Akita-Yakeyama Volcano

• Geology

  According to an explanation of the geologic map of Akita-Yakeyama volcano (Suto, 1992), the geology and tectonics of the volcano are summarized as follows:

  Akita-Yakeyama volcano is considered to be younger than 1 Ma by K-Ar dating, a paleomagnetic study and volcanostratigraphy. Most of the volcanic rocks from the volcano are quartz and/or olivine-bearing pyroxene andesite, whereas small amounts of them are composed of dacite and rhyolite. A piston cylinder type depression is observed right beneath Akita-Yakeyama volcano, which is named as the Pre-Yakeyama caldera, and is considered to have collapsed during the eruption of the Tamagawa Welded Tuffs. Another caldera is also recognized south of the volcano: the Old-Tamagawa caldera. The caldera is thought to have collapsed during the eruption of the Old-Tamagawa Welded Tuffs in the Neogene, resulting two stage depressions filled up with welded tuffs with the thicknesses exceeding 1,000 m.

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Fig. 1 Onigajyo lava dome at the summit of Akita-Yakeyema volcano (1986)

• Magnetization Intensity Map of Akita-Yakeyama Volcano

Fig.1 Magnetization intensity map of Akita-Yakeyama volcano with a topographic shading (Okuma, 1998)

The map area corresponds to the small box in Fig. 1 at the home page of the Sengan Geothermal Area. Contour interval is 0.2 A/m. The areas bounded by solid lines indicate hydrothermal altered areas. Solid and open circles locate volcanic rocks which are normally and reversely magnetized, respectively. Paleomagnetic data (Suto, 1985; Suto, 1987; Suto and Mukoyama, 1987; Suto, 1992) which have stable magnetization intensities (ｳ1.0 A/m ) with magnetic pole latitudes (ｳ| ｱ50ｰ|) were plotted. Red broken lines indicate caldera rims of the Pre-Yakeyama caldera (north) and the Old-Tamagawa caldera (south).

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The distribution pattern of magnetization intensity of Akita-Yakeyama volcano is different totally from that of typical young volcanoes in the Sengan Geothermal Area. Usually a magnetization high is distributed on the edifice of each volcano. Instead, a magnetization low lies right on the southern flank of the volcano, surrounded by magnetic highs.

• Forward Modeling for Magnetic Structure of Akita-Yakeyama Volcano
  

1. Known information about structural and rock magnetic parameters
   
   In conjunction with rock magnetic data from surface volcanic rocks and core samples from geothermal exploration wells, a magnetic model which accounts for the magnetic anomalies was constructed.
   
   1) Drilling showed the existence of Tertiary granitic intrusions at an altitude of sea level below the northern flank of the volcano.
   2) Rock magnetic measurements indicate that the granitic rocks have the magnetic susceptibility (4 p x 2.5 x 10 ^-3 SI) strong enough to cause intensive magnetic anomalies, though the natural remanent magnetization (NRM) of the rocks is negligibly small (｣ 2 x 10 ^-1 A/m) (NEDO, 1986a).
   
   3) Drilling also showed that the surface volcanic rocks on the southern flank of the volcano are underlain by the Old-Tamagawa Welded Tuffs (NEDO, 1986b).
   
   4) According to paleomagnetic measurements (Suto, 1985; Suto, 1987; Suto and Mukoyama, 1987; Suto, 1992), most of the Old-Tamagawa Welded Tuffs are reversely magnetized as well as the Tamagawa Welded Tuffs to the south.
   
   
2. Method
   Magnetic anomalies which are caused only by the volcanic edifice (Fig. 2a) were extracted from the observed anomalies (Fig. 3 in the home page of the Sengan Geothermal Area). The magnetic anomalies (Fig. 2c) were calculated from the magnetization intensity data within an area bounded by a thick solid line (Fig.2b). Next, synthetic anomalies (Fig. 2d) caused by a terrain model (Fig. 2a) with a constant thickness of 500 m and a top corresponding to the ground surface were calculated by taking into account of the existence of hydrothermally altered areas: the altered areas were assumed to be non-magnetic. Then, residual anomalies (Fig. 3a) were calculated by subtracting synthetic terrain anomalies (Fig. 2d) from the extracted observed anomalies (Fig.2c). Finally, optimal synthetic anomalies (Fig. 3b) were calculated to fit the extracted observed anomalies (Fig.3a) by employing five horizontal polygons (Talwani, 1965). The goodness-of-fit ratio (r) (Blakely, 1995) was employed to indicate a goodness of fittings between two data sets.

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   Fig.2
   (a) Topographic map of Akita-Yakeyama volcano and its vicinity (Okuma, 1998)
   The map area corresponds to the large box in Fig. 1 at the home page of the Sengan Geothermal Area. The patterned area bounded by thick solid lines indicates a terrain model which has a constant thickness of 500 m and its top is the ground surface, excluding hydrothermal altered areas which correspond to non magnetic areas. Contour interval is 100 m.
   (b) Magnetization intensity map of Akita-Yakeyama volcano and its vicinity
   Contour interval is 20 x 10 -2 A/m. Solid and broken lines show positive and negative values, respectively. See also Fig. 2a.
   (c) Extracted total intensity aeromagnetic anomalies (IGRF residuals) of Akita-Yakeyama volcano
   The anomalies were calculated from prism models within a thick solid line in Fig. 2a, which have own magnetization intensities in Fig. 2b and have a flat bottom at an altitude of 5 km below sea level. Contour interval is 10 nT. See also Fig.2b.
   (d) Synthetic total intensity magnetic anomalies calculated from a terrain model with a boundary denoted by a thick solid line in Fig.2a
   Magnetization intensity of the terrain model is assumed to be uniformly 2.0 A/m except for the local hydrothermal altered areas which are non-magnetic.Contour interval is 10 nT. See also Fig. 2b.

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   Fig.3
   (a) Residual total intensity magnetic anomalies between the observed (Fig. 2c) and synthetic magnetic anomalies (Fig. 2d)
   Contour interval is 10 nT. See also Fig. 2b.
   (b) Synthetic total intensity magnetic anomalies which best fit the residual anomalies (Fig. 3a)
   Contour interval is 10 nT. Goodness-of-fit ratio is 1.8. Numbers 1 - 5 denote horizontal polygons. Circles show the locations of geothermal exploration wells. See also Fig. 2b.

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   Fig.4 N-S cross section of synthetic total intensity magnetic anomalies (Fig. 3b) with the magnetic model (Okuma, 1998)
   Contour interval is 10 nT. Goodness-of-fit ratio is 1.8. See also Fig. 3b.

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   Summary of a magnetic modeling

   A magnetic modeling, with known structural and magnetic parameters, was conducted to reveal a detailed subsurface structure of Akita-Yakeyama volcano. The magnetic model is composed of five polygons: three of them correspond to granitic intrusions beneath the northern flank, while the rest of them corresponds mainly to the buried Old-Tamagawa Welded Tuffs beneath the southern flank (Figs 3b and 4).
   
   (1) The northern polygons show the depth extent, about 2,000 m below sea level, of the granitic intrusions or the local Curie isothermal depth.
   
   (2) Geothermal exploration wells, SN-5 and SN-7D, where high temperatures more than 200 ｡C were observed, and the Sumikawa geothermal power plant are located just above horizontal boundaries of polygon 2 and 3, respectively (Fig. 3b), implying the existence of concealed faults and fractures where hydrothermal fluids can flow easily.
   
   (3) The southern polygons indicate a subsurface convex structure (Fig. 4), implying the existence of a concealed old volcano associated with the Old-Tamagawa Welded Tuffs and/or the Pre-Yakeyama Andesite Pyroclastic Rocks.

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