are supplied from deeper pressure source (B). Given that C1 extends to a greater depth than the conductive zones beneath other faults, and its resistivity is lower than that of other zones, it is possible that earthquakes have occurred repeatedly on the Chijiwa Fault. R1 is interpreted as the cooled conduit of Unzen volcanism and consists of lava dykes and veins related to eruptions of various ages. Based on the correlation between the resistivity and seismic reflection structures, we conclude that the interconnected pore network controls the relationship between resistivity and reflections.
AT, KA, and YT designed the field survey. AT carried out the analysis and drafted the manuscript. All authors contributed to MT data acquisition and discussion. All authors read and approved the final manuscript.
We thank forestry officers for granting access to conduct field campaigns. We appreciate S. Matsumoto and H. Shimizu for extensive discussions and suggestions while interpreting the results. We appreciate H. Tsutsumi, M. Ueda, and Geospatial Information Authority of Japan for supplying the fault data. SEVO students Y. Hayashida, K. Takeishi, Y. Yuasa, S. Watanabe, and Y. Odasaki are thanked for their help with the MT survey. Our gratitude goes to Y. Ogawa for supplying 2-D inversion code and W. Siripunvaraporn for supplying his 3-D inversion code. We used the computer systems of the Earthquake and Volcano Information Center of the Earthquake Research Institute, the University of Tokyo. Comments from two anonymous reviewers were helpful in improving the manuscript. This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program, and funded by JSPS KAKENHI (JP15K17751). AT sincerely thanks the LPDP Scholarship (Indonesian Education Scholarship) for funding her education at Kyushu University.
The authors declare that they have no competing interests.
Availability of data and materials
The datasets are available from the corresponding author AT on reasonable request.
Consent for publication
Ethics approval and consent to participate
This study was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, under its Earthquake and Volcano Hazards Observation and Research Program and KAKENHI (Grant No. 15K17751). AT sincerely thanks the LPDP Scholarship (Indonesian Education Scholarship) for funding her education at Kyushu University.
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