Bull. Volcanol. Soc. Japan
Vol. 59 (2014) No. 1, pp. 1-11

ArticleResistivity Structure of the Showa-Shinzan Domeat Usu Volcano, Hokkaido, Japan
Yoshihiko GOTO＊ and Akira JOHMORI＊＊(Received October 30, 2013 ; Accepted January 18, 2014)
A controlled source audio frequency magnetotelluric (CSAMT) survey was conducted over Showa-Shinzan Domeat Usu Volcano, Hokkaido, Japan, in order to investigate its internal structure. The Showa-Shinzan Dome (800-1000 m
across, 350 m high) is a partly extruded cryptodome that formed in AD 1943-45 due to the uplift of pre-existing rocks
and sediments by the intrusion of dacitic magma. The dome comprises a flat-topped cryptodome called ‘Yaneyama’ anda dacitic lava dome projecting above the Yaneyama cryptodome. The CSAMT survey was carried out on a 1600-mlong line that crosses the Showa-Shinzan Dome in an east-west orientation. Two-dimensional inversion of the CSAMT
data revealed the resistivity structure at depths less than 1000 m beneath the dome. The resistivity structure suggests theexistence of a sub-spherical dacite intrusion (resistivity 50-130 Ω･m; ~400 m across) below the summit of the ShowaShinzan Dome. The dacite intrusion may represent the solidified dacitic magma emplaced in AD 1943-45. The
Yaneyama cryptodome only comprises pre-existing rocks and sediments uplifted by the intrusion of dacite magma. The
upper zone of the Yaneyama cryptodome consists of the Usu Somma Lava (>100 Ω･m), whereas the lower zone
consists of Quaternary pyroclastic flow deposits and sedimentary rocks (<30 Ω･m), such as the Toya pyroclastic flow
deposits, the Fukaba Formation, the Takinoue welded tuff, the Sobetsu pumice flow deposits, and the Yanagihara
Formation. There is no dacite intrusion beneath the Yaneyama cryptodome. This structural model is consistent with the
distribution of active fumaroles on the Showa-Shinzan Dome, and also with historical records of dome growth. The
geophysical data provide new insights into the formation mechanism of the Showa-Shinzan Dome.
Key words : resistivity survey, CSAMT method, Showa-Shinzan Dome, internal structure, Usu Volcano

1．Introduction
Resistivity surveying provides valuable information on
the underground geological structures of active volcanoes(e.g., Aizawa et al., 2008, 2009; Aizawa, 2010; Fikos etal., 2012; Matsushima et al., 2001; Nishida et al., 1996;
Ogawa et al., 1998; Risk et al., 2003; Srigutomo et al.,2008; Yamaya et al. 2009). We have conducted a controlled source audio frequency magnetotelluric (CSAMT)
survey (Milsom, 2003; Sandberg and Hohmann, 1982) of
the Showa-Shinzan Dome of Usu Volcano, Hokkaido,
Japan, in order to investigate its internal structure. The
Showa-Shinzan Dome is a partly extruded cryptodome
formed in AD 1943-45 due to the uplift of pre-existingrocks and sediments by the intrusion of dacitic magma(Katsui, 1988; Mimatsu, 1962; Minakami et al. 1951; Soyaet al., 2007; Yokoyama et al., 1973). The internal structureof this dome has previously been studied using various
geophysical techniques, including seismology (Kato andShoji, 1949; Hayakawa et al., 1957; Nemoto et al., 1957),
magnetic surveying (Nishida and Miyajima, 1984), and＊College of Environmental Technology, Graduate School ofEngineering, Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido 050-8585, Japan＊＊
Neo Science Co. Ltd, Tarui 4-2-30, Sennan, Osaka 590-

muon radiography (Tanaka et al., 2007; Tanaka andYokoyama, 2008), but still remains poorly constrained.
Herein, we present the results of a CSAMT survey of the
Showa-Shinzan Dome and discuss the nature of thesubsurface geology beneath the dome.
2．Showa-Shinzan DomeThe Showa-Shinzan Dome is located at the eastern footof Usu Volcano (Fig. 1). The dome is elliptical in planview with a diameter ranging from 800 m (N-S) to 1000 m(E-W), and it rises 350 m above the surrounding area (Fig.
2). The highest point of the dome is 398 m above sea level.
The dome consists of a flat-topped cryptodome called‘Yaneyama’ and a pyramidal dacitic lava dome projecting
above the Yaneyama cryptodome (Fig. 2A).
The Yaneyama cryptodome is pancake-shaped, 8001000 m across, and 200 m thick. The surface of the cryptodome mainly comprises andesitic lava blocks (2-5 m insize) of the Usu Somma Lava (Yokoyama et al., 1973) andunconsolidated sediments (soil, clay, and volcanic ash) that
0521, Japan
Corresponding author: Yoshihiko Gotoe-mail: ygoto@mmm.muroran-it.ac.jp

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Yoshihiko GOTO and Akira JOHMORI

GS-R1

Fig. 1. Location map of the Showa‒Shinzan Dome (yellow area) in the eastern part of Usu Volcano, southwestern
Hokkaido, Japan. Also shown is the location of the CSAMT survey line (red line). The points marked A and B are theends of the grounded wire (dipole source) of the transmitter for the CSAMT survey (black line). Location of the GS-R1
hole (see Fig. 8) is also shown. The base map was taken from the 1：50,000 scale topographic maps ‘Toyako-onsen’ and‘Date’ issued by the Geospatial Information Authority of Japan. The topographic contour interval is 20 m.

were uplifted during dome growth. The geology of theinterior of the Yaneyama cryptodome is poorly understood
due to dense vegetation cover (Fig. 2B) and the absence ofcross-sectional exposures. The Yaneyama cryptodome hosts
no active fumaroles, apart from the region in the immediate vicinity of the lava dome (Symonds et al., 1996).
The dacitic lava dome projects from the western side of
the Yaneyama cryptodome (Fig. 2A). The dome has apointed top and steeply sloping sides, and is 300-400 m indiameter and rises 150 m above the Yaneyama cryptodome. The lava dome consists of fresh coherent dacite(SiO2 =69 wt.%; Oba et al., 1983) that contains phenocrysts of plagioclase and hypersthene. Most of the lava
dome is covered with reddish brown, clayey rocks(‘natural brick’ ; Mimatsu, 1962) that were formed by theheating of uplifted sediments in contact with hot dacitic
magma. The lava dome hosts a number of active fumaroles(Symonds et al., 1996) and its surface is almost bare (Fig.
2B).
Minakami et al. (1951) and Mimatsu (1962) documented that the formation of the Showa-Shinzan Dome

was preceded by a series of severe earthquakes on 28
December 1943. In January 1944, the ground at the easternfoot of Usu Volcano began to inflate and many cracks
opened in the ground. On 23 June 1944, the first phreatic
explosion took place in this area, and explosions repeatedly occurred until October 1944. By late October, theamount of inflation had reached 150 m, thereby forming aflat-topped cryptodome (Yaneyama). In November 1944, adacitic lava dome was extruded upwards on the westernside of the Yaneyama cryptodome and was accompanied
by further growth of the Yaneyama cryptodome. The
growth of the lava dome continued until September 1945.
3．CSAMT survey
The CSAMT survey was performed in order to obtain aresistivity structure at depths of up to 1000 m beneath theShowa-Shinzan Dome. The survey was carried out on aneast-west line passing over the Yaneyama cryptodome andthe lava dome (Fig. 3). The CSAMT survey was performed
following the ‘scalar CSAMT’ method (Matsuoka, 2005;
Yokokawa, 1984; Fig. 4), whereby a transmitter injects

Resistivity Structure of the Showa‒Shinzan Dome at Usu Volcano, Hokkaido, Japan

Fig. 2. Photographs of the Showa‒Shinzan Dome viewed
from the south (A) and west (B). The dome consists ofa flat-topped cryptodome named ‘Yaneyama’ and apyramidal dacitic lava dome projecting above the Yaneyama cryptodome.

electrical currents into the ground at audio and near-audio
frequencies via a grounded wire (dipole source), whilst a
receiver records the electric field parallel to the groundedwire and the magnetic field perpendicular to the groundedwire (Fig. 4).
The CSAMT survey was carried out using a highresolution electromagnetic system (Geo-SEM; Neoscience
Co. Ltd, Osaka, Japan) consisting of a transmitter and a
receiver (Fig. 5). The transmitter (Fig. 5A) comprises atransformer, rectifier, switching circuit, GPS clock, andgenerator. A grounded wire that is 1.5 km long with 25-35
electrodes at each termination was connected to thetransmitter. The receiver (Fig. 5B) consists of an amplifier,filter, data logger, GPS clock, and set of sensors. The
sensors are a pair of electrodes and a coil. The transmitterand receiver were synchronized with a high-precision
quartz clock system using GPS to an accuracy of 1×10−6
s. The specifications of the Geo-SEM system are given in
Table 1 and further details of the system are described in
Johmori et al. (2010).
The transmitter was positioned 8. 5 km south of the
Showa-Shinzan Dome, with the 1. 5-km-long grounded3

wire oriented N67° W (Fig. 1). The survey line was 1600 m
long and oriented N80° W (M1-12; Fig. 3). The 12 receiver stations (M1-12) were horizontally spaced at intervals of 65-225 m. The M1-2 and M12 receivers werelocated outside of the Showa-Shinzan Dome. The M3 andM6-11 receivers were located on the Yaneyama cryptodome, with the M6-10 receivers being located on the flat
top of the cryptodome. The M4-5 receivers were locatedon the lava dome. The positions of the receiver stations
were confirmed using GPS. The distance between the transmitter and receiver stations was 8.5 km (Fig. 1), which is
8.5 times larger than the depth of interest (1000 m), andsufficient to obtain CSAMT data in the far-field region
beyond the near-field region (e.g., Sandberg and Hohmann,
1982).
The CSAMT measurements were performed during 2-6
November 2012. The transmitter injected 1-8 A electricalcurrents into the ground at frequencies of 1, 2, 4, 8, 16, 32,
64, 128, 256, 512, 1024, 2048, 4096, and 8192 Hz, andanother series at frequencies of 20, 40, 80, 160, 320, 640,
1280, 2560, and 5120 Hz, in order to minimize the noise
related to the commercially used frequency of 50 Hz andits higher harmonics. The receiver recorded the electricand magnetic fields parallel (N67° W) and perpendicular,
respectively, to the grounded wire. The measurement timeat each receiver station was 1 h (2 min at 8192 Hz; 8 min at
1 Hz). The CSAMT data were then processed using aband-pass filter, a Fourier transform, and stacking to
remove noise. The number of waves for stacking was >400,000 at 8192 Hz or >300 at 1 Hz. The apparent resistivity and phase were then calculated from the electric andmagnetic fields. Measurement errors for the apparent
resistivity and phase were not calculated, as frequency
analyses for the CSAMT data were performed for a longtime-series of data that was not divided into plural datasets,in order to increase the frequency resolution (see Johmori
et al., 2010).
The obtained CSAMT data comprising apparent resistivity and phase are shown in Fig. 6. The CSAMT data areinferred to have been measured in a far-field region that isbeyond the near-field region, as: (1) the phase angles range
from 30° to 65° at 1 Hz, with no angles of 0°; and (2) theapparent resistivity does not show abrupt V-shaped
decreasing and increasing trends at low frequencies (i.e., it
does not show a ‘notch’ or ‘transition zone’; Hishida andTakasugi, 1998).
4．Data analysis
The resistivity structure beneath the Showa-Shinzan
Dome was calculated by 2D inversion of the CSAMT datausing a finite element method, following Sasaki (1986).
The 2D inversion was performed in transverse magnetic(TM) mode (Sasaki, 1986), along a section oriented N80°
W and 1600 m in length (survey line; Fig. 3). The topographic model for the 2D inversion was obtained from 1：

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Yoshihiko GOTO and Akira JOHMORI

Fig. 3. Topographic map of the Showa‒Shinzan Dome showing the survey line for the CSAMT survey (red line) andreceiver stations (red circles; M1-12). The survey line is 1600 m long and oriented N80° W. The base map was takenfrom the 1:5000 scale topographic maps ‘Usuzan III’ and ‘Usuzan VI’ issued by the Geospatial Information Authority ofJapan. The topographic contour interval is 5 m.

Fig. 4. Schematic of the electromagnetic system (GeoSEM) used for the CSAMT survey. The groundedwire for the transmitter is 1.5 km long and has 25-35
electrodes at each termination. The receiver has a setof sensors consisting of a pair of electrodes and acoil. The distance between the grounded wire andreceiver stations is 8.5 km.

5000 scale topographic maps (‘Usuzan III and VI’) issued
by the Geospatial Information Authority of Japan. The
mesh size of the finite element method was 20 m (element
size=20×20 m). Each inversion block representing theunit used to calculate resistivity by 2D inversion (Sasaki,1986) consisted of four elements. The 2D inversion was
carried out by comparing the field data (apparent resistivity and phase angles) with the calculated results using
the non-linear least-squares method. The iteration for the
2D inversion was run eight times, using a uniform
resistivity (25 Ω･m) as the initial model. The value of 25
Ω･m was determined from the average apparent resistivity
at all the measurement locations. The effects of topography
and static shift on the CSAMT data were reduced by 2D
inversion. Static shift correction was not performed on the
CSAMT data. Even if a near-surface resistivity anomaly
was present, it was probably detected by the highfrequency CSAMT data (up to 8192 Hz) that were measured at closely spaced receiver stations (intervals 65-225m), and therefore may not have caused distortion of theresistivity model (see Takasugi et al., 1991).
Figure 6 compares the CSAMT field data (apparent

Resistivity Structure of the Showa‒Shinzan Dome at Usu Volcano, Hokkaido, Japan
Table. 1.

5

Specifications of the exploration system used for the CSAMT survey (Geo SEM).

resistivity and phase angles) with the calculated 2D inversion results, showing good overall agreement. A root meansquare (RMS) value was obtained from the field data(apparent resistivity) and the calculated results, in order toinvestigate quantitatively the match between the CSAMT
field data and calculated results. The RMS value (δ) is
defined as δ=[Σ{ln(ρaf)−ln(ρac)}2/n]1/2, where ρaf is thefield measurement (apparent resistivity), ρac is the calculated result, and n is the number of data (n=276). According to this definition, an RMS value equal to zeromeans that the calculated result perfectly matches the field
data, whereas an RMS value equal to 0. 1 means that
almost 90 % of the calculated result matches the field data
(i.e., ~10 % error). The obtained RMS value of 0.10 indicates good agreement between the field data and thecalculated result.
The depth of penetration was determined from the skindepth (Cagniard, 1953). The skin depth is defined as thedepth at which the amplitude of electromagnetic waves
decreases to 1/e (where e is the base of the natural
logarithm). The skin depth calculated from the lowestfrequency electric currents (1 Hz) and its average apparent
resistivity (5 Ω･m) yields a penetration depth of 1100 m.
The depth of penetration is thus ca. 1000 m below theground surface.

Fig. 5. Photographs of the transmitter (A) and receiver(B) of the electromagnetic system (Geo-SEM) used
for the CSAMT survey. Photograph (B) was takenon the Showa‒Shinzan Dome (receiver station M4;
Fig. 3).

5．Results and interpretation
Processing of the CSAMT data revealed the subsurface
resistivity structure at depths of < 1000 m beneath theShowa-Shinzan Dome (Fig. 7A). The resistivity structure
is divided into four zones based on the resistivity values(zones A-D in Fig. 7B). In general, the resistivity of rocks
and sediments is lowered by the presence of conductive
minerals (e. g., smectite-series clays), thermal waters inpores and fractures, and high temperatures (e.g., Risk etal., 2003). Geological interpretations of the four zones (AD in Fig. 7B) are as follows.
Zone A, which has a resistivity of >100 Ω･m, extends
subhorizontally along the ground surface and is 50-100 m
thick. This zone is present just below the surface of the6

Yoshihiko GOTO and Akira JOHMORI

Fig. 6. Apparent resistivity and phase angles of the CSAMT field data (blue lines). Calculated 2D inversion results
for the apparent resistivity and phase angles are also shown (red lines). Location numbers (M1-12) correspond to
those shown in Fig. 3.

Resistivity Structure of the Showa‒Shinzan Dome at Usu Volcano, Hokkaido, Japan

7

Fig. 7. Resistivity section beneath the Showa‒Shinzan Dome (A) and its geological interpretation (B). Zone A is interpreted
to represent andesitic lava blocks of the Usu Somma Lava. Zone B is inferred to be the dacite intrusion emplaced in AD
1943-45. Zone C (above zone D) is assigned to Quaternary pyroclastic flow deposits and sedimentary rocks, such as the
Toya pyroclastic flow deposits, Fukaba Formation, Takinoue welded tuff, Sobetsu pumice flow deposits, andYanagihara Formation. Zone D is inferred to be the Tertiary Osarugawa Formation, comprising intensely altered tuff
breccia. The location and size of the feeder dyke is hypothetical.

Yaneyama cryptodome, but is absent at the lava dome.
Given that the surface geology of the Yaneyama cryptodome is dominated by andesitic lava blocks of the Usu
Somma Lava that were uplifted during the dome growth,zone A is interpreted to represent these lava blocks. The
absence of zone A at the lava dome may be attributed tolateral migration of the lava blocks from the lava dome
area to the Yaneyama cryptodome area during extrusion of

8

Yoshihiko GOTO and Akira JOHMORI

the lava dome.
Zone B (50-130 Ω･m) is located below the summit of
the Showa-Shinzan Dome and is a semi-circular domainthat is ~400 m across in cross-section. Given that this zone
is located immediately beneath the dacitic lava dome, weinterpret it as a dacite intrusion within the Showa-Shinzan
Dome (i.e., the dacite magma emplaced in AD 1943-45).
Considering the emplacement age the Showa-Shinzan
Dome, the dacite intrusion is probably solidified and notmolten. However, the presence of active fumaroles on thelava dome suggests that the dacite intrusion is still at hightemperatures. The intermediate resistivity (50-130 Ω･m)of zone B suggests that the dacite intrusion is partlyhydrothermally altered. A fresh, unaltered dacite intrusion
would have much higher resistivity (see Murase, 1962).
We therefore infer that zone B represents a solidified hightemperature dacite intrusion that has been partly altered.
The location and size of the dacite intrusion are consistent
with the fact that active fumaroles occur on the lava dome
and its immediate surrounds, but not on the Yaneyama
cryptodome. The calculated volume of the dacite intrusion
is 3.3×107 m3, assuming that the intrusion has a perfectspherical shape with a diameter of 400 m.
Zone C (15-30 Ω･m) is located below zone A andoccupies the interior of the Yaneyama cryptodome. The
thickness of zone C (above zone D) is up to 250 m. As
zone A is interpreted to represent lava blocks of the Usu
Somma Lava, zone C must be geological units underlying
the Usu Somma Lava. The geological section of a borehole
drilled 1 km southwest of the Showa-Shinzan Dome (GSR1 hole; length 376 m; Fig. 8) comprises the following
units from base to top (Sato, 1967): the Tertiary Osarugawa Formation (36 m of intensely altered tuff breccia),
the Quaternary Yanagihara Formation (121 m of volcanic
sandstone), the Sobetsu pumice flow deposits (9 m ofdacitic pumice), the Takinoue welded tuff (57 m of densely
welded pumice), the Fukaba Formation (11 m of conglomerate), the Toya pyroclastic flow deposits (59 m ofrhyolitic pumice), the Usu Somma Lava (72 m of andesitelava), and volcanic ash (11 m). Given that the lowermost
Tertiary Osarugawa Formation corresponds to zone D(described later), zone C (thickness <250 m) is interpreted
to represent the sequence from the Yanagihara Formation
through to the Toya pyroclastic flow deposits. The low
resistivity of zone C (15-30 Ω･m) suggests that these
Quaternary pyroclastic deposits and sedimentary rocks arehydrothermally altered. The thickness variations of Zone C(Fig. 7) imply that these pyroclastic deposits and sedimentary rocks are highly deformed. We are unable to infer
the nature of the deeper part of zone C (below zone D), as
there are no drilling data at depths of > 376 m below
ground level (Sato, 1967).
Zone D (< 5 Ω･m) is present 300-400 m below the
Showa-Shinzan Dome and is ~200 m thick. This zone
extends subhorizontally but branches upwards below theFig. 8. Stratigraphic section of the GS-R1 hole drilled 1 km
southwest of the Showa‒Shinzan Dome (see Fig. 1).
Modified from Sato (1967).

M7 receiver location (Fig. 7B). Zone D is characterized byextremely low resistivity and is interpreted to be alteredpyroclastic deposits containing abundant conductive clay
minerals such as smectite. Other interpretations, such as
the presence of thermal waters or high temperatures,
cannot readily explain the location and shape of zone D.
Considering the depth of zone D (300-400 m) and thegeology of the GS-R1 drillhole (Fig. 8; Sato, 1967), zone
D is interpreted to be the Tertiary Osarugawa Formation(intensely altered tuff breccia) that contains abundant clay
minerals (Oshima and Matsushima, 1999; Takakura et al.,2009). We attribute the upward branching of zone D below
M7 (Fig. 7B) to lateral displacement (i.e., eastward migration) and uplift of the Osarugawa Formation that resulted
from emplacement of the dacite intrusion. Uplift of the
Tertiary Osarugawa Formation is consistent with the local
exposure of the Takinoue welded tuff and Fukaba Formation (i.e., the units underlying the Usu Somma Lava;
Fig. 8) in a small area (100×150 m) on the Yaneyama
cryptodome in the regions around the M6-7 receivers(Nemoto et al., 1957).

Resistivity Structure of the Showa‒Shinzan Dome at Usu Volcano, Hokkaido, Japan

6．Discussion
Previous studies have proposed various models for the
subsurface geological structure of the Showa-Shinzan
Dome. Nakamura and Mori (1949) carried out laboratory
analog modeling experiments of the Showa-Shinzan Domeusing sand and iron rods, and concluded that the morphology the dome could be explained by the ascent of acylindrical dacite intrusion (~300 m across) beneath thelava dome, and another, smaller, cylindrical dacite intrusion beneath the Yaneyama cryptodome. Hayakawa et al.
(1957) conducted a seismological study of the ShowaShinzan Dome and inferred the existence of a dacite
intrusion located beneath the dome that extends laterally tothe deeper part of the Yaneyama cryptodome. Nishida andMiyajima (1984) conducted a magnetic survey over Usu
Volcano, including Showa-Shinzan Dome, and inferred
the existence of a dacite intrusion 400 m across beneath thelava dome. Nishida and Miyajima (1984) also invoked thepresence of another dacite intrusion beneath the Yaneyama
cryptodome, following the model of Nakamura and Mori
(1949). Symonds et al. (1996) considered that the sizeable
uplift associated with the Yaneyama cryptodome represents the large volume of magma intruded beneath thecryptodome, and proposed a model in which a large dacite
intrusion occupies the interior of the cryptodome. The
study of Hernandez et al. (2006) adopted the model ofSymonds et al. (1996). Tanaka and Yokoyama (2008)
performed a muon radiography survey of the ShowaShinzan Dome and proposed that the lava dome decreases
in diameter downward, and grades into a volcanic conduit
with a diameter of 100±15 m at 260 m above sea level and
50±15 m at 217 m above sea level.
Our model is characterized by the existence of a subspherical dacite intrusion (~400 m across) beneath the lava
dome, and the absence of a dacite intrusion beneath the
Yaneyama cryptodome (Fig. 7B). This model differs fromthe previous models described above, in terms of thelocation and size of the dacite intrusion. We discount thepossible existence of a dacite intrusion beneath the Yaneyama cryptodome, as the interior of the cryptodome ischaracterized by low resistivity (Fig. 7). Our model canalso readily explain the spatial distribution of active fumaroles, which occur on the lava dome and its surroundings,
but not on the Yaneyama cryptodome. We consider that
the Yaneyama cryptodome formed by uplift and lateralmigration of pre-existing rocks and sediments due to theintrusion of dacite magma. Such a formation mechanism isconsistent with the growth history of the Showa-Shinzan
Dome (Mimatsu, 1962).
7．Conclusions
A CSAMT survey has revealed the subsurface resistivity structure at depths of <1000 m beneath the ShowaShinzan Dome. The obtained resistivity structure suggests
the presence of a sub-spherical dacite intrusion that is ~400

9m across, representing solidified magma emplaced beneaththe lava dome in AD 1943-45. The Yaneyama cryptodome
only comprises pre-existing rocks and sediments upliftedby the intrusion of dacitic magma and is not underlain by adacite intrusion.
Acknowledgements
This research was financially supported by the MuroranInstitute of Technology. S. Mimatsu (Mimatsu Masao
Memorial Hall) is thanked for allowing our field survey of
the Showa-Shinzan Dome. We thank T. Takahashi (NeoScience) for assistance in the field. Y. Nishida (Hokkaido
University) is thanked for constructive discussions about
the internal structure of the Showa-Shinzan Dome.
Comments by two anonymous referees and T. Hashimoto(Hokkaido University) significantly improved the manuscript.
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(in Japanese)(Editional handling Takechi Hashimoto)Resistivity Structure of the Showa‒Shinzan Dome at Usu Volcano, Hokkaido, Japan

北海道有珠火山，昭和新山ドームの比抵抗構造
後藤芳彦・城森明

北海道南西部，有珠火山東部に位置する昭和新山は，1943-1945 年の噴火で形成され，屋根山と呼ばれる扁平な潜在ドームと，ピラミッド型の溶岩ドームからなる．昭和新山の内部構造を解明するため，CSAMT法による比抵抗構造探査（深度<1000 m）を行った．探査は昭和新山を東西方向に横断する測線上（測線長1600 m，受信点 12 箇所）で行い，データ解析は有限要素法を用いた 2 次元逆解析で行った．その結果，溶岩ドーム直下に，直径約 400 m のデイサイト貫入岩体が存在することが明らかになった．この貫入岩体は
1943-1945 年に貫入したマグマが固結したものであると考えられる．屋根山はデイサイト貫入岩体により押し上げられた既存の岩石や堆積物（有珠外輪山溶岩，洞爺火砕流堆積物など）からなり，屋根山内部にデイ
サイト貫入岩体は存在しない．

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