Many hot springs are gushing from the inside of the Unzen Graben which traverses the central part of Unzen Volcano in east-west direction. Their geochemical characteristics are of salt type at the western foot, of sulfur type at the central mountain area and of bicarbonate type at the eastern foot of the volcano. They are hot in the west and warm in the east. A hydrothermal system at Unzen Volcano was presented by Ohta (1972) that the original magmatic emanations are supplying from a magma reservoir which lies beneath the Tachibana Bay assuming Chijiwa Caldera located at western side of volcano, and also that magmatic emanations have been ascending obliquely toward the east generating each type of thermal waters corresponding to the degree of differentiation. But this hydrothermal system was not accepted widely. Unzen Volcano began to erupt in November 1990 after 198 years dormancy. The forgoing earthquake swarm occurred beneath the Tachibana Bay in November 1989, and the foci migrated toward the summit of the volcano repeatedly for One year. They are arranged in semicircle at the southern part of Tachibana Bay and in a parallel two lines which run from the bay to the summit as if indicating the caldera rim and the conduit, respectively. Deformation of ground during the eruptions also showed that pressure sources were underlain beneath the inside of the parallel foci lines, and final pressure source was detected beneath the Tachibana Bay (Matsushima et al., 2003, 2005). So, the hydorothermal system presented by Ohta (1972) may be proved to be true because of these new facts. Based on these, modified supplying model of magmatic emanations were proposed here.
This paper summarizes the recent results of geological and petrological research at Shimabara peninsula. The Shimabara peninsula is located at about 100 km behind the volcanic front of the Kyushu island. No subduction related seismicity is observed beneath the peninsula. There are a number of E-W trending normal faults in the peninsula due to the N-S extension. This resulted in the formation of the Unzen graben. Volcanic succession in the Shimabara peninsula is divided into two stages; Pre-Unzen volcano (4 Ma - 500 ka) and Unzen volcano (500 ka - present). The nature of volcanism of Pre-Unzen and Unzen volcano differs clearly; the former represents a monogenetic volcano group which is exposed in the southern Shimabara peninsula, whereas the latter is a composite volcano which is distributed east to west along the Unzen graben. The eruptive products of Pre-Unzen volcano have also been recognized beneath the the volcanic succession in central and northern parts of the peninsula by borehole surveys. Pre-Unzen volcanic rocks are composed of olivine basalt and two-pyroxene andesite lava flows and pyroclastics. The evolution of Pre-Unzen volcanic rocks can be basically explained by olivine-dominant fractional crystallization for basalts, and the combination of plagioclase + pyroxenes + magnetite fractional crystallization for andesites. Unzen volcano has been divided into three substages; the older Unzen (500-300 ka), the middle Unzen (300- 150 ka) and the younger Unzen (150 ka - present). Unzen volcanic rocks are composed of hornblende andesite to dacite lava domes, lava flows and pyroclastics. The evolution of Unzen volcanic rocks can be explained by magma mixing between aphyric basalt and phenocryst-rich dacite magma in various ratios. The existence of mafic inclusions with positive Nb anomalies indicates an injection of ocean-island type basaltic magmas. This suggests continuous basaltic magma plumbing system throughout the eruptive history of the Shimabara peninsula.
In this study, a model for hydrothermal system and formation processes of hot springs (Obama, Unzen and Shimabara hot springs) in Unzen Graben of the Shimabara Peninsula was constructed on the basis of a detailed analysis of hydro-geochemical data obtained by Ohsawa et al. (2002) and NEDO (1988). Both Obama and Unzen hot springs at the western part of the graben, which are located at the southwest side of the volcanic center of Unzen Volcano, are generated by one active liquid-dominant hydrothermal fluid of Na-Cl type. Fumarolic area adjoining Unzen hot spring being on the uplands is developed by a steam separated from the hydrothermal fluid at some 300°C, and hot water of H-SO4 type of Unzen hot spring is formed by mixing of the secondary steam into shallow groundwater at about 150°C. Heat and material source of the liquid-dominant hydrothermal fluid is a high-pressure magma-derived hydrothermal fluid in which chloride is contained in Na-form. The “residual” deep thermal water migrates laterally toward the western coast while conductively cooled, and finally the mixing of seawater into the thermal water results in the formation of boiling or hot water of Obama hot spring at approximately 200°C. Warm water of HCO3 type of Shimabara hot spring at the eastern part of the graben, which is located at the east side of the volcanic center of Unzen Volcano, is formed at less than loot by mixing of a low-temperature magma-derived C02-dominant gas into shallow groundwater of meteoric origin: This low-temperature gas might be derived from a solidified magma relevant to a monogenetic volcano, Mt. Mayu-yama formed 3000 years before present, but the formation mechanism of such gas is still unaccounted for. This hydro-geochemical study cannot pinpoint the precise location of the primary magma chamber of Unzen Vocalno which is still hotly argued, however the conclusion of this study for the aspect of geochemical formation mechanism of hot spring waters agrees to the previous model (Ohta, 1973; 1975) on the whole.
The Unzen Scientific Drilling Project (USDP) had been conducted by the Science and Technology Agency (FY1999-2000) and the Ministry of Education, Culture, Sports, Science and Technology, Japan (FY2001-2004). In this project, one of the objectives of our study was to construct a comprehensive hydrothermal model of beneath Unzen Volcano by using a numerical simulation. Four large geothermal systems are known in the Shimabara Peninsula (Obama hot springs, Unzen fumarolic field, Shimabara hot springs and the West Unzen High Temperature Body [WUHTB]). Three pressure source locations (“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 reservoir. We attempted to explain the existence mechanism 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 indicated that it would be difficult to develop the Unzen fumarolic field only by Source C ; therefore, we set another heat source just beneath the fumarolic field. Consequently, 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 to generate heat discharges at the fumarolic field.
There are more than 240 thermal springs in Algeria. The geothermal energy in Algeria is generally of a low enthalpy type. The total heat discharge from the main springs and existing wells is approximately 642 Mwt. Three geothermal zones have been delineated according to some geological and thermal considerations: 1) The Tlemcenian dolomites in the northwestern part of Algeria, 2) carbonate formations in the northeastern part of Algeria and 3) the sandstone Albian reservoir in the Sahara (south of Algeria). The northeastern part of Algeria is geothermally very interesting. Application of gas geothermometery to northeastern Algerian gases suggests that the reservoir temperature is around 198°C. Two conceptual geothermal models are presented, concerning the northern and southern part of Algeria. The principal utilizations of the hot water are balneology and space and greenhouse heating. Eight hot springs are used as public thermal resorts for medical purposes.
Fracture characterization was attempted using two-dimensional crack model in the Australian Hot Dry Rock field. The characterization method, which is based on the dynamics of crack model, allows the estimation of crack aperture, length and interfacial stiffness due to partial contact between upper and lower crack surfaces. The method compares peak frequencies obtained from field data with eigen frequencies estimated by the crack model to search for a set of data which gives an optimum fit between the peak and eigen frequencies. For the characterization, a seismic event observed during drilling was used. This event was observed at around 23:30 on March 14, 2003, before massive hydrofracturing in this field. Depth of hypocenter of this event is estimated to be roughly 4000m, and position of epicenter is estimated to be roughly 400m north of the injection well (Habanero#1). In this work, we assume that the event occurred along the single natural crack in granite rock and the crack is filled with water. It was found that the half length and the aperture of the crack are about 32m and 1.0mm, respectively. The effects of properties of the rock and the effects of pressure and temperature of the water on the fracture characterization were also examined. Density and Poisson's ratio of rock have some effects on estimated crack length. Temperature and pressure of water in the crack have weak effects on the estimated crack length and aperture.
We numerically investigated early cooling process of a magma chamber in order to construct a comprehensive model of geothermal system, which includes both a deeper heat source and a shallower hydrothermal system. Conservation of mass, momentum, energy, and dissolved and exsolved volatile components were taken into account in 2-D space, emphasizing the role of exsolution process with nucleation and growth of bubbles. The calculation for 182 years considering a granitic magma chamber containing 4wt% of water at a depth of 4km was achieved and showed a stable zone at the upper part of the magma chamber because of exsolution of volatile components, though the calculation time span is too short to discuss the evolution of a geothermal system. The appearance of the stable zone affected not only flow velocity of the magma but also distributions of all parameters. The instability of numerical calculation after 182 years seems to be attributed to the formation of the stable zone. Some improvement of numerical method may be needed. Taking account of values suggested by previous studies, the obtained degassing rate might be possible. However there still are matters to be considered such as crystallization in vicinity of magma-crust boundary or compression by vesiculation.