Recently geothermal fluid flows have become to be regarded as mainly controled by fracture permeability rather than formation permeability, especially on a reservoir scale. In order to clarify the role of the fracture system for geothermal fluid flow and reservoir behaviour, the properties of fractures and their associated hydrothermal veins were analyzed on surface outcrops and drill core samples in the Yuzawa-Ogachi geothermal field, Akita, Japan. This field can be divided structurally into the Okumaemori-Oyasudake upheaval belt and the Kijiyama subsidence belt by the Doroyu and associated faults. The former is characterized by high-angle-fractures in NW-SE trend, mid-angle-fractures in N-S trend, and hydrothermal veins in W-E trend, and the latter is characterized by high-angle-fractures in WSW-ENE and WNW-ESE trends, low-angle-fractures, and hydrothermal veins which are oblique to dominant fracture trends. Bore hole temperature logging data have revealed the detailed subsurface thermal structure in this field, and the isothermal contours suggested the coupling of two convection cells around the Doroyu and the Oyasu, and two geothermal heat source regions around the Takamatsu-dake and the Oyasu-dake. Based on these fracture and subsurface isothermal contour data, the deep seated geothermal fluids around the Doroyu can be inferred to ascend obliquely north-northwestward from the geothermal heat source region to geothermal reservoir zones along N-S and NW-SE trending, high-dipping fractures at deeper part, and preserved and/or deposited as hydrothermal veins in W-E trend fractures at shallow part. It is assumed that the essential geothermal fluids obliquely ascend by transferring several fractures rather than passing through a single fracture beneath the reservoir. Whereas around the Oyasu, the Sanzugawa formation consists of impermeable siltstone, which can be inferred to play an important role on fluid flow control as a cap rock to the geothermal reservoirand a lateral geothermal fluid flow. Based on the stress analysis of conjugate faults, the low angle fractures around the Oyasu can be regarded to be formed as the reverse faults under NE-SW trending compressive stress, and other high angle fractures around the Doroyu and the Oyasu can be regarded to be formed as the normal or lateral faults.

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A numerical experiment with two-dimensional elasto-plastic finite element method was applied to an analysis of geologic structure and fracture systems in the geothermal area. The experiment is based on the Virtual Basement Displacement Method (Kodama et a1., 1985). The virtual basement is preliminarily assumed, and deformation of upper-layer strata caused by its movement is experimentally reproduced for each step of deformation. The location selected as a model area is the northern part of the Kurikoma geothermal area, Akita prefecture, Japan, which has many active geothermal fields. The geology of this area consists of Quaternary volcanic rocks, Sanzugawa Formation (Pliocene lake deposits), Minasegawa Formation (late Miocene volcanic rocks), Doroyu Formation (middle Miocene marine sediments), and basement rocks (granite and schist). The basement rocks broadly subsided from Akinomiya to Minase River. This subsided zone is filled with thick tuff layers of Minasegawa Formation (> 1000m), and it has been considered to be a caldera (Utada and Ito, 1986). The central part of this subsided zone was raised after collapse of the caldera. This experiment was executed in a geologic section trending NE-SW, perpendicular to the general geologic structure trending NW-SE. The development processes of geologic structure are divided into the following five steps:step 1: sedimentation of Doroyu Formationstep 2: sedimantation of Minasegawa Formationstep 3: uplifting of the central area (1)step 4: sedimentation of Sanzugawa Formationstep 5: uplifting of the central area (2) Calculated deformation and fracture distribution in each step shows that the highly fractured vertical zones are formed along edges of the uplifted zone in step 3. It also suggests that radially distributed fracture systems are formed around the uplifted zone in step 5. Deep geothermal reservoirs have been found at Uenotai, located at the eastern edge of the basement uplifted zone. There fracture systems are distributed in the basement rocks and around dikes in Minasegawa Formation (Naka et at, 1987). Kawarage and Arayu are also active geothermal fields along this section. The result of this experiment indicates that underground strata at these geothermal fields have been fractured with basement uplifting in step 3 and 5. The distribution of the fracture system shown by this numerical experiment is considered to represent the actual fracture distribution. This method is useful when the fracture systems are not found through surface mapping, for example, the surface is covered with Quaternary volcanic rocks, such as the Uenotai field.

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1997年に発生した澄川地すべりの研究を契機として, 東北地方に見い出される大規模な地すべり地形はカルデラ火山に関係しているものが多いことが明らかになってきた。 東北地方には後期中新世から後期更新世にかけて形成されたカルデラが80箇所ほど存在している。 これらカルデラと大規模地すべり地形との関係をカルデラ火山の解体期における削剥過程に沿って次の3亜期に区分して概観した。 1) 後カルデラ火山体および外来火山岩削剥亜期では, 中小規模の地すべりや火山体の山頂部での変形がみられる。 2) 湖成堆積物削剥亜期では, 湖成堆積物の上位に残存している火山体には大規模な地すべり地形が認められ, また, グラーベンと放射状地すべりからなる変形がある。 湖成堆積物の地表面では中小規模の地すべりが多発して不規則な地形を示す。 3) カルデラ形成期の火砕流堆積物削剥亜期では, 崩壊や土石流の発生が著しく, 土砂災害としても注目される。 なお, 湖成堆積物は, その塑性変形をしやすい物性, 上位の後カルデラ期火山岩類とによるキャップロック構造の形成, 差別的侵食作用, 熱水作用の側方への拡大など, 地すべり形成に対して多面的に重要な素因となっている。

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