Bull. Volcanol. Soc. Japan
Vol. 60 (2015) No. 1, pp. 35-46

ArticleInternal Structure of Kuttara Caldera, Hokkaido, Japan
Yoshihiko GOTO＊ and Akira JOHMORI＊＊(Received August 7, 2014 ; Accepted January 30, 2015)
A controlled-source audio-frequency magnetotelluric (CSAMT) survey was conducted across the caldera of
Kuttara volcano, Hokkaido, Japan, to investigate its subsurface structure. The caldera is 3 km in diameter and contains acircular lake (Lake Kuttara) 2.5 km across. The CSAMT survey was conducted along a 12-km-long east‒west-trendingtransect that crossed the volcano and passed over the caldera. A total of 23 receiver stations were distributed along thesurvey line, including 7 stations in the caldera. Unique on-boat measurements were obtained at the surface of Lake
Kuttara. A two-dimensional inversion of the CSAMT data, which revealed the resistivity structure to depths of up to
1000 m beneath the caldera, suggested the existence of a low-resistivity region (<30 Ω･m) beneath the eastern caldera
floor, extending subvertically for >1000 m and with a width of 1300-1500 m. The structure is interpreted to be a region
filled with lava blocks and pyroclasts, which subsided during collapse of the caldera and which has been affected byhydrothermal alteration. The location of the low-resistivity region corresponds to an oval basin in the eastern part of thelake, implying that caldera subsidence occurred mainly beneath this basin. The western part of the caldera floor tilts
gently to the east and has a rugged surface, suggesting that the western caldera rim was significantly enlarged as a resultof landsliding during caldera formation. We thus infer that the Kuttara caldera was produced by asymmetric caldera
collapse, which is attributed to the geological heterogeneity of the Kuttara volcano, with an eastern part comprised
mainly of andesitic lavas and a western part comprised mainly of dacitic pyroclastic deposits.
Key words : resistivity survey, CSAMT method, caldera, internal structure, Kuttara volcano

1．Introduction
The Kuttara caldera (3 km in diameter), Kuttara volcano,southwestern Hokkaido, Japan, is one of the smallest Quaternary calderas in Japan (Fig. 1). The caldera contains aremarkably circular freshwater lake, Lake Kuttara (Fig.
2A). Subsurface geological structures of the caldera havebeen previously studied by a gravity survey (Yokoyama
et al., 1967), but details of the structures remain poorly
constrained. We conducted a controlled-source audiofrequency magnetotelluric (CSAMT) resistivity survey(Milsom, 2003; Sandberg and Hohmann, 1982) to investigate its internal structure. As the Kuttara caldera contains alake, we obtained unique on-boat CSAMT measurementsfrom the lake surface. A two-dimensional inversion of the
CSAMT data revealed the resistivity structure at depths of
up to 1000 m beneath the caldera. We here present theresults of the resistivity survey, and discuss the nature of
the subsurface geological structures beneath the caldera.
2．Kuttara caldera
The Kuttara caldera is located at the summit of Kuttara
volcano (Fig. 2). Volcanic activity on Kuttara volcano,which occurred from 80 to 40 ka, included an early period＊

College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido 050-8585, Japan.
＊＊
Neo Science Co. Ltd, Tarui 4-2-30, Sennan, Osaka 590-

of silicic explosive activity followed by the construction ofan andesitic stratovolcano (Katsui et al., 1988; Moriizumi,
1998; Moriya, 2003; Yamagata, 1994). The stratovolcano(elevation, 549 m above sea level; base diameter, 9 km)
consists mainly of andesitic lavas and scoria, and associated dacitic pyroclastic deposits (Katsui et al., 1988).
The andesitic lavas and scoria are distributed on thenorthern, eastern, and southern parts of the stratovolcano,whereas the dacitic pyroclastic deposits are distributed on
the western part (Fig. 2B).
The Kuttara caldera is subcircular (diameter, 3 km) andretains its primary morphological features, such as acaldera rim and inner caldera wall (Fig. 2A). The caldera
formed during violent and explosive silicic eruptions at ca.
40 ka (Moriizumi, 1998; Yamagata, 1994) that resulted ina large-volume dacitic pyroclastic fall and flow deposits
around the caldera (Kt-1 tephra; Yamagata, 1994). Lake
Kuttara is 2.5 km in diameter with a maximum depth of
148 m (water level, 258 m above sea level). The lake wateris clear and transparent to a depth of 22 m, as measured in1991 (Environment Agency of Japan, 1993). The caldera
has never been drilled, and its subsurface geology is poorly
known. Gravity data (Yokoyama et al., 1967) show that
0521, Japan.
Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp

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Yoshihiko GOTO and Akira JOHMORI

Fig. 1. Location map of the Kuttara volcano, Hokkaido, Japan. The CSAMT survey line (red line) is 12 km long and isoriented E‒W, passing over Kuttara caldera (Lake Kuttara) and the Noboribetsu geothermal field. Locations of the bipole
sources (A1, A2, and A3; black lines, and black circles showing the locations of electrodes) and receiver stations (redcircles; M1-23) are shown. The base map is from the 1:25,000 scale topographic maps (the ‘Noboribetsu-onsen’ and‘Tobiu’ quadrangles), issued by the Geospatial Information Authority of Japan. The topographic contour interval is 10 m.

the caldera has a negative gravity anomaly.
Post-caldera volcanism, which occurred on the western
slope of the stratovolcano at ca. 15 ka, formed theNoboribetsu geothermal field (Goto and Danhara, 2011;
Katsui et al., 1988; Fig. 1). The geothermal field (1×1.5km) is characterized by a dacitic cryptodome (theHiyoriyama cryptodome; 130 m high and 350-550 m
across), a number of small explosion craters (up to 210 m
across), active fumaroles, and hot springs (Goto et al.,2011). Previous geological studies suggest that more than
12 phreatic eruptions have occurred in this field during thepast 8500 years (Goto et al., 2013).
3．CSAMT survey
The CSAMT survey was conducted to obtain theresistivity structure beneath the Kuttara caldera at depthsof up to 1000 m. The survey was conducted along a 12-kmlong east‒west-trending transect crossing Kuttara volcano

and passing over the Kuttara caldera and the Noboribetsu
geothermal field (Fig. 1). The CSAMT procedure followed
the ‘scalar CSAMT’ method (Matsuoka, 2005), whereby atransmitter injects electrical currents into the ground ataudio and near-audio frequencies via a grounded wire(bipole source), and a receiver records both the electricfield parallel to the grounded wire and the magnetic field
perpendicular to the wire (Fig. 3A).
The CSAMT survey was conducted using a highresolution electromagnetic system (Geo-SEM; Neoscience
Co. Ltd, Osaka, Japan) consisting of a transmitter and a
receiver (Fig. 4A and B). The transmitter (Fig. 4A) consisted of a transformer, rectifier, switching circuit, GPSclock, and generator. The grounded wire connected to thetransmitter was 1.3-2.6 km long and contained 30 electrodes
at each termination (each electrode consisted of a stainlesssteel rod, 600 mm long and 13 mm across) (Fig. 3A). The
receiver (Fig. 4B) comprised an amplifier, filter, data

Internal Structure of Kuttara Caldera, Hokkaido, Japan

Fig. 2. (A) Photograph of the Kuttara caldera viewed fromthe southwest. The caldera contains a circular freshwaterlake (Lake Kuttara), 2.5 km across. (B) Three-dimensional
topographic image of Kuttara volcano showing its main
geological features. The northern, eastern, and southernparts of the volcano are comprised mainly of andesitic
lavas, whereas the western part is comprised of dacitic
pyroclastic deposits (within the broken yellow line). The
Noboribetsu geothermal field (NGF) lies on the western
slope of the volcano (red circle). LT, Lake Tachibana(volcanic crater). The base map is from the red relief
image map, RRIM10 of Asia Air Survey (Chiba et al.,2007), using 10-m DEM data of the Geospatial Information Authority of Japan (GSI).

logger, GPS clock, and set of sensors. The sensors
consisted of a pair of electrodes and a coil; each electrodeconsisted of a copper rod, 280 mm long and 15 mm across.
The transmitter and receiver were synchronized using ahigh-precision GPS quartz clock system to an accuracy of
1×10‒6 s. The specifications of the Geo-SEM system aregiven in Table 1; further details of the system are describedin Johmori et al., (2010).
The CSAMT survey line was 12 km long and oriented
N88° E. The 23 receiver stations (M1-23) were horizontally spaced at intervals of 300-1000 m (Fig. 1); M1-9

37

Fig. 3. (A) Schematic diagram of the electromagnetic
system (Geo-SEM) used for the CSAMT survey foron-land stations (stations M1-10 and M16-23). The
bipole sources (grounded wire) for the transmitters are
1.3-2.6 km long and have 30 electrodes at each termination. The receiver has a set of sensors consisting of apair of electrodes and a coil. The distances between the
grounded wire and receiver stations were 3.5-6.0 km.
(B) Schematic diagram of the CSAMT resistivity survey on Lake Kuttara (stations M11-15), obtained byon-boat measurements. A pair of electrodes (distance,5 m) was set at either end of surveying poles that
extended from the boat, which were adjusted parallelto the bipole source to measure the electric field. The
coil was set at the nearest lakeshore. (C) Cross-sectionof the CSAMT resistivity survey on Lake Kuttara. The
boat is fixed to the measuring location with an anchor.

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Yoshihiko GOTO and Akira JOHMORI

Fig. 4. (A) Photograph of the transmitter of the electromagnetic system (Geo-SEM) used for the CSAMT survey. (B)A Geo-SEM receiver. The white coil is 72 cm long. (C) CSAMT survey on Lake Kuttara (receiver station M11;
Fig. 1). A manned fiberglass boat was used to measure the electric field on the lake. A pair of electrodes (distance,5 m) was set at either end of surveying poles that extended from the boat. (D) A coil was set on the lakeshore tomeasure the magnetic field (M16 station, for substituting the magnetic fields at M13, M14 and M15).

Table 1.

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

were located on the western slope of the stratovolcano(M4-7 being located in the Noboribetsu geothermal field),
M10-16 were located in the Kuttara caldera (M11-15
being located on Lake Kuttara), and M17-23 were locatedon the eastern slope of the stratovolcano. Positions of thereceiver stations were determined by GPS.

The bipole sources (grounded wires) were placed atthree locations (A1, A2, and A3; Fig. 1) so as to cover theentire 12-km length of the survey line. Source A1 (formeasurements at M1-10) was 1.3 km long and oriented
N73° W. Source A2 (for measurements at M11-16) was
2.3 km long and oriented N87° E. Source A3 (for meas-

Internal Structure of Kuttara Caldera, Hokkaido, Japan

urements at M17-23) was 2.6 km long and oriented N86°
W. The distances between the bipole sources and thereceiver stations were as follows: 3.5-6.0 km between A1
and M1-10, 6.0 km between A2 and M11-16, and 6.0 km
between A3 and M17-23.
The resistivity measurements were carried out in two
different ways, depending on whether the receiver station
was on land or water. At stations on land (M1-10 and
M16-23), the electrodes and coil were placed on the
ground surface (Fig. 3A). A pair of electrodes (separated
by a distance of 20 m) were set parallel to the bipole source
to measure the electric field, whereas the coil was set
perpendicular to the bipole source to measure the magnetic
field. At stations on Lake Kuttara (M11-15), a manned
fiberglass boat (4.5 m long, 1.5 m wide) was floated on thelake (Figs. 3B and 4C) and anchored at each measuringlocation (Fig. 3C). A pair of electrodes (each consisting ofa copper rod, 280 mm long and 15 mm across) was
mounted at either end of a surveying pole that extended
from the boat (separation distance, 5 m). The poles were
adjusted so that they were parallel to the bipole source, sothat the electrodes could correctly measure the electricfield (Fig. 4C). The on-boat measurements of the electricfield were possible because the waves on the lake were
sufficiently small that their effect on the electric fieldmeasurements was minor. In contrast, measurements of themagnetic field were significantly influenced by small
waves, and therefore for magnetic measurements the coil
was placed on the ground surface on the lake shore(locations M10 and M16; Fig. 1), and was set perpendicular to the bipole source (Fig. 4D). We substituted themagnetic field at M10 for the fields at M11 and 12, andsubstituted the magnetic field at M16 for the fields at M13,
14, and 15. These substitutions were based on the observation that magnetic fields up to a given distance from abipole source were nearly identical, assuming theresistivity structure to be 1-D or 2-D. Suzuki et al. (2009)
performed a similar on-lake CSAMT survey on a dammed
lake. The on-boat survey method is environmentallyfriendly and inexpensive.
The CSAMT survey was conducted between 20 Julyand 2 August 2011. The transmitter injected 1-8-A
electrical currents into the ground at frequencies of 1, 2, 4,
8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, and 8192
Hz, and another series at frequencies of 20, 40, 80, 160,
320, 640, 1280, 2560, and 5120 Hz, to minimize noise
related to the commercially used frequency of 50 Hz andits associated higher harmonics. The receiver recorded theelectric and magnetic fields parallel and perpendicular to
the grounded wire, respectively. The measurement time ateach receiver station was 1 hour (minimum of 2 minutes at
8192 Hz; maximum of 8 minutes at 1 Hz).
The CSAMT data were processed using a band-passfilter, a Fourier transform, and stacking to remove noise.
The number of waves for stacking was >400,000 at 8192

39

Hz to >300 at 1 Hz. The apparent resistivity and phasewere then calculated from the electric and magnetic fields.
Measurement errors for the apparent resistivity and phasewere not calculated because to increase the resolution
frequency, the frequency analyses of the CSAMT data
were performed for long time-series of data that had notbeen divided into plural datasets (see Johmori et al., 2010).
The quality of the data at each measurement station waschecked by examining the ρa ‒f curve, where ρa is theapparent resistivity and f is the frequency. The ρa‒f curves
for stations M1 and M3-23 were smooth, and generally
exhibited an increase in apparent resistivity in direct
proportion to the frequency. The smooth curves imply thatthe data from stations M1 and M3-23 are acceptable forresistivity calculations. In contrast, the ρa‒f curve for M2
showed an abrupt decline in apparent resistivity at around
50 Hz, which is the commercial frequency in this area,
implying that the M2 data were influenced by artificial
noise. The M2 station is located near a high-voltage power
line; therefore, the M2 data were rejected.
The possibility of discrepancies in the data caused by
the use of three bipole sources with slightly different
azimuths (A1, N73° W; A2, N87° E; A3, N86° W) was
also considered. Two sets of data, measured at station M10
station using sources A1 and A2, were compared using theρa‒f and ϕ‒f curves, where ϕ is the phase angle and f is thefrequency. The curves were in good agreement with one
another, indicating that discrepancies related to source
azimuth angles were absent or minimal.
Given the short distances between the bipole source andthe receivers for stations M3-10 (3.5-4.5 km), the lowfrequency data (<8 Hz) at these stations could potentiallybe affected by the near-field effect (cf. Sandberg andHohmann, 1982). Therefore, after checking the CSAMT
data, a near-field correction was applied to the CSAMT
data, following the method of Sasaki (1988). The corrected
CSAMT data (apparent resistivity and phase angles) are
shown in Fig. 5.
4．Data analysis
The resistivity structure beneath the Kuttara volcano
was calculated using a 2-D inversion of the CSAMT data,assuming a 2-D structural model. The 2-D inversion was
performed using a finite element method (following
Sasaki, 1986) by setting the results of the 1-D inversion as
the initial model. The 1-D-inversion model was determined from the apparent resistivity and phase angles of the
CSAMT data, using a non-linear least-square method
based on the analytical theory of Ward and Hormann(1987).
The 2-D inversion was performed in transverse magnetic(TM) mode (Sasaki, 1986) along a 12-km-long section
oriented N88° E (survey line, Fig. 1). The amount of calculation required for the 2-D inversion of this section was
beyond the capacity of our computing resources. We

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Yoshihiko GOTO and Akira JOHMORI

Fig. 5. Apparent resistivity and phase angles of the obtained CSAMT responses (blue lines) and the calculated responses forthe apparent resistivity and phase angles (red lines) for receiver stations M1-23 (see Fig. 1 for station location). The M2
data are not shown because of the influence of artifacts due to 50-Hz commercial noise (see Section 3).

Internal Structure of Kuttara Caldera, Hokkaido, Japan

Fig. 5.

Continued.

41

42

Yoshihiko GOTO and Akira JOHMORI

therefore divided the section into three parts (M1-12,
M9-18, and M17-23) and performed the calculations foreach part separately. The 2-D inversion results for the threeparts were then joined together (at 4000 m and 8300 m
from the western termination of the survey line) to reformthe 12-km-long section. No discrepancies were observed
between the calculated results (resistivity models) at thepositions of the joined segments.
The 2-D inversion included a topographic model constructed from elevation data of 1:25,000 scale topographic
maps (the ‘Noboribetsu-onsen’ and ‘Tobiu’ quadrangles)
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’, which
represents the unit used in the 2-D inversion to calculate
resistivity (Sasaki, 1986), consisted of four elements. The
2-D inversion was performed by comparing the field data(apparent resistivity and phase angles) with the calculated
results, using a non-linear least-squares method. The
iteration for the 2-D inversion was run eight times. The
2-D inversion reduced the effects of topography on the
CSAMT data. The 2-D inversion may also have reduced
distortions of the resistivity structure due to static shifts.
Figure 5 shows that the obtained CSAMT responses(apparent resistivity and phase angles) were in overall
agreement with the calculated responses, except for valuesof the response at station M7, located on the edge of theNoboribetsu geothermal field, which showed differingvalues at high frequencies. These differences were probably the result of the steep rugged topography at thisstation; however, the station is located far from the Kuttara
caldera, and thus the differences should not have adversely
impacted the results of the resistivity structure beneath thecaldera.
A root mean square (RMS) value was calculated fromthe obtained CSAMT responses (apparent resistivity) andthe calculated responses to quantitatively investigate thematch between them. Phase values were not used in thecalculations, as the phase is functionally dependent on theapparent resistivity. The RMS value (δ) is defined as δ=[Σ {ln (ρa f) − ln (ρa c)}2 /n]1/2, where ρa f is the obtained
CSAMT response (apparent resistivity), ρa c is the calculated response, and n is the number of measurements.
According to this definition, an RMS value of zero
indicates that the calculated response perfectly matches theobtained CSAMT response, whereas an RMS value of 0.1
indicates that ~90 % of the calculated response matches theobtained CSAMT response (i.e., ~10 % error). The RMS
values were calculated for three data sets (M1-12, M9-18,
and M17-23). The obtained RMS values are 0.133 forM1-12, 0.084 for M9-18, and 0.046 for M17-23, indicating a good agreement between the obtained CSAMT
responses and the calculated responses.
Depth of penetration was determined from the skindepth (Cagniard, 1953) and the Bostick depth (Murakami,1987). The skin depth is defined as the depth at which theamplitude of electromagnetic waves decreases to 1/e,
where e is the base of the natural logarithm. The Bostick
depth is defined as 1/ 2 of the skin depth, and isconsidered to represent a more accurate depth of penetration than the skin depth. The skin depth calculated from thefrequency of electric currents for the deepest layer (1 Hz),and from the corresponding apparent resistivity (mostly
10-100 Ω･m) for the CSAMT data, yields depths ofpenetration of 1500-5030 m, while the Bostick depth
calculated from the same data yields penetration depths of1060-3556 m. The depth of penetration was therefore
conservatively considered to be 1000-1500 m below theground surface.
5．Results
Processing of the CSAMT data revealed the subsurface
resistivity structure beneath Kuttara volcano, including
Kuttara caldera, to depths of up to 1000 m below thesurface (Fig. 6A). Prominent features of the resistivity
structure (outlined in Fig. 6B) are: a layer of high resistivity extending along or near the ground surface across theentire volcano (zone A); a layer of high resistivity at Lake
Kuttara (zone B); a columnar region of low resistivity
beneath Lake Kuttara (zone C); a broad columnar region of
low resistivity beneath the Noboribetsu geothermal field(zone D); and an extensive region of low resistivity at
depth below the eastern part of the volcano (zone E).
Among these zones, zone C is of particular importancefor understanding the caldera's internal structure because it
is located just beneath the caldera. We therefore performed
a sensitivity test for zone C, by substituting a virtual
resistivity value of 20 Ω･m for the original resistivity
value, and re-calculating the RMS value for M9-18 (thevalue of 20 Ω･m was based on the resistivity value aroundzone C). The sensitivity test resulted in an increase in the
RMS value to 0.142, as compared with the original RMS
value of 0. 084, indicating the reliability of our results
regarding the existence of zone C.
6．Geological interpretations
Generally, the resistivity of rocks and sediments islowered by the presence of conductive minerals (e.g.,alteration minerals such as smectite-series clays), thermal
waters in pores and fractures, and high temperatures (e.g.,
Milsom, 2003; Takakura, 1998). These observations,suggesting that resistivity data show marked changes in thevicinity of hydrothermal alteration zones and faults (e.g.,
Martyn et al., 1997; Risk et al., 2003), are the basis for thegeological interpretations of the five zones (A‒E in Fig.
6B) presented below. Because the resistivity section
obtained in this study (Fig. 6A) offers only a twodimensional perspective, we have combined the data withthe distribution of topographic features to make inferences
regarding the architecture of the caldera, particularly with

Internal Structure of Kuttara Caldera, Hokkaido, Japan

43

Fig. 6. (A) Resistivity structure beneath the Kuttara volcano, including the Kuttara caldera, calculated by a 2-D
inversion. (B) Annotated version of the resistivity structure, showing zones A‒E. Zone A is interpreted torepresent andesitic lavas, scoria, and pyroclastic deposits. Zone B corresponds to lake water. Zone C is inferred to
be a domain filled with lava blocks and pyroclasts, which subsided during caldera collapse and were subject tohydrothermal alteration. Zone D is interpreted to be a hydrothermally altered zone beneath the Noboribetsu
geothermal field. Zone E is inferred to be a hydrothermally altered zone. (C) Topographic map showing locationsof the receiver stations and the main bathymetric features at the bottom of Lake Kuttara.

respect to the three-dimensional extent of geologicalstructures.
Zone A (resistivity>100 Ω･m) extends subhorizontally
along the ground surface and varies in thickness from 200
m at the western part of Kuttara volcano to 800 m at theeastern part. The surface of the volcano consists of fresh
andesitic lavas, andesitic scoria, and minor dacitic
pyroclastic deposits (Katsui et al., 1988; Moriizumi, 1998),
suggesting that zone A corresponds to these rock types.
The andesitic lavas and scoria are distributed mainly in thenorthern, eastern, and southern parts of the volcano,whereas the dacitic pyroclastic deposits are distributed in
the western part (Fig. 2B; Katsui et al., 1988; Moriizumi,1998). Variations in the thickness of zone A may reflect
these changes in rock type.
Zone B (resistivity>100 Ω･m), which is 2500 m wideand ~150 m thick, coincides with the location of Lake

Kuttara, which is 2.5 km across and 130-148 m deep. Thissuggests that zone B corresponds to lake water, as the highresistivity values (>100 Ω･m) are consistent with those of
fresh (non-salt) water. The resistivity of lake water
sampled from the lake surface in July 2011 and measured
in our laboratory was 143.5 Ω･m, which is consistent with
the interpretations of the field data. The correspondence
between the lake depth (130-148 m) and the thickness ofzone B (~150 m) indicates that the results of the on-board
CSAMT survey on Lake Kuttara are reliable.
Zone C (resistivity<30 Ω･m) is located beneath Kuttara
caldera, and extends subvertically for up to 1000 m and is1300-1500 m wide. Because of its location beneath thecaldera, we interpret this zone as a domain filled with lava
blocks and pyroclasts that subsided during caldera collapse
and was subjected to hydrothermal alteration. Zone C
comprises a higher-resistivity core (10-30 Ω･m) and a

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Yoshihiko GOTO and Akira JOHMORI

lower-resistivity rim (< 10 Ω･m), a structure that could
reflect the stability and presence of smectite (stable attemperatures of <200℃; Aizawa et al., 2008, 2009; Leeet al., 2010; Takakura et al., 2000), showing that smectite
is absent in the core of the domain but present on its edges.
The eastern boundary of zone C is sharp and linear,suggesting the presence of a distinct subvertical fault(probably a ring fault) at the eastern edge of the domain.
The western boundary of zone C is diffuse, suggesting a
gradual geological contact or the presence of an indistinct
ring fault.
Zone C is located toward the eastern edge of Lake
Kuttara rather than at the center of the lake. Bathymetricisobaths (Fig. 6C) indicate that an oval basin is present in
the eastern part of the lake (1000×1500 m, elongate N‒S)
and that this basin represents the deepest part of thecaldera; the western part of the caldera, on the other hand,is tilted gently to the east and has a relatively irregularsurface. The location of zone C thus corresponds to thelocation of the oval basin, suggesting that this was thecenter of caldera collapse. This inference is consistent withthe gravity data of Yokoyama et al. (1967), who reported anegative gravity anomaly (1.5 milligal low) at the eastern
center of the caldera floor, indicating the existence of lowdensity materials at depth. Topographic data (Fig. 2B)show that a steep curviplanar cliff demarcates the eastern
inner caldera wall, whereas the western caldera wall isslightly irregular and has several scallop-shaped indents.
We thus infer that the western caldera rim wassignificantly enlarged as a result of landsliding during
caldera formation.
Zone D (resistivity < 5 Ω･m) is located beneath theNoboribetsu geothermal field, and extends subverticallyfor up to 1000 m and is 1000-1500 m wide. Because of its
location beneath the geothermal field, we interpret thatzone D is a hydrothermal alteration zone produced by
upwelling high-temperature fluids (Goto and Johmori,2011). The location and size of the hydrothermal alteration
zone are consistent with the distribution of geothermalfeatures observed at the surface of the geothermal field(e.g., active fumaroles, hot springs, and hydrothermal
alteration zones). Notably, zones C and D are distinct andseparate entities beneath the Kuttara caldera and theNoboribetsu geothermal field, respectively (Fig. 6B). The
distance between them (1500-2000 m) implies that, at least
within the vertical resolution of the data (up to 1000 m
below the surface), different pathways exist for the ascentof high-temperature fluids beneath Kuttara volcano.
Zone E (resistivity < 5 Ω･m) is located beneath theeastern slope of Kuttara volcano and extends subhorizontally for more than 4000 m at a depth of ~1000 m. We
cannot provide an adequate interpretation for this lowresistivity region, as no drilling data are available from thisarea (NEDO, 1991); however, the occurrence of hydrothermal alteration may provide a possible explanation for

the zone. Zone E extends sharply upward below station
M18 (Fig. 6B), a feature which could represent theexistence of a fault. Zones D and E are both roughly
symmetrical with respect to the Kuttara caldera, suggestingthat they may represent hydrothermal alteration developed
along large concentric fractures (~5.5 km across) encircling the Kuttara caldera.
7．Discussion
The resistivity structure obtained in this study (Fig. 6B)
reveals that the internal structure of the Kuttara caldera isasymmetric. We infer that caldera collapse occurred below
the eastern part of the caldera floor and that the perfectcircular outline of Lake Kuttara resulted from the enlargement of the western caldera rim due to mass wastingprocesses during caldera formation. Such asymmetricstructures are common in relatively small calderas, such as
in Miyakejima, Japan (Geshi, 2009). Asymmetric structures have also been reported in ancient eroded calderas(e.g., Kumano, Japan; Miura, 1999). Analog modelingexperiments simulating caldera collapse (e.g., Geyer et al.,2006; Kennedy et al., 2004) also suggest that caldera
subsidence commonly begins with a downsag flexure(Branney, 1995) followed by the formation of an asymmetric ring fault. We infer that the asymmetric internal
structure of the Kuttara caldera is caused by geologicalheterogeneities within the volcano, whereby the easternpart comprises mainly andesitic lavas and the western partcomprises mainly dacitic pyroclastic deposits. As suggested earlier, the thickness variation of zone A (Fig. 6B)
may reflect these variations in rock type.
Asymmetric caldera collapse was well documented
during the 2000 AD eruption of Miyakejima volcano,Japan (Geshi, 2009; Geshi et al., 2002). There, a small
circular caldera (1.6 km across) was formed by asymmetricsubsidence of the volcano summit and by repeatedlandsliding, rockfall, and small-scale debris avalanching of
the resultant caldera wall. Caldera subsidence occurred
mainly in the northwestern part of the caldera floor,
whereas the mass wasting processes occurred mainly in thesoutheastern part of the caldera. The resulting caldera wasasymmetric, contained an oval basin in the northwesternpart of the caldera floor (600×800 m, elongate NE‒SW),
and had a steep slope in its southeastern part. Geshi (2009)
suggested that asymmetric collapse at the Miyakejima
2000 caldera was a result of the heterogeneous structure of
the volcano. The Kuttara caldera shares similarities to the
Miyakejima 2000 caldera in terms of its size andmorphology; thus, based on this modern-day analogue wesuggest the Kuttara caldera was produced by similar
asymmetric subsidence and mass-wasting processes.
8．Conclusions
A CSAMT survey revealed the subsurface resistivity
structure of the caldera on Kuttara volcano at depths of up

Internal Structure of Kuttara Caldera, Hokkaido, Japan

to 1000 m. The resistivity structure showed that theinternal structure of the caldera is asymmetric. Caldera
collapse occurred beneath an oval basin located toward theeastern part of the caldera floor, and the western caldera
rim was significantly enlarged as a result of landsliding
during caldera formation. The asymmetry of caldera
collapse may be attributed to geological heterogeneities
within the volcano.
Acknowledgements
This research was sponsored by the Ministry of Education, Culture, Sports, Science and Technology of Japan(MEXT), and was supported financially by the MuroranInstitute of Technology. We thank N. Johmori, T. Kondou,
and T. Takahashi (Neoscience Co. Ltd) for assistance inthe field, and K. Kizaki (Docon Co. Ltd) for constructive
discussions on the geology of Kuttara caldera. Comments
by N. Geshi (AIST), an anonymous referee, and editing by
K. Aizawa (Kyushu University) significantly improved themanuscript.
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北海道南西部クッタラカルデラの内部構造
後藤芳彦・城森明

北海道南西部に位置するクッタラカルデラは，クッタラ火山の山頂部に形成された径約 3 km の小型カルデラである．このカルデラの内部構造を解明するため，CSAMT 法による比抵抗構造探査を行った．探査はクッタラカルデラを東西方向に横断する測線上（測線長 12 km，受信点 23 箇所）で行い，カルデラ内のクッ
タラ湖では小型ボートを用いた湖上測定を行った．データ解析は，有限要素法を用いた 2 次元逆解析を用いた．その結果，深度 1000 m までの比抵抗構造が得られた．クッタラカルデラの下には，鉛直方向に伸長す
る低比抵抗領域（<30 Ω･m，深度 1000 m 以上，東西径 1300-1500 m）が存在し，この領域はカルデラ形成により落ち込んだ溶岩ブロックや火山砕屑物等が熱水変質を受けたものであると推定される．この低比抵抗
領域は，クッタラ湖の中心からやや東に位置する湖盆地形の直下にあり，カルデラ陥没は主にこの地点で行われたと考えられる．カルデラの西部は地滑り等により拡大された可能性が高い．クッタラカルデラの内部構造は東西非対称であり，非対称なカルデラ陥没はクッタラ火山の非対称な地質に起因すると考えられる．