Journal of Volcanology and Geothermal Research 175 (2008) 35–44

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Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s

Numerical model of the hydrothermal system beneath Unzen Volcano, Japan Yasuhiro Fujimitsu a,⁎, Sachio Ehara a, Ryosuke Oki b, Ryohei Kanou b a b

Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

A R T I C L E

I N F O

Article history: Accepted 24 March 2008 Available online 5 June 2008 Keywords: numerical model hydrothermal system heat source geothermal fluid Unzen Volcano Shimabara Peninsula

A B S T R A C T Drilling of the volcanic conduit in the Unzen Scientific Drilling Project (USDP) was completed in 2004. Some conduit materials of the 1990–95 eruption were encountered at the bottom of Well USDP-4 (150 m below sea level), and a bottom temperature of about 200 °C was estimated using logging data, although before drilling a temperature of about 700 °C had been inferred. Accordingly, conduit cooling from the initial temperature (850 °C) to the estimated temperature (about 200 °C) was evaluated by numerical simulation. The drilling provides constraints for the numerical model. The drilling indicates that the N–S width of the conduit of the latest eruption is 20 to 30 m and that it occupies a zone of about 300 m, which includes conduits of past eruptions. The process of cooling in the conduit, from an initial temperature of 850 °C in 1995 (the end of the eruption) to 200 °C in 2004 (completion of the conduit drilling), was replicated in models with permeabilities of 1 and 10 mdarcys. This result demonstrates that a highly permeable volcanic body surrounding a small conduit is required to explain the estimated bottom temperature. Our study also aimed to use a numerical simulation to construct a comprehensive hydrothermal model beneath Unzen Volcano. There are four large geothermal systems in the Shimabara Peninsula (Obama hot springs, Unzen fumarolic field, Shimabara hot springs and the West Unzen High Temperature Body [WUHTB]). Three pressure sources (“Sources A”, “B” and “C” from shallow to the deep) were determined by geodetic data during the 1990–95 eruption. Source C is located at about 8 km deep at WUHTB, and is considered to be a magma body. We attempted to explain the existence of the four geothermal systems from the large-scale structures (the topography of the Shimabara Peninsula and Unzen Graben) and the various heat sources. We first set a heat source around Source C and changed its position and size. This numerical model produced the upflow zones at the Obama and Shimabara hot springs and WUHTB; however, the Unzen fumarolic field became a recharge area. This result indicates that another heat source is required to explain the Unzen fumarolic field and that two heat sources beneath WUHTB and the Unzen fumarolic field are involved in the formation of the four hydrothermal systems in the Shimabara Peninsula. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Unzen Volcano, located on the Shimabara Peninsula, Nagasaki Prefecture, Western Kyushu (Fig. 1), is one of Japan's active volcanoes. The latest period of eruption began in 1990 and finished in 1995. During the period, a lava dome appeared at a crater about 500 m east of the summit of Mt. Fugen. The summit of the lava dome is now about 130 m higher than that of Mt. Fugen (Fig. 2). The geothermal development promotion survey in the western Unzen area had been conducted by the New Energy Development Organization (NEDO) from 1984 to 1987. In this survey, many kinds of geological (geological reconnaissance, XRD, K–Ar dating, fission-track

⁎ Corresponding author. Fax: +81 92 802 3368. E-mail address: [email protected] (Y. Fujimitsu). 0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.03.032

dating, etc.), geophysical (CSAMT, SP, Schlumberger survey, gravity survey, etc.), and geochemical (Hg and CO2 in soil gas, componential analysis etc. in water of hot springs) investigations, investigation drillings (three 400 m-class holes and seven 1000 m-class wells), hydraulic well tests with well-core investigations, and environmental research (microseismic activity, meteorological observation, etc.) were carried out, and the results were compiled as a report of 1060 pages (NEDO, 1988). After the 1990–95 eruption, the Unzen Scientific Drilling Project (USDP) has been conducted by the Science and Technology Agency (STA) and the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan since 1999. In Phase I (April 1999 to March 2002) of the project, two flank holes and a pilot hole were drilled in the lead up to the conduit drilling (Phase II). In Phase II (April 2002 to March 2005), the drilling of the conduit of the 1990–95 eruption by Well USDP-4 was completed in 2004. We participated in both phases of USDP, and as part of the project studied the conduit cooling after the latest eruption of Unzen Volcano

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Fig. 1. Location of Unzen Volcano and the active volcanoes in Kyushu Island.

Fig. 2. Topographical map of Unzen Volcano and the locations of four hydrothermal systems in Shimabara Peninsula with horizontal extent of the modeling areas. a: Mt. Fugen (1359 m asl), b: the summit of the lava dome (1486 m asl), 1: Obama hot springs, 2: Unzen fumarolic field, 3: Shimabara hot springs, 4: West Unzen High Temperature Body (WUHTB), Solid rectangle: the conduit cooling model, Broken line rectangle: the comprehensive hydrothermal model. Line A–B indicates the position of the slice in Fig. 13.

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and the development of the hydrothermal systems in the Shimabara Peninsula. 2. Conduit cooling after the 1990–95 eruption 2.1. Summary of temperature logging at Well USDP-4 The wellhead of Well USDP-4 is sited about 1.3 km north of the summit of Mt. Fugen; the hole length is 1995.75 m. Some conduit materials of the 1990–95 eruption were encountered at the bottom of USDP-4 (150 m below sea level), and a true formation temperature of about 200 °C at the bottom was estimated from logging data by using the Horner plot method (Sakuma et al., 2005; Kajiwara et al., 2005), although a temperature of about 700 °C had been inferred in a previous study (Fujimitsu and Kanou, 2003). Therefore, the first aim of our study is to assess predisposing factors for the rapid cooling of the conduit by inferring permeability structure of Unzen Volcano with numerical simulation of conduit cooling from the initial temperature (850 °C) to the estimated temperature (about 200 °C). 2.2. Numerical model In this study, we have to calculate the behavior of supercritical fluid in the ground because of the high temperature conduit. Therefore, we used HYDROTHERM (Hayba and Ingebritsen, 1994) Version 2.2 for numerical modeling, because this computer program calculates the three-dimensional, multiphase flow of pure water and heat, over a temperature range of 0 to 1200 °C and a pressure range of 0.5 to 10,000 bars, by a finite difference method. The summit of Mt. Fugen was set at the center of the model; the boundaries extend horizontally 5 km (E–W) by 4.6 km (N–S) (Fig. 2) and vertically from 3 km below sea level to the ground surface (Fig. 3). The model was divided into two layers, made up of Unzen Volcanic Rocks (Layer I) and a basement rock layer (Layer II). Layer I was divided into vadose and watersaturated zones in this study (Fig. 4). Each layer has unique thermal and hydrological properties as shown in Table 1, using values based on

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NEDO (1988). The vadose zone was simulated by setting a low porosity for Layer I, because HYDROTHERM can only treat the behavior of H2O as liquid and gas. Before transient analysis, we constructed a steady state model with no magma penetration in order to calculate the background temperature and pressure distributions of the region. The bottom was made impermeable with a constant heat flux of 120 mW/m2 as a boundary condition constraint. The atmospheric pressure and annual average temperature of 15 °C were assigned to the ground surface. Lateral boundaries were thermally insulated and impermeable. The numerical model of a previous study of ours (Fujimitsu and Kanou, 2003) was used as the starting point in this study, and then revised using the new information obtained from the conduit drilling. In the model of the previous study, the conduit width in the N–S direction is 100–300 m, because this width is based on the results of seismic survey data by Shimizu et al. (2002). They conducted seismic reflection surveys in 2001, and detected a vertical low-reflecting zone of about 200–300 m width on a reflection profile of an N–S traverse line that passed through the western side of Mt. Fugen. This lowreflecting zone was inferred as the conduit of Unzen Volcano. However, the result of geological research on the core data of USDP4 concluded that the latest conduit probably has a N–S width of about 20 to 30 m at 150 m below sea level, and exists in the conduit zone of about 300 m N–S width that includes conduits of past eruptions, although USDP-4 did not completely pass through the conduit of the latest eruption (Nakada et al., 2005; Goto et al., 2005; Sakuma et al., 2005). Therefore, the revised model has a conduit of 25 m width at the center of a conduit zone of 300 m width in N–S direction (Fig. 5). 2.3. Transient analysis In the transient analysis, we calculated the cooling of the conduit, which had an initial temperature of 850 °C, from 1995 (the end of the eruption) to 2004 (completion of the conduit drilling), and we monitored the temperatures of the conduit and the surroundings at the bottom of USDP-4. Two cases were calculated for conduit widths

Fig. 3. 3-D finite difference blocks for the conduit cooling model.

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Fig. 4. E–W slice No. 6, which includes the summits of the lava dome and Mt. Fugen (left figure), and N–S slice No. 15, which includes the summit of the lava dome (right figure), of the conduit cooling model.

in an E–W direction (100 and 300 m), because there is no information to constrain these widths. The E–W width of the conduit zone was assumed to be the same as the conduit width (Fig. 5). The steep westdipping orientation of the conduit on the E–W slice No. 6 is indicated in Fig. 6, and it was estimated from the hypocenter distribution of volcanic earthquakes that occurred before and during the eruption. Umakoshi et al. (1994) proposed a magma ascent path from the point at about 13 km depth beneath the western shore of the Shimabara Peninsula, rising to the east with an angle of 40–50°, based on the distribution of volcano-tectonic (VT) earthquakes during the seismic observation period from 1989 to 1993. Nakada et al. (1999) classified the seismic events that had occurred from 1989 to 1996 into six types; VT earthquakes, high-frequency earthquakes, low-frequency (LF) earthquakes, an explosion earthquake, volcanic tremors and seismic waves excited by rock fall or pyroclastic flow events. And they indicated the hypocenter region of isolated volcanic tremors that were the most significant precursors of the eruptions and that of LF earthquakes during the period of growing of the lava dome. The hypocenter region of the isolated tremors was located about 1.5 to 0.5 km below the summit of Mt. Fugen, western side of the lava dome, and that of LF earthquakes extended from the lava dome to the depth of about 0.5 km descending to the west. Nakada and Shimizu (2000) concluded that the hypocenter regions of the tremors and the LF earthquakes coincide with the conduit of the 1990–95 eruption considering that the isolated tremors occurred from ascending magma coming into contact with an aquifer and/or vesiculation and degassing of the magma, and that the LF earthquakes were caused by fracturing when a new lobe was intruding, reflecting breaking of the carapace of the lava dome due to increasing excess pressure inside the dome or the top of the conduit. Other conditions of the model were the same as those of the steady state model.

Initially, the permeability of the volcanic body (both Layers I and II except the blocks of the conduit zone) was fixed at 1 mdarcy; the conduit zone was set as a parameter, with assigned values of 1, 10, 30 and 50 mdarcys. However, the temperature of the monitoring block (Fig. 6) proved to be insensitive to the permeability of the conduit zone and was higher than the observed data; therefore, the permeability of the volcanic body was also treated as a parameter, and the same values as the conduit zone were assigned. In the case of the conduit width of 300 m in the E–W direction, the conduit temperature of the monitoring block did not cool down to the observed data even if permeability of the volcanic body was increased. Therefore, we evaluated only the case of a 100 m wide conduit.

Table 1 Rock properties of each layer for the conduit cooling model Layer I (Unzen Volcanic Rocks)

Specific heat (J/kg K) Density (kg/m3) Thermal conductivity (W/m K) Porosity (−) ⁎W–S Zone: Water-saturated Zone.

Vadose Zone

W–S Zone⁎

8.0 × 102 2.5 × 103 1.9 0.1, 0.05

8.0 × 102 2.5 × 103 1.9 0.2

Layer II (Basement Rock)

8.0 × 102 2.5 × 103 2.6 0.1

Fig. 5. Block configurations for the conduit of the 1990–95 eruption in the conduit zone. (a): the case of the conduit width of 100 m in E–W direction, (b): the case of the conduit width of 300 m in E–W direction. It is assumed that the conduit and the conduit zone are of identical width in E–W direction.

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Fig. 6. Block layout of the conduit on the E–W slice No. 6 in the case of the conduit width of 100 m in an E–W direction. The white rectangle represents the monitoring block, and the temperature distributions in the area of the broken line rectangle are indicated in Fig. 7.

2.4. Discussion In the simulation, high permeability alone in the conduit zone was insufficient to completely cool the conduit temperature of the monitoring block. Therefore, we inferred that the volcanic body also had a higher permeability than we initially assumed. Moreover, according to the simulation result, the E–W width of 300 m was too large to obtain the measured temperature. Hence, we estimated that the conduit possibly has an E–W width of 100 m. Table 2 shows the simulated temperature at the center of the conduit (the center of the monitoring block) and the interpolated temperature of the conduit surface (northern side surface of the monitoring block) for each permeability value. As a result, a temperature of about 200 °C was obtained in the conduit of the monitored block under permeability conditions of 1 and 10 mdarcys (Fig. 7) in the case of the conduit width of 100 m in an E–W direction. This result demonstrated that a highly permeable volcanic body and a small conduit would be required to explain the estimated temperature of about 200 °C. 3. Comprehensive hydrothermal model of Shimabara Peninsula 3.1. Geothermal manifestations in Shimabara Peninsula There are three conspicuous geothermal manifestations in the Shimabara Peninsula: the Obama hot springs, the Unzen fumarolic field (Unzen Jigoku) and the Shimabara hot springs (Fig. 2). Heat discharge by hot water is dominant at the Obama and Shimabara hot springs. On the other hand, most heat is discharged as steam at the Unzen fumarolic field. The Obama hot springs discharge chloride

Table 2 Simulated temperature of the conduit center and the interpolated temperature of the conduit surface at the monitoring block Permeability of the volcanic body (mdarcys)

1

10

30

50

Temperature at the center of the conduit (°C) Temperature on the surface of the conduit (°C)

370 220

232 156

187 125

164 110

Fig. 7. Calculated temperature distributions in 1995 and in 2004 in E–W slice No. 6 in the case of 10 mdarcys. (a): in 1995, at the end of the 1990–95 eruption, (b): in 2004, at the completion of Well USDP-4. The curved line in this figure shows the projected trace of Well USDP-4. The blue trace indicates the part of the well behind the slice, and the red trace indicates that in front of the slice.

waters, and those of the Unzen fumarolic field and the Shimabara hot springs discharge acid sulfate and bicarbonate waters, respectively (Ohta, 1973). Yuhara et al. (1981) have conducted infrared measurements by a helicopter-borne thermocamera over the Unzen fumarolic field for the heat discharge estimation calculated from the thermal images by using the heat balance technique (Sekioka and Yuhara, 1974), and some other heat discharge estimation methods (the Benseman's method, the conduction probe method, etc.) in 1978. They estimated with the research results on the flow rates of the hot springs in the three geothermal areas by the Unzen Meteorological Station (1978) that the heat discharge rates from the Obama, Unzen and Shimabara geothermal areas were about 50 MW, 21 MW and 0.4 MW, respectively. In addition to these three geothermal areas, the geothermal development promotion survey in the western Unzen area conducted by NEDO revealed another high temperature geothermal system about 4 km west of Mt. Fugen (NEDO, 1988). The underground temperature of the system exceeds 200 °C at 1 km depth although there are no geothermal manifestations at the surface. We named this hydrothermal system “the West Unzen High Temperature Body (WUHTB)” (Fig. 2). Ohta (1973) proposed a qualitative model, in which there is a magma reservoir at a depth below the Chijiwa caldera configurating Tachibana Bay, and magmatic emanations have been ascending on a slant to the east on the basis of the correlation between the mode of occurrence of volcanic hot springs and geologic structure or earthquake in the Unzen volcanic region. In this model, the variation of the chemical characteristics of the thermal waters is attributed to the differentiation of the magmatic emanations as well as to mutual reactions with wall-rocks and blending of ascending thermal water with marine water or descending ground water. This model was the first to integrate interpretation of the Obama, Unzen and Shimabara geothermal areas, however, other models are proposed (e.g. Shigeno and Abe, 1986; Ohmi and Lees, 1995; Notsu et al., 2001), and the subject is still debated. Therefore, the second aim of our study is to construct a comprehensive hydrothermal model beneath Unzen

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Volcano by using a numerical simulation in order to evaluate the development of the four hydrothermal systems (the Obama, Unzen and Shimabara geothermal areas and WUHTB) present in the Shimabara Peninsula. 3.2. Numerical model In this study, we again used HYDROTHERM Version 2.2 for numerical modeling. The analytical area for the comprehensive model extends horizontally 22 km (E–W) by 16 km (N–S) (Fig. 2) with a buffer area outside it to absorb the effects of the conditions on the lateral boundaries (Fig. 8), and extends vertically −11 km to 1.25 km above sea level (the ground surface) to represent a broad structure of the Shimabara Peninsula (Fig. 9). The model was divided into four layers, made up of Unzen Volcanic Rocks (Layer I), the Kuchinotsu Group (Layer II), the Paleogene rock layer (Layer III) and an abyssal rock layer (Layer IV). Each layer has some thermal and hydrological properties, as shown in Table 3. These values are also based on the research by NEDO (1988), and the properties for Layers III

and IV are assumed values as there is a lack of data. In this model, Unzen Graben was represented by the distributions of these four layers (Fig. 10). However, relatively small-scale structures like a high permeable zone with a fault were not expressed because the horizontal size of the block in the analytical area is 1 km × 1 km. Therefore, we attempted to explain the existence of the four geothermal systems from the large-scale structures (the rough topography of Shimabara Peninsula and Unzen Graben) and some heat sources. Before transient analysis, we constructed a steady state model with no heat sources like the magma chambers in order to calculate the background temperature and pressure distributions. Boundary conditions at the bottom had a constant heat flux of 120 mW/m2 and an impermeable boundary. Atmospheric pressure and an annual average temperature of 15 °C were assigned to the ground surface. Lateral boundaries were fixed with the hydrostatic pressure because the Shimabara Peninsula is surrounded by the sea, and were fixed with the temperature distribution that indicates the gradients for the heat flux of 120 mW/m2.

Fig. 8. Plan view of the finite difference blocks for the comprehensive hydrothermal model. The numbered inner rectangle indicates the analytical area shown in Fig. 2, and the blocks outside the analytical area are part of a buffer area.

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Fig. 9. 3-D finite difference blocks for the analytical area applied in the comprehensive hydrothermal model.

3.3. Transient analysis Three pressure sources were determined by the analysis of the vertical displacement data obtained from repeated leveling in the Shimabara Peninsula from 1986 to 1992 (Ishihara, 1993); “Sources A”, “B” and “C” (from shallow to the deep, Fig. 11). In addition, the GPS measurements by using seven GPS stations around Unzen Volcano since 1991 detected ground deformation caused by magma intrusion and lava discharge, and the location of a pressure source estimated by the analysis using the horizontal displacement at each GPS station was identified in the proximity of Source C (Nishi et al., 1999). Source C is located at about 8 km deep at WUHTB and is considered to be a magma body that has had its high temperature maintained for a long period of time by replenishments of new magma such as in the 1990–

Table 3 Rock properties of each layer for the comprehensive hydrothermal model Layer I

Layer II

Layer III

Layer IV

(Unzen Volcanic (Kuchinotsu (Paleogene (Abyssal Rocks) Group) Rock) Rock) Specific heat (J/kg K) Density (kg/m3) Thermal conductivity (W/m K) Porosity (-) Permeability (mdarcy)

8.0 × 102 2.3 × 103 1.9 0.15 5

8.0 × 102 2.3 × 103 1.9 0.1 3

8.0 × 102 2.5 × 103 2.6 0.05 1

8.0 × 102 2.5 × 103 2.6 0.01 1.0 × 10− 4

95 eruption. The location of Source C is near the central part of the old body of Unzen Volcano (Hoshizumi et al., 2002b). Hence we consider that Source C is the most important magma reservoir and also the most important heat source for the evolution of the Shimabara Peninsula hydrothermal system. We then attempted to construct a model of a hydrothermal system with a heat source around Source C. According to the formative history of Unzen Volcano, five large eruptions with pyroclastic flows occurred at intervals of about 4000–5000 years (19 ka, 14 ka, 9–10 ka, 4 ka and 1990–95) (Hoshizumi et al., 2002a). Therefore, we assumed that the high temperature heat source has been maintained for the past 20,000 years, and so simulated the behavior of the geothermal fluid in the body of the Shimabara Peninsula for the past 20,000 years from the initial conditions. The background pressure and temperature distributions, which were the results of the steady state modeling, were used as the initial conditions and the temperature of the heat source was fixed at 850 °C. During the simulation, we changed the size of the heat source by trial and error to produce upflow zones for the four geothermal areas, and to bring the calculated heat discharge rates of the Obama, Unzen and Shimabara geothermal areas close to the values estimated by Yuhara et al. (1981). In the most suitable model, the size of the heat source extends horizontally for 1 km × 1 km and has a vertical height of 8 km, with the center of the heat source located at the 7.5 km below sea level. This model produces an upflow zone at WUHTB as well as two upflow zones near the western and eastern coasts coinciding with the Obama

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Fig. 10. E–W slice No. 9 that includes the summit of the lava dome and Mt. Fugen (left figure), and N–S slice No. 16, which indicates Unzen Graben (right figure), in the analytical area of the comprehensive hydrothermal model.

and the Shimabara hot springs, respectively. However, the Unzen fumarolic field becomes a recharge area where a downflow of ground water occurs because of a topographical effect (Fig. 12). This result means that it is difficult to develop the Unzen fumarolic field through just Source C. In order to generate the upflow of geothermal fluid at the Unzen fumarolic field, we set another heat source just beneath the fumarolic field. We assumed that the new heat source is temporary, younger, and shallower than Source C, because we thought that the new heat source would be derived from Source C. Therefore, we named this heat

source Source C'. According to Ishihara (1993), the intensity of the pressure source B is about 1/10 of Source C, and that of A is about 1/10 of Source B. So it is expected that the assumed source derived from Source C is much smaller than Source C. However, the minimum block size of this model below −500 m asl is 1 km × 1 km × 1 km. Therefore, we set that Source C' was a 1 km sided cube with an initial temperature of 1000 °C. We calculated in accordance with a scenario that had the existence of just Source C for 20,000 years followed by the existence of Source C and C' for another 1000 years. The initial state of Source C' was assumed to be molten magma, and the source has cooled for 1000 years. However, HYDROTHERM is not available for treatment of magma solidification, so the initial temperature of Source C' was set higher than the fixed temperature of Source C (850 °C) to represent the latent heat of magma. In the simulation, the location of the center of Source C' was set as a parameter and values of 1, 2 and 3 km below sea level were assigned. The areas set in the model for the calculations of heat discharge rates from the Obama hot springs, Unzen fumarolic field and Shimabara hot springs were larger than the actual areas due to the rough block splitting of the numerical model; therefore, the model tends to overestimate heat discharge calculations. The results of the simulation indicate that Source C' produces the upflow zone at the Unzen fumarolic field (Fig. 13), and in all cases heat discharges occur at the Obama, Unzen and Shimabara geothermal areas (Table 4); however, the case of 3 km below sea level is the closest match to values estimated by Yuhara et al. (1981). 3.4. Discussion As demonstrated above, both the heat source beneath WUHTB (Source C), and that beneath the Unzen fumarolic field (Source C') are involved in the formation of the four hydrothermal systems in the Shimabara Peninsula. Source C' is essential to generate heat discharges at the Unzen fumarolic field. Moreover, Source C' may be identical with the pressure source “B”, since the position of Source C' is close to that of source “B”. Source C explains part of the heat flow from the Obama hot springs, in spite of the tendency for overestimation in this model. This result allows for the possibility of supply from another heat source, such as a magma reservoir beneath Tachibana Bay. Source C also produces the upflow of hot water from the Shimabara hot springs. However, a high concentration of magmatic gas in the hot spring waters (Kazahaya et al., 2005) and the detection of a low resistivity body near the Shimabara hot springs (Kagiyama et al., 2005) may show the contribution of other magma. 4. Conclusions

Fig. 11. Three pressure sources determined by geodetic data obtained during the 1990– 95 eruption (after Ishihara, 1993).

In conjunction with the Unzen Scientific Drilling Project, we studied conduit cooling after the latest eruption of the Unzen Volcano,

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Fig. 12. Temperature distribution and mass flux on the E–W slice No. 9, which includes Source C. The downflow of ground water occurs in the vicinity of Unzen fumarolic field (in the elliptical area).

and the development of the hydrothermal systems in the Shimabara Peninsula. An evaluation of conduit cooling at Unzen Volcano from an initial temperature (850 °C) to a temperature estimated at the bottom of USDP-4 (about 200 °C) was conducted by numerical simulations. From the results, a temperature of 200 °C was obtained in the conduit of the area monitored under permeability conditions of 1 and 10 mdarcys for a conduit width of 100 m in the E–W direction. This result demonstrated that a highly permeable volcanic body and a small conduit are required to explain the estimated bottom temperature.

The comprehensive hydrothermal model showed that two heat sources beneath WUHTB and the Unzen fumarolic field are involved in the formation of the four hydrothermal systems in the Shimabara Peninsula. Especially, the heat source beneath the Unzen fumarolic field is essential for the generation of heat discharges at the Unzen fumarolic field. Acknowledgements The authors would like to thank Dr. Jun Nishijima, Dr. Koichiro Fukuoka and the students of the Laboratory of Geothermics,

Fig. 13. Temperature distribution and mass flux on the A–B slice in Fig. 3, which includes Sources C and C'. An upflow of hot water occurs at the Unzen fumarolic field (in the elliptical area).

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Table 4 Calculated heat discharge rates from Obama, Unzen and Shimabara geothermal areas and the values estimated by Yuhara et al. (1981) Position of Source C' Heat discharge Simulated value rate (MW) 1 km below sea level 2 km below sea level 3 km below sea level Yuhara et al. (1981)

Obama Unzen Shimabara H. S. F. F. H. S. 19.2 23.7 19.3 50

71.1 98.1 52.5 21

11.0 11.0 11.0 0.4

⁎ H. S.: Hot Springs, F. F.: Fumarolic Field.

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