Earth Planets Space, 61, 345–356, 2013

Resistivity and density modelling in the 1938 Kutcharo earthquake source area

along a large caldera boundary

Hiroshi Ichihara∗, Toru Mogi, Hideaki Hase, Tomonori Watanabe, and Yusuke Yamaya

Institute of Seismology and Volcanology, Hokkaido University, N10W8 Kita-ku, Sapporo, Japan

(Received April 5, 2012; Revised October 11, 2012; Accepted October 14, 2012; Online published March 3, 2013)

Abstract We present the crustal structure around the fault zone pertaining to the 1938 Kutcharo earthquake ( M 6.0),

northern Japan, to consider why large earthquakes have occurred around calderas. The study was based on gravityanomalies and magnetotelluric and direct-current (DC) electrical-resistivity survey data. The density structureobtained from gravity anomalies indicated that the fault plane corresponded to the main depression boundary ofthe Kutcharo caldera. The resistivity section, based on audio-frequency magnetotelluric surveys, indicated thatthe estimated fault plane was located along the boundary of resistivity blocks, which also corresponded to thedepression boundary. A detailed resistivity section in the ruptured zone revealed by a DC electrical-resistivitysurvey showed a discontinuity of layers, implying cumulative fault displacements. These results indicate that the1938 earthquake was an abrupt slip along the main depression boundary of the Kutcharo caldera. The most likelyhypothesis pertains to fluid intrusion along the depression boundary. However, additional seismic and geodeticstudies are required to identify other feasible earthquake mechanisms.Key words: Magnetotellurics, gravity anomaly, DC electrical-resistivity survey, inland earthquake, Kutcharocaldera.

1. Introduction lian (1983) indicated that several earthquakes in 1980 wereLarge earthquake events ( M > 5) have often been re-

ported around large volcanic calderas, such as the LongValley caldera in the United States (e.g., Hill et al., 2003;Prejean et al., 2003), the Onikobe caldera in north-easternJapan (Umino et al., 1998), and the Rabaul caldera in PapuaNew Guinea (Mori and McKee, 1987). Several studies haveidentified possible mechanisms for the occurrence of earth-quakes around large calderas. For example, Umino et al.(1998) suggested that M 5.7 and M 5.9 earthquakes at theOnikobe caldera in 1996 were fault-slip earthquakes, whichmight have occurred along structural discontinuities aroundthe caldera boundary. Because many inland earthquakes oc-curring in the upper crust have been considered abrupt slipswithin large discontinuous zones (e.g., Jackson, 1980; Katoet al., 2006), the large structural boundaries along calderasmay also be important factors for the occurrence of earth-quakes, suggesting that the caldera structure affects the oc-currence of large inland earthquakes. However, the detailedrelationship between caldera structures and fault activityhas not yet been clarified.

Volcanic activity may be associated with the occurrenceof large earthquakes around calderas as has been suggestedfor the Long Valley caldera (e.g., Julian, 1983; Savage andCockerham, 1984; Prejean et al., 2003). For example, Ju-

∗Present address: Earthquake Research Institute, the University ofTokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.

Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sci-ences (SGEPSS); The Seismological Society of Japan; The Volcanological Societyof Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sci-ences; TERRAPUB.

caused by fluid injection into dykes through a mechanismexplained by the compensated linear-vector dipole model.Savage and Cockerham (1984) suggested that dyke inflationgenerated tension across the vertical plane that became theruptured surface of the earthquake swarm. This reduced thefrictional stress on the rupture plane and perhaps triggeredthe slip that caused the swarms.

The 1938 Kutcharo earthquake ( M 6.0) occurred withinthe Kutcharo caldera, which is located in the Akan-Shiretoko volcanic belt in the south-western Kurile arc ofnorthern Japan (Fig. 1(a)). This area lies within one of theactive inland earthquake areas of the Hokkaido district, the

Teshikaga area, where eight large earthquakes ( M > 5)occurred between 1938 and 1972 (Fig. 1(b)). The 1938earthquake formed numerous rupture zones, mostly strikingnorth-west to south-east (Fig. 1(c)). Based on the distribu-tion of surface ruptures, the earthquake fault was estimatedto be along a north-west–south-east strike azimuth betweenthe Kushiro River and the Wakoto Peninsula (Tanakadate,1938; Tsuya, 1938; Kato, 1940; Fig. 1(c)). Some of theruptures showed left-lateral strike-slip and a maximum dis-placement of 2.5 m (Tsuya, 1938; Fig. 1(c)). Vertical dis-placements were also reported in the ruptures and formeda depression zone in the largest rupture zone (Fig. 1(c)).According to Tsuya (1938), the north-eastern side of theslip on the eastern flank of Maruyama subsided a maximum0.8 m. A change in the lake level was also observed af-ter the 1938 earthquake at the south part of the Kutcharocaldera. Tanakadate (1938) and Kato (1940) reported sub-sidence along, and upheaval, at the north-east side of the1938 estimated fault (Fig. 1(c)). The vertical deformation

345

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y (m

Gal

)

30a

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(a)

H. ICHIHARA et al.: RESISTIVITY AND DENSITY STRUCTURE OF THE 1938 KUTCHARO EARTHQUAKE

to

z

Ak olca

(c) LakeKutcharo

200

2 km

100 km Pacificpla te

Wakotopeninsula

Borehole(TS-1)

Rishirilava dome

90a

(b)

Borehole(TS-6)

80a

LakeKutcharo

Mt. Atosanupuri50a

60a

55a

A3070a 400

20

0 Oputakeshukelava dome

Nakajimavolcano

LakeMashu

400

Maruyamalava dome

20a(A20)

40a45a

Borehole(TS-3)

Kushiro

Riv er

1938 Kutcharoearthquake (M 6.0)

Altitude (m)

10a

200

10 km 0 100 300 700 1300

Fig. 1. (a) Tectonic setting around the study area. Red triangles denote active volcanoes. (b) Topography of the study area (red rectangle). Red dashedlines, Kutcharo caldera rim after Katsui (1962); stars, epicentres of the inland plate earthquakes that occurred between 1938 and 1967 (Suzuki andHirota, 1973); red solid dots, hydrothermal activity. (c) Location of MT stations and rupture zones of the 1938 earthquake (light blue rectangle).Grey lines, altitude; pink areas, rupture zones; red dashed line, estimated fault of the 1938 Kutcharo earthquake ( M 6.0; Tanakadate, 1938); blue plus,upheaval areas; blue minus, depression areas (Kato, 1940); Red dots, hydrothermal activity; circles, wideband MT stations; triangles, AMT stations;blue line, DC electrical resistivity imaging survey.

43.7

43.65

100

43.6

TS-1

TS-6

90

80

70

6043.55

50

43.5

TS-3

5 km

40

30

144.25 144.3 144.35 144.4 144.45 144.5

Fig. 2. Bouguer anomaly around Kutcharo caldera (assumed density, 2400 kg/m3). Red squares, gravity stations; pink line, MT survey profile; red

dashed line, estimated fault of the 1938 Kutcharo earthquake ( M 6.0; Tanakadate, 1938).

Bou

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H. ICHIHARA et al.: RESISTIVITY AND DENSITY STRUCTURE OF THE 1938 KUTCHARO EARTHQUAKE

(a)

0

-20

-40

-60

347

(b) RTS-3 TS-6 TS-1

unit A-2(2430 kg/m3)

unit A-1(1660 kg/m3)

0

1

2

3

4

unit C (2620 kg/m3)

unit B (2090 kg/m3)

5 km5

Fig. 3. (a) Bouguer anomaly along profile A–A in Fig. 2. Circles, observed gravity anomaly at a station within 3.0 km of the profile; solid line,calculated gravity anomaly of the final 2-D density model. (b) The final 2-D density model along profile A–A . Inverted triangles, borehole sitelocations; inverted solid triangles, surface geological boundaries; R, rupture zone location of the 1938 Kutcharo earthquake.

indicated by the changing lake level seems to contradict thatof the rupture displacement. Nevertheless, no quaternaryfault activities have been identified around the study area.This may be due to the thickness of the pyroclastic flowsand tuffs created by volcanic events that bury evidence offault activity.

The activity around the Kutcharo caldera is divided intothree stages: the precaldera, caldera-forming, and post-caldera stages (Hirose and Nakagawa, 1995). The pre-caldera stage activity (1.8–0.87 Ma) formed somma lavasbetween the rims of the Kutcharo caldera (Hirose and Nak-agawa, 1995). In the caldera-forming stage (0.34 Ma–31 ka), numerous eruptions caused pyroclastic flows thatformed the Kutcharo caldera (e.g., Katsui, 1962; Koshimizuand Ikushima, 1989). Postcaldera activity formed theMt. Atosanupuri volcano and the lava domes denoted inFig. 1(c), and this volcano has shown recent activity. Hy-drothermal activity has also been identified, especially onthe eastern side of the Kutcharo caldera (Fig. 1(b)).

Yokoyama (1958) reported a clear negative Bougueranomaly in the Kutcharo caldera, in which the gravity wasabout 40 mGal lower at the centre of the caldera than at

porosity as well as the salinity and interconnectivity of thefluid (e.g., Archie, 1942). In addition, resistivity imagingis particularly sensitive to clay minerals, such as montmo-rillonite, produced by alteration because they significantlyreduce resistivity of the host rock (e.g., Takakura, 1995),and detecting such clay minerals supports the presence ofhydrothermal activity (e.g., Nurhasan et al., 2006). Thus,resistivity imaging helps identify fluid distribution in vol-canic areas, which may be related to the occurrence ofearthquakes, such as the earthquake swarms in the LongValley caldera. In addition, resistivity reflects geologicalheterogeneity (e.g., Park and Wernicke, 2003), so we alsointerpreted the resistivity structure to identify the geologicalstructure.

Additionally, we performed a two-dimensional (2-D)direct-current (DC) electric resistivity survey within theruptured zone of the 1938 fault, which detects vertical dis-placement from fault activity when resistive or conductivelayers are distributed beneath fault zones (e.g., Suzuki et al.,2000). With this approach, the historical activity along the1938 fault, which is not known, can possibly be clarified.

the rim. This implies a significant density anomaly around 2. Density Structurethe Kutcharo caldera. Detailed investigations by densitymodelling could identify the geometry of the geologicalboundary, which may be related to fault activity. Therefore,we modelled the density structure using densely distributedgravity data. Surface geology and borehole data were usedas constraints to obtain a robust density model.

We also conducted audio-frequency magnetotelluric(AMT) surveys and modelled the electrical-resistivity dis-tribution. Imaging of the resistivity distribution is effec-tive for estimating fluid distribution because it depends on

2.1 Bouguer anomalyWe calculated the Bouguer anomaly around the Kutcharo

caldera using gravity data reported by Yokoyama (1958),the Japan Geothermal Energy Development Centre (1980),Ichihara et al. (2006), Yamamoto and Ishikawa (2004)and Hasegawa et al. (2006). We assumed that the den-sity for the Bouguer and the terrain corrections were2400 kg/m3 because Yamamoto and Ishikawa (2004) esti-

mated a 2400±130 kg/m3 optimum surficial density basedon a method developed by Murata (1993) using Akaike’s

348 H. ICHIHARA et al.: RESISTIVITY AND DENSITY STRUCTURE OF THE 1938 KUTCHARO EARTHQUAKE

Table 1. Summary of the lithology and petro-physical properties of the boreholes drilled by NEDO (1985). The density value is the average of measuredcore samples from four boreholes drilled in the caldera. The P -wave velocity (V p ) and resistivity are from the velocity found by electrical logging(100 cm normal) at borehole TS-3. The resistivity range is based on standard deviations in logarithms of the resistivity. Calculations were based onthe average of V p and density listed in the left columns, and 0.25 was assumed for Poisson’s ratio.

Unit

Unit A-1

Unit A-2

Unit B

Unit C

Lithology

Pumice andPyroclastic deposits

Andesitic lavas

Tuff and

volcanic breccias

Altered andesite and

pyroclastic rocks

Age

Quaternary

QuaternaryPliocene

–Quaternary

Middle

Miocene

Density(kg/m3)

1660±320

2400±130

2090±280

2620±120

V p(km/s)

2.00

—

2.74

3.88

Young’smodule (GPa)

6.1

—

14.1

32.6

Resistivity(ohm-m)

68–370

—

18–83

290–980

Bayesian Information Criterion (ABIC).The Bouguer anomaly showed a steep gradient around

the 1938 estimated fault (Fig. 2). At the north-east sideof the fault, the estimated anomaly was 60 mGal lowerthan that at the south-west side, and the gravity gradientexceeded 10 mGal/km near the estimated fault. This con-siderable gravity variation indicated a large density contrast

deep at the north-east side and approximately 900 m atthe south-west side of the rupture zone. Borehole TS-3limited the sharp change in depth. If the depth of theboundary between units B and C changes gradually,then the boundary depths between A-1/B and B/C mustsignificantly exceed the constraint depths of boreholesTS-3 and TS-6.

or a deeply extending density boundary. In addition, thestrike of the gravity contours was parallel to the strike of the1938 estimated fault. These results implied a relationshipbetween the fault plane and density heterogeneity causedby the caldera formation.2.2 Two-dimensional density modelling

Using 2-D forward processing (Talwani et al., 1959), wemodelled the density structure along the A–A profile toidentify the density distribution around the ruptured zones(Fig. 2), which cross the estimated fault of the 1938 earth-quake. The observed Bouguer anomalies within 3 km fromthe profile are presented in Fig. 3(a).

Based on samples from three boreholes drilled to a depthgreater than 1 km by the New Energy and Industrial Tech-nology Development Organization (NEDO, 1985), the ge-ology of the Kutcharo caldera was categorised into unitsA-1, A-2, B, and C in the model (Table 1). The geome-try of the boundaries between each unit was estimated inthe model by assuming a fixed density within each unit (Ta-ble 1). Three boreholes drilled by NEDO near the profile(TS-1, TS-3 and TS-6) provided information on the depthof the unit boundaries. The boundary depths for unit A-1/Bwere 545, 697, and 507 m in boreholes TS-3, TS-6, and TS-1, respectively. The boundary depths for unit B/C were 742and 1366 m in boreholes TS-3 and TS-6, respectively. UnitA-2 (Atosamupuri somma lava) was between depths of 416and 610 m in borehole TS-1. These boundary depths andthe surface geological boundary (Matsunami and Yahata,1989; Fig. 3(b)) were used as a model constraint.

Features of the final density model (Fig. 3(b)) and theirvalidity are as follows:

3)

4)

5)

3.

The bottom depth of unit B was estimated to be3500 m, but two reasons suggest that the true depthwas probably deeper. First, the density contrast be-tween units C and B (530 kg/m3) should decrease withdepth because unsolidated sediment (unit B), but notsolidated rock (unit C), is compacted with depth. Asmaller density contrast requires a deeper boundarybetween units B and C. Second, although Bougueranomalies clearly implied a three-dimensional (3-D)structure, we assumed a 2-D structure in this mod-elling (Fig. 2). The 2-D analysis assumed that den-sity anomalies (units A-1, A-2, and B) are distributedinfinitely along the strike zone; however, if the low-density area has a limited distribution along the strike,the bottom depth of unit B should be deeper thanshown by the 2-D model. The deeper boundary indi-cates that the gradient of the unit B/C boundary underthe ruptured zone is steeper than that indicated by thefinal density model.Because the density models that followed the con-straints explained in 1) and 2) did not completely fitthe observed gravity data, the calculated gravity wasslightly lower than the observed gravity around bore-hole TS-6 (Fig. 3(a)). This inconsistency may be alle-viated when the compaction of units A and B is furtherinvestigated or a 3-D model is developed.No borehole data were available for the unit bound-aries at the north-east of borehole TS-1, so the depthswere not determined.

Resistivity Structure Derived from Magnetotel-luric Observations

1) The boundary depth between units A-1 and B did notvary significantly around the rupture zones of the 1938earthquake, which is validated by the borehole data(545, 697, and 507 m in boreholes TS-3, TS-6, andTS-1, respectively).

2) The boundary depth between units B and C changedsharply around borehole TS-6; it was over 3500 m

3.1 Magnetotelluric survey and data processingAMT surveys were conducted across the rupture zone of

the 1938 earthquake fault at 10 sites in May and Novem-ber 2006 (Fig. 1(c)). A time series of the MT data wererecorded using the MTU2000 system manufactured byPhoenix Geophysics, Ltd. (Toronto, ON, Canada). Theelectric fields were measured at all sites using orthogo-

min

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ncy

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0 H

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goL

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H. ICHIHARA et al.: RESISTIVITY AND DENSITY STRUCTURE OF THE 1938 KUTCHARO EARTHQUAKE 349

1100 Hz

0.3

km

9.4 Hz

0.3

km

75

60

45

300 1 2 0 1 2

15

97 Hz

0.3

km

1.02 Hz

0.3

km

10

0

0 1 2 0 1 2

-10

Fig. 4. Phase-tensor ellipses and Parkinson’s induction vectors (1100, 97, 9.4, and 1.03 Hz). Colour scales of the outer ellipse, min skew angles; greyscales of the inner ellipse, phase-tensor skew angles; red arrows, induction vectors; red dots, hydrothermal activity. AMT and wideband MT data(Ichihara et al., 2006) were used for the analyses of 1100, 97, 9.4 Hz, and 1.03 Hz, respectively.

(a)Observed

(b)Observed

A20 55a A20 55a10a /20a 30a 40a 45a 50a 60a A30 70a 80a 90a 10a /20a 30a 40a 45a 50a 60a A30 70a 80a 90a

3

2

1

0

3

2

1

0-4 -3 -2 -1 0 1 2 3 4 5 -4 -3 -2 -1 0 1 2 3 4 5

3

2

1

CalculatedDistance (km)

3

2

1

CalculatedDistance (km)

Distance (km)

Resistivity (Log10 ohm-m)

5Distance (km)

Phase (degree)

4 5

1.0 1.5 2.0 2.5 3.0 20 30 40 50 60 70

Fig. 5. Pseudo sections of the observed and calculated apparent resistivity and impedance phase for the MT inversion. Black dots, data used for theinversion procedure.

nal dipoles with Pb-PbCl2 electrodes attached at both ends.The magnetic fields at all sites were measured using threeorthogonal-induction coils. Wideband magnetotelluric datawere also acquired at stations 40a and 90a using inductioncoils oriented for wideband sensing.

The recorded AMT time-series data were converted into

impedance tensors in the frequency domain through a cas-cade decimation technique (Wight and Bostick, 1980). Thefrequency of the converted impedance for the AMT dataranged from 3300 to 0.35 Hz. High-quality data were ob-tained at most sites, but the lower frequency band for the

AMT impedance (<2 Hz) was not well identified, espe-

350 H. ICHIHARA et al.: RESISTIVITY AND DENSITY STRUCTURE OF THE 1938 KUTCHARO EARTHQUAKE

cially for sites 10a, 30a, 55a, 70a, and 80a. In addition, wedid not obtain impedances lower than 10 Hz at site 60a. Theimpedances in the so-called dead bands (5000–1000 Hz)were also not well determined at some sites. Wideband MTdata at sites 40a and 90a (Fig. 1(c)), and A20 and A30 (Ichi-hara et al., 2006) were converted into impedance tensors be-tween 320 and 0.00055 Hz through the same process usedfor the AMT data. High-quality data were obtained at allsites and for most frequency bands.3.2 Estimation of the 2-D strike azimuth and dimen-

sionality of the MT dataThe magnetotelluric-phase tensors (Caldwell et al.,

2004) and Parkinson’s induction vectors were calculatedto verify two-dimensionality and to determine the 2-D az-imuth for the inversion. Phase tensor is an important toolto recognise the dimensionality of the subsurface structure.The phase tensor is defined as

= X−1Y,

where X and Y are the real and imaginary componentsof the impedance tensor (Z), respectively. A significantadvantage is that phase tensor is not affected by galvanicdistortion. A phase tensor is written in the form

Figs. 5 and 6, respectively. The inversion code estimates themodel based on the ABIC least-squares method and com-prehensively evaluates the root-mean-square (RMS) mis-fit between the observed and calculated impedance, themodel smoothness and static shifts. Impedances in lower

frequency bands (<1 Hz) were not used for the inversionfor the following reasons. First, the low frequency dataat all of the wideband MT sites were affected by a largephase-tensor skew angle exceeding 10◦ and anomalous off-diagonal phases exceeding 90◦ (Ichihara et al., 2006). Sec-ond, a NE–SW azimuth of the regional strike of easternHokkaido district (Satoh et al., 2001) is not consistent withstrike of our profile.

We used only the TM-mode impedances for the inver-sion for the following reasons. High phases around 100 Hzat sites 40a–55a also indicated a conductor around this area(Fig. 5). However, induction vectors oriented to the north-west between 1100 and 9.4 Hz (Fig. 4) implied that the con-ductor had a 3-D geometry. Ledo et al. (2002) and Siripun-varaporn et al. (2005) tested the 2-D inversion for a sec-tion in which a synthetic 3-D structure conductor was dis-tributed in and out of the survey line, similar to the situa-tion in this study. When the 3-D conductor is out of theprofile, Siripunvaraporn et al. (2005) showed a phantom of

= RT (α − β)max

0

0

minR(α + β),

the 3-D conductor among the inversions using TM, TE, and

TM + TE mode impedances. However, Ledo et al. (2002)showed that the phantom is not clear in the TM + TE mode

where max, min and β are co-ordinate invariants and R(θ )is a rotation matrix. The phase-tensor ellipse is the graph-ical expression of the phase tensor in which the major axis

( max) and minor axis ( min) of the ellipse depict the princi-pal axes and the tensor values. The orientation of the major

axis is specified by the angle α − β . In the 2-D case, themajor axis of the phase tensor is aligned parallel or perpen-dicular to the strike of the regional resistivity distribution.

The phase-tensor skew angle, β , measures asymmetry ofthe phase response. For the detailed explanations, see Cald-well et al. (2004).

inversion compared to the TM mode inversion. Thus, TM

+ TE mode data seemed to be appropriate in our situation.When the 3-D conductor is in the profile, Siripunvarapornet al. (2005) showed that inversions using only TM-modeimpedances identified an appropriate structure, whereas in-

versions using only the TE mode or the TM + TE modesproduced a strong phantom conductor beneath the true 3-D conductor. Ledo et al. (2002) also indicated that the

TM mode was superior to the TM + TE data. Therefore,whether the 3-D conductor is in or off the profile determinesthe appropriate mode. Because resistivity logging in TS-3

The azimuths of max in the phase-tensor ellipses were showed a conductor in the profile between a depth of 500–

mostly directed to the same azimuth as the MT profile(N55◦E–S55◦W) or its perpendicular azimuth (Fig. 4). Be-

cause max and min varied within the profile, the 2-D strikeazimuth seemed to be oriented N35◦W–S35◦E. Based onthe above two possibilities, the induction vectors (Parkin-son’s vectors) support the N35◦W–S35◦E azimuth as the 2-D strike (Fig. 4). Therefore, N35◦W–S35◦E was adoptedas the strikes of the 2-D inversions. However, the induc-tion vectors near the 1938 earthquake rupture zone orientedtoward the north at 1100–9.4 Hz (Fig. 4). These featuresindicate the existence of a 3-D conductor in the northernpart of the 1938 earthquake fault zone. The unsalted waterof Lake Kutcharo was excluded as a candidate for the 3-Dconductor.3.3 Two-dimensional inversion of the MT data

After the impedances were rotated into the estimated 2-D azimuth, the apparent resistivity and phase in the fre-quency band between 3300 and 1 Hz were inverted usingthe 2-D inversion code developed by Ogawa and Uchida(1996). Pseudo sections of the data using the inversionand sounding curves of sites 40a and 55a are described in

700 m (NEDO, 1985; Fig. 1(c)), the conductor seemed tobe distributed in the profile. Therefore, the TM mode waspreferable for the inversion.

The inversion was started from a 100-ohm-m homoge-neous half-space model. A 5% error floor in the apparentresistivity and phase was applied. The observed impedanceswere mostly explained by the inversion procedure (Fig. 5).The RMS misfit in the initial model (16.92) was reduced to2.50 in the final iteration. The inverted resistivity section(Fig. 7) showed the following features:

1) A conductive body was imaged under the ruptured

zone (C1, 10–30 ohm-m). The C1 conductor wasrequired to explain large phases over 60◦ that wereobserved around 100 Hz at sites 40a–55a.

2) The C2 conductor (3–20 ohm-m) was distributedaround the north-eastern edge of the profile, where hy-drothermal activities have been observed (Fig. 1(b)).Low apparent resistivity at site 90a (Fig. 5) clearly in-dicated the C2 conductor.

3) A resistive area was imaged below 1000 m at thesouth-west side of the 1938 Kutcharo earthquake rup-

resi

stiv

ity [l

og10

ohm

-m]

resi

stiv

ity [l

og10

ohm

-m]

phas

e [d

eg]

phas

e [d

eg]

Dep

th (

km)

Re

sist

ivity

(Lo

g10

ohm

-m)

10 10 10 10 10 10 10 10 10 10

45a 50a 55a 60a A30 70a

H. ICHIHARA et al.: RESISTIVITY AND DENSITY STRUCTURE OF THE 1938 KUTCHARO EARTHQUAKE 351

(a)3

Site 40a(b)

3Site 50a

2.5

2

1.5

obscal

test s2test s1-2test c1 2.5

2

1.5

obscal

test s3test s3-4test c1

7560

45

30

60

45

154 3 2 1

frequency [Hz]

030

4 3 2 1

frequency [Hz]

0

Fig. 6. Observed and calculated sounding curves for the apparent resistivity and phase of sites 40a and 55a. Dots with error bars, observed apparentresistivity and phase of TM mode; black solid line, inverted model; coloured lines, hypothetical model.

S55 W N55 E

0

-1

10a

A2020a

30a40a

C1

R

R2

80a90a

4.0

3.5

3.0

2.5

-2 R1 S-1 S-2 S-3 S-4C2 2.0

1.5

-31.0

-4 -3 -2 -1 0 1 2 3 4 5 0.5

Distance (km)

Fig. 7. Inverted 2-D resistivity image along the MT profile crossing the rupture zone of the 1938 Kutcharo earthquake ( M 6.0). Red rectangle, locationof the DC electrical survey in Fig. 8; R, location of a 1938 Kutcharo earthquake ( M 6.0) rupture zone.

ture zone (R1, 500–1000 ohm-m). High apparent re- 3.4 A sensitivity test of the inverted resistivity struc-sistivity and low phase between 1 and 10 Hz at sites10a, 20a/A20, A30 and 40a required the R1.

4) The resistive zone (R2) was detected from the surfaceto a depth of 200–300 m at the eastern part of the pro-file. R2 obviously corresponded to unit A-2 consistingof andesitic lavas.

Resistivity logging at TS-3, drilled near site 40a, mea-

sured a resistivity of 40–300 ohm-m in unit A (0–545 mof depth), 10–50 ohm-m in unit B (545–742 m) and 100–

500 ohm-m in unit C (>742 m; Fig. 8). The inverted re-sistivity image was consistent with the vertical resistivityvariations measured in borehole TS-3 (Fig. 8). Thus, theinverted image seemed to be reliable around borehole TS-3.

turesThe final inverted resistivity model from the MT data

showed a conductive body under the rupture zone (C1, 20–50 ohm-m; Fig. 7). The sensitivity test confirmed the exis-tence of the C1 conductor. Based on the final model, C1 ishypothesised to have been converted to 100 ohm-m (Fig. 7).The responses provided by the hypothetical model were sig-nificantly inconsistent with that of the final model and theobserved 100 Hz at sites 40a–55a (Fig. 6). The RMS errorof the hypothetical model increased from 2.50 to 4.84 in thefinal model.

The position of the boundary between R1 and the moder-ate resistivity zone was also important to the caldera struc-ture and the 1938 earthquake, but the location is tenuousbecause the C1 conductor covered this area. Thus, wetested the sensitivity of the location with four hypotheticalmodels. First, the S-2 region of the final resistivity model

0 0

Uni

t A-1

m)(

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(m)e

D

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352 H. ICHIHARA et al.: RESISTIVITY AND DENSITY STRUCTURE OF THE 1938 KUTCHARO EARTHQUAKE

(a)Resistivity (ohm-m)

(b)Density (kg/m3)

0 200 400 600 1000 2000 3000

200

400

600

800

1000

200

400

600

800

1000

Fig. 8. Measured and modelled resistivities and densities along borehole TS-3 (NEDO, 1985). The location of the borehole is shown in Fig. 2. (a)Dotted lines, logged (25 cm normal) resistivity from the 2-D inversion; solid lines, estimated resistivity. (b) Dots, core sample wet density; solid line,estimated density (Table 1).

(Fig. 7) was filled with 50 ohm-m. The response producedby this hypothetical model was almost the same as the finalmodel at all sites (response at site 40a is drawn with a bluedashed line in Fig. 6(a)). The second model indicated thata region between S-1 and S-2 was filled with 50 ohm-m.The model response showed significantly higher phases of

around 10 Hz and lower apparent resistivity (<3 Hz) com-pared to the final model and the observed data (a blue dottedline in Fig. 6(a)). These results indicate that high resistivitywas required at S-1 but possibly not at S-2. For the thirdhypothetical model, the S-3 region in Fig. 7 was filled with300 ohm-m. These responses explained the sharp phase un-dulations of around 10 Hz at sites 50a and 55a comparedto the final model (blue dashed line in Fig. 6(b)). Theimpedances at the other sites were almost the same as thefinal model. Finally, the S-3 and S-4 regions were filledwith 300 ohm-m. The results differed significantly fromthe observed data and the third hypothetical model. Teststhree and four indicated sharp resistivity changes betweenS-3 and S-4, but the smoothness constraint did not allowconvergence into a sharp structure. Consequently, the loca-tion of the R1 margin was around S-2, S-3, and S-4, andthe sharper resistivity boundary may have been distributedat the R1 margin.

near site 50a (Fig. 1(c)). Then, DC electrical resistivi-ties were measured from 748 combinations of transmittingand receiving dipoles in Wenner and Eltran arrays by us-ing electrical-resistivity imaging instruments manufacturedby Chiba Electronics Co. Interelectrode spacing (a) variedfrom 10 to 160 m. The sensitive zone of the Wenner ar-ray was concentrated at the dipole centres but was narrowalong the horizontal direction. In contrast, the Eltran ar-ray had a relatively high sensitivity at the horizontally widearea but had reversed sensitivity at the outside of the dipolesystem. Both arrays had weaker sensitivity at areas deeperthan a. Pseudo sections of the observed apparent resistivi-ties are shown in Fig. 9. The observed apparent resistivitydata were inverted to a 2-D resistivity image using the nu-merical inversion code developed by Uchida (1991). Theinversion started from the 100 ohm-m homogeneous half-space model. An excellent fit of the observed data and thecalculated pseudo sections was noted (Fig. 9). The RMSmisfit was reduced from 46.6 to 1.34 in the final iteration.

Several features were recognised in the inverted re-sistivity section under the 1938 earthquake rupture zone

(Fig. 10). One was a deep (>100 m) high-resistivity body(R3, <1000 ohm-m) found at the south-west side of the rup-ture zone. Because R3 was not found in the north-east sideof the rupture zone, a thickness of the R3 resistive body

4. DC Electrical Survey Around the Ruptured greater than 30 m indicates that more than 30 m of verticalZone

To reveal the detailed resistivity structure around the1938 earthquake rupture zone, we performed a 2-D DCelectrical-resistivity survey in November 2006. We laid a490-m transect as an AMT survey line with 50 electrodes

movement occurred along the 1938 earthquake fault. Pastactivity along this plane may have been supported by anapproximately 10-m vertical offset, distributed around 50-m depth, between the relatively low resistivity anomaliesof C3 and C4. The 10-m offset is not precise because the

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H. ICHIHARA et al.: RESISTIVITY AND DENSITY STRUCTURE OF THE 1938 KUTCHARO EARTHQUAKE 353

0

50

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150 OBS (Wenner) 150 OBS (Eltran)0 100 200 300 400 500 0 100 200 300 400 500

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150 CAL (Wenner) 150 CAL (Eltran)0 100 200 300 400 500 0 100 200 300 400 500

Distance (m)

Resistivity (log10 ohm-m)

Distance (m)

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

Fig. 9. Pseudo sections of the observed and calculated apparent resistivity phase for the DC electrical survey.

0

50

100

R4A ruptured zone ofthe 1938 earthquake

C3

C4

R3

3.5

3

2.5

2

0 100 200 300 400 500

Distance (m)

Fig. 10. DC resistivity image around the fault zone of the 1938 Kutcharo earthquake (6.0). A rupture zone created by the earthquake was reported nearthe 170-m point (Tsuya, 1938).

(a)

010a

A2020a

TS-3

30a40a

TS-6R

55a45a 50a

C1

60a A30 70a

TS-180a

R2

90a

(b)

0

topographiccaldera wall

the 1938 Kutcharo earthquake (M 6.0)

Hydrothermal activities

conductor (C1)

-1 1

-2 R1 C2 2conductor

(C2)

-3-4 -3 -2 -1 0 1 2 3 4 5

3 Basement rocks

Distance (km) Resistivity (Log10 ohm-m)

1.0 2.0 3.0 4.0

4

5

transportation

of aqueous fluid

? ?

Magma

6 Intrusion

-4 -3 -2 -1 0 1 2 3 4 5

Distance (km)

Fig. 11. (a) Comparison of the density and resistivity structure. The image created by the black mesh marks denotes the final density model in Fig. 3.The colour image describes the resistivity structure in Fig. 6 projected to profile A–A in Fig. 2. (b) A simultaneous image of the Kutcharo calderaand the estimated fault of the 1938 Kutcharo earthquake ( M 6.0) along the same profile as shown in (a).

354 H. ICHIHARA et al.: RESISTIVITY AND DENSITY STRUCTURE OF THE 1938 KUTCHARO EARTHQUAKE

block size of the resistivity model was 10 m; however, it isthought to be adequately large compared to the vertical dis-placement caused by the 1938 earthquake (0.8 m; Tsuya,1938). The lack of R3 on the north-east side and the offsetbetween C3 and C4 may indicate subsidence at the north-east side as a result of repeated earthquake activity. Theseoffsets are consistent with the vertical displacement of the1938 earthquake surface ruptures. However, the changesin lake level after the 1938 earthquake indicate subsidencealong or on the south-west side and upheaval at the north-east side of the ruptured zone (Kato, 1940; Fig. 1(c)). De-tailed surveys, such as a fault trench, are required to clarifythe fault movements.

A resistive zone (R4) was also identified immediately be-low the surface near the Maruyama dome at the south-westside of the profile. Because the area around the Maruyamalava dome has a higher elevation, R4 was probably associ-ated with Maruyama lava.

the lowermost layer among the outcroppings of the Neo-gene basement rocks in the Teshikaga area (Matsunamiand Yahata, 1989). NEDO (1985) and Koshimizu andKim (1986) suggested that the fission and track ages ofthe Ikurushibe formation are 13.1 and 7.9–8.2 Ma, respec-tively. The resistivity logging showed high resistivity (290–980 ohm-m; Table 1) in unit C. The inverted resistivity fromthe AMT survey along borehole TS-3 revealed high resistiv-ity consistent with the logged data (Fig. 8), indicating thatthe resistive zone (R1) corresponded to unit C. In addition,the spatial distribution of the resistive zone (R1) and unitC is consistent (Fig. 11(a)). Therefore, we can assume thatthe basement rock of the Ikurushibe or older formations isdistributed on the south-western side of the 1938 fault zone.The area between 500 and 1000 m under site 10a shows rel-atively low resistivity. This conductive zone may indicatean older formation underlying the Ikurushibe formation orsediments within the Ikurushibe formation (Matsunami andYahata, 1989), which generally shows low resistivity com-

5. Discussion pared to andesites (NEDO, 1985).

5.1 Interpretation of resistivity and density structures

The well-log (TS-3) drilled by NEDO (1985) providedinformation on the lithology, density, and resistivity of thesite (Fig. 8). Based on this borehole data and other refer-ences, the calculated density and resistivity models can beinterpreted as follows.

The final inverted resistivity model from the MT datashowed a conductive body under the rupture zone (C1,20–50 ohm-m; Fig. 7). The sensitivity test confirmed theexistence of the C1 conductor. The C1 resistivity (10–30 ohm-m) was consistent with the logging resistivity ofboreholes TS-3 and TS-6. A comparison of the resistivitybetween the final model and the TS-3 logging data is shownin Fig. 8. Montmorillonites were identified in core sam-ples obtained near the C1 conductor (NEDO, 1985). Be-cause montmorillonite significantly reduces the resistivityof host rock (e.g., Takakura, 1995), C1 is interpreted asa montmorillonite-rich zone and implies hydrothermal ac-tivity under the rupture zone because they are commonlyproduced by hydrothermal alteration. The C1 conductorseems to be distributed toward the north-west of the profile,but not the south-west side because the induction vectorspointed north at 1100, 97, and 9.4 Hz near C1 (Fig. 4). Sig-nificant hydrothermal activity has been recognised aroundMt. Atosanupuri as well as along the 1938 estimated fault(Fig. 1(b, c) and Fig. 4), implying that the C1 conductoralong the 1938 fault was produced by hydrothermal alter-ation. The margin of the depression zone may have been apath for water that subsequently induced the C1 hydrother-mal alteration.

The magma intrusion has been estimated to be about6 km deep at 2 km north of site 90a (Fig. 11(b); Geographi-cal Survey Institute, 2006). In addition, hydrothermal areasand an active volcano (Mt. Atosanupuri) are distributed atthe surface of the magma-intruded zone (Fig. 1(b)), sug-gesting that the C2 conductor was produced in associationwith these activities.

Unit C (Ikurushibe formation) consists mainly of an-desite and tuff breccias, most of which are altered in thecore samples. The Ikurushibe formation is assumed to be

The low Bouguer anomaly in the Kutcharo caldera canbe explained by the thick sediment (units A-1, B; Fig. 3).The low to moderate resistivity zone (10–200 ohm-m), in-cluding the C2 conductor north-east of R1, correspondedto units A-1 and B that were inferred as the sediment-fill-covered Kutcharo caldera (Fig. 11(a)). Based on the hy-pothetical testing discussed in 3.4, the R1 margin was mostlikely located between S-2 and S-4, and a sharper resistivityboundary may have been distributed there than indicated bythe inverted resistivity model. These features are consistentwith the steep incline zone of the density boundary betweenunits B and C, and indicate that the resistivity and densitydistributions image the geological boundary of past calderaactivities.5.2 The relationship between the 1938 Kutcharo earth-

quake and the Kutcharo calderaThe detailed resistivity image produced by the DC elec-

tric survey data suggests cumulative fault activity in theQuaternary along the 1938 rupture zones (Fig. 10). Thedensity model constrained by boreholes TS-3, TS-6, andTS-1 showed a steeply dipping boundary between unitsB and C beneath this area (Fig. 3). The resistivity sec-tion obtained from the MT data also supports this bound-ary (Figs. 7, 11(a)). In general, many inland earthquakesare the result of an abrupt slip within large discontinuouszones. Therefore, this clear spatial correspondence betweenthe main depression boundary of the Kutcharo caldera andthe 1938 fault indicates that an abrupt slip along the calderamargin caused the 1938 earthquake.

Normal faulting with the collapse of the caldera, the re-lease of regional stress due to a crustal heterogeneity andfluid intrusion are three possible mechanisms for the 1938earthquake. The first possibility is unlikely because de-pression (outer sides) and upheaval (inner sides) of thecaldera occurred after the 1938 earthquake and were basedon changes in the lake-water level (Tsuya, 1938; Kato,1940; Fig. 1(c)). In addition, the 1938 earthquake causedleft-lateral strike-slip displacement.

The second possibility may be appropriate because largecontractive strain (about 8 ppm during 1883–1994) has been

H. ICHIHARA et al.: RESISTIVITY AND DENSITY STRUCTURE OF THE 1938 KUTCHARO EARTHQUAKE 355

accumulating in the eastern Hokkaido area (Hashimoto andTada, 1988) generated by the oblique subduction of the Pa-cific plate along the Kurile trench. As Table 1 shows, ge-ological differences between the caldera fill (units A andB) and basement rock (unit C) create a clear elastic bound-ary (Young’s modulus). Generally, the local elastic hetero-geneity in the regional-stress field induces a localised ac-cumulation of stress resulting in fault ruptures. Many in-land earthquakes were explained by this theory (e.g. Iio andKobayashi, 2002; Ichihara et al., 2008). The 1938 earth-quake was a release of accumulated regional stress aroundan area of strong elastic heterogeneity due to the calderastructure; however, the north-west–south-east azimuth ofthe contractive axis is nearly parallel to the strike azimuthof the 1938 fault. Thus, the small shear stress is believed tohave induced fault slip.

The third possibility is that fluid injection from associ-ated magma activity induced or enhanced the 1938 earth-quake. Fluid injection can directly cause fault rupture asdiscussed by Julian (1983). He indicated that a dyke intru-sion along or near the vertical fault plane of the Long Val-ley caldera caused several M 6 earthquakes. This hypoth-esis is based on mechanisms that cannot be explained by adouble-couple mechanism, but can be explained by a com-pensated linear-vector dipole mechanism induced by dykeintrusion. Savage and Cockerham (1984) obtained a cor-responding source model from geodetic surveys. No spe-cific seismic and geodetic studies have been conducted inthe Teshikaga area; however, magma intrusion relating tothe 1938 earthquake is supported by the following points.First, the depression boundary beneath the rupture zone caneasily become a pathway of intrusion. Second, the C1 con-ductor, which indicates hydrothermal activity beneath thedepression boundary, suggests the presence of magma ac-tivity.

Fluid injection can also enhance double-couple earth-quakes. High-pressure fluid injection into the fault planeof the San Andreas Fault reduced shear-fracture strengthand allowed fault slip in the small shear-stress environment(e.g., Rice, 1992; Sasai, 1997). The high-pressure fluidalong the depression boundary of the 1938 fault is consis-tent with this notion because the C1 montmorillonite-richzone could behave as an impermeable cap for the suppliedfluids transported through the 1938 fault plane (Nurhasanet al., 2006). Thus, the 1938 earthquake may have resultedfrom a similar mechanism as described for the San AndreasFault. Consequently, the mechanism for the earthquakearound the Kutcharo caldera can be explained by the fluidintrusion hypothesis. This mechanism and stress accumu-lation due to crustal heterogeneity do not conflict, and thuscould have acted synergistically to induce the 1938 earth-quake. A dense seismicity and geodetic network aroundthe caldera is expected to contribute to the understanding ofthe complex relationship between fluid behaviour and earth-quake occurrence.

6. ConclusionDensity and resistivity structures were investigated

around the seismogenic zone of the 1938 Kutcharo earth-

quake ( M 6.0) to identify the relationships between the

earthquake and the caldera structure. The final den-sity model revealed a highly inclined geological bound-ary corresponding to a main depression-zone margin in theKutcharo caldera that was underneath the estimated fault.The bottom of the caldera was inferred to be over 3.5 kmdeep. The resistivity structure, derived from AMT surveys,supported the geological boundary at the depression bound-ary. A detailed resistivity section, revealed by a DC electricsurvey, showed discontinuities under the rupture zone, indi-cating cumulative vertical displacement generated by faultactivity. The discontinuity seemed to extend to the depres-sion boundary. Therefore, the 1938 earthquake probably re-sulted from fault activity along the main depression marginof the Kutcharo caldera. We propose that the 1938 earth-quake may have been caused by the collapse of the calderaas a result of normal faulting, the release of regional stressenhanced by a crustal heterogeneity, or fault rupture relatedto fluid intrusion. The first possibility was excluded becausethe depression and upheaval observed after the 1938 earth-quake was inconsistent with the collapse of the caldera. Thesecond possibility also seems unlikely because the maxi-mum principal-stress axis and inferred fault strike are al-most parallel. The last possibility suggests that fluid in-trusion induced a compensated linear-vector dipole mech-anism into the dyke. This high-pressure intrusion into thefault reduced fracture strength and allowed fault-slip due tosmall shear stress. However, little evidence exists to supportthese mechanisms. Thus, stationary seismic and geodeticobservations are needed around the Kutcharo caldera.

Acknowledgments. We thank Prof. Yasuo Ogawa of the TokyoInstitute of Technology and Prof. Masaaki Mishina of Tohoku Uni-versity for providing many of the instruments used in the MT sur-vey. We also thank landowners in the study region for allowing usto establish observation sites on their land. The editor, Dr. AkiraTakada, and two reviewers, Prof. Yoichi Sasai and another anony-mous reviewer, helped us to improve the manuscript. Helpful dis-cussions with Prof. Takeshi Hashimoto, Prof. Yasunori Nishida,Dr. Ryo Honda, Dr. Hiroaki Kamiyama and Prof. Hiroaki Taka-hashi of Hokkaido University helped improve the content of thispaper. GMT software (Wessel and Smith, 1998) was used to drawsome of the figures.

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