Journal of Natural Disaster Science, Volume 35, Number 2, 2014, pp55-66

Use of magnetic surveying in landslide analysis at the boundary between the granite
region and the green tuff region in southwestern Toyama Prefecture
Toshihide SUGIMOTO*
Kazuo KAWASAKI**
Hideo SAKAi***MLIT Hokuriku Regional Development Bureau, River Department**University of Toyama, Division of Earth and Environmental Systems l(Received March 5, 2014 Accepted September 25, 2014)
ABSTRACT
A helicopter-borne magnetic survey is conducted at the landslide-prone area of theboundary zone between the Hida belt and the green tuff in southwestern Toyama Prefecture,and the effectiveness of the airborne magnetic survey is considered in relation to its ability
to verify the magnetic structure in the slope regions. Analysis using the reduction-to-thepole (RTP) method is applied to airborne magnetic anomaly data in the survey after diurnal
correction and removal of the International Geomagnetic Reference Field (IGRF)-7. In
addition, the magnetic properties are measured, and x-ray powder diffraction of boringcores drilled in the region of the Funatsu granite is examined. According to the magnetic
anomaly distribution using the RTP method, the area of igneous rock distribution showshigh magnetic anomaly, and the hydrothermal alteration zone and colluvial deposit area oflandslides show low magnetic anomaly. It has become clear through the study of boringcores that pole magnetic anomaly by airborne magnetic survey reflects the distribution ofmany magnetization substances in the shallow part. Aeromagnetic surveys are an efficientmethod of identifying both the distribution of collapsed soil sediments in the Neogene andareas of high landslide potential in Funatsu granitic rock.
Keyword: Magnetic prospecting, Magnetic anomaly, Susceptibility, Hydrothermal
alteration, Landslide, Igneous rock

1. Introduction
A large number of landslide disasters have occurred in Japan in recent years due to the increasing
occurrence of localized heavy rainfall and earthquakes. There is a concern that more frequent landslides and deep-seated landslides will occur in the future due to the high probability of increasing
frequency of earthquakes and heavy rainfall. In orderto prepare for such landslide disasters, it is necessary

to gain an understanding, in advance, of the locations
and scales of landslides and deep-seated landslides
that can result from earthquakes and heavy rainfall.
Therefore, numerous studies on risk evaluation methods for slope failure have been conducted (Khasbaatar
et al., 2009; Hayashi et al., 2011; Higaki et al., 2011;
Uchida et al., 2011). Landslide and slope failure research is generally performed by conducting topographical and geological reconnaissance and boring55

T. SUGIMOTO, K. KAWASAKI, H. SAKAI

on site, taking into consideration the results of synoptic surveys based on bibliographic surveys, aerial
photointerpretation, and high-precision topographicalinformation. These are then followed by considerationof ground properties and structural analysis (Japan
River Association, 1977; Tamura et al., 2008). However, such investigations take many days to perform,
are costly, and require specialized techniques basedon experience. Taking into consideration the demand
for landslide and slope failure investigations in the future, technologies for efficiently detecting unstableslopes from slopes present over a wide area, as wellas investigative techniques for gaining an understanding of the geology of slopes over broad areas, are required. For this reason, new technologies such as lightdetection and ranging (LIDAR), electromagnetic
studies, and remote sensing are now being used togain information on topographic surfaces and underground structures over broad areas (for instance, Suzuki et al., 2009; Uchida et al., 2010; Shiozaka et al.,2010).
An airborne magnetic survey is a method of investigating geological structures from the air. Thissurvey utilizes an aircraft to measure magnetic fields
and is used to investigate geological structures and lithology by scanning the distribution of magnetic bodies in the subsurface. The method can be used to obtain a synopsis of the geology and related factors ofslopes over a wide area in a short time and is, therefore, considered effective for detecting the occurrenceof primary landslides on slopes that manifest no
symptoms of changes in topography or where deepseated landslides are expected. Recent improvementsin the measurement accuracy of magnetic sensors andthe global positioning system (GPS) have made it
possible to perform detailed surveys at lower altitudes
and shorter survey line intervals. What was previously
a means to explore iron deposits can now be used as
an effective means to unravel underground structuresand is, in fact, currently being used to survey underground structures of volcanoes (e.g., Okuma et al.,
2001; Tanahashi et al., 1995; Hasegawa, 2010). Whileairborne electromagnetic surveys, which are ordinarily conducted at the same time as airborne magnetic
surveys, are now commonly applied in geologicalsurveys over broad areas (Konishi et al., 2001; Nakazato et al., 2004), there have been no studies to clarify
the significance of airborne magnetic surveys for de-tecting landslides, unstable slopes, or boring magnetism by comparing data between the two types of survey.
Sugimoto et al. (2013) conducted an airborne
magnetic survey to determine the unstable sloping
landslide area southwest of Toyama Prefecture, where
there is a boundary between the Hida belt and thegreen tuff region. The magnetic anomaly values observed in 2000 (after diurnal correction and removalof the International Geomagnetic Reference Field(IGRF)-7) were compared with geographical features
and the site geology and they concluded that the magnetic anomalies were correlated with the areas oflandslide.
The shape of a magnetic anomaly depends on
several factors, including: a) the shape of the causative body; b) the inclination and the declination of
the body’s magnetization; c) the inclination and thedeclination of the local earth’s magnetic field; and d)
the orientation of the body with respect to the magnetic north (Nabighian et al., 2005). The IGRF correction could only remove the effect of the local earth’s
magnetic field. Therefore, the magnetic anomaly values after IGRF correction by Sugimoto et al. (2013)
are less accurate in presenting local magnetic anomalies. Hence, the reduction-to-the-pole (RTP) method
is applied to airborne magnetic anomaly data in Sugimoto et al. (2013) to enhance the anomalies and makethe relationship between the magnetic anomalies andthe area of landslide clearer. The RTP method has
been shown to be an effective method for interpretingcorresponding magnetic anomalies (Okuma et al.,
1990; Hasegawa et al., 2009). In addition, we used inlaboratory techniques, including the magnetic properties and the x-ray powder diffraction of the boringcores drilled at the study site to support the analyzed
data. We then compared the geology with the airborne
magnetic anomaly data after transformation with RTP
to investigate the hydrothermal alteration and shear
zone area of the landslide.
2. TOPOGRAPY AND GEOLOGY OFSURVEYED SITES
2.1 Summary of geology
The surveyed sites are located on a boundary between the Hida belt and the region of the Neogene
green tuff layers in the southwestern section of Toyama Prefecture. The green tuff layers form the Toga
56

Use of magnetic surveying in landslide analysis at the boundary between the granite region and the green tuff region in southwestern Toyama Prefecture

The topographic distribution of the landslides in
Figs. 3 and 4 is made based on interpretation from themicrotopography of the landslides that specificallycomprises the gentle slope of the front and the scarpof the head, side cliff, and end climax, as seen fromaerial photographs of 20,000 to 1 scale as well as ageological reconnaissance and existing boring survey
results. In the study area, the five major landslide areas of Oshiba, Kita-mametani, Ohmametani, Toga,and Iwabuchi are distributed in a row from the northto the south. It is noted that the distribution of landslides is regulated by the underlying rocks, and mostof the landslides are distributed in the region of the
Iwaine formation or Nirehara formation (Toyama Prefecture, 1992). Large-scale landslides are scarcely
distributed in Funatsu granite rocks, except in the Oshiba district. However, regional surface slope failures
are likely to occur in Funatsu granite distribution areas that are degraded by bedrock hydrothermal alteration. Although many large-scale compound slides are
distributed on the right-bank side of the Toga Riverand the southern part, small-scale landslides are only
locally distributed on the left-bank side. These largescale landslides slope down to the west on the dip
slope as a whole.
The large-scale landslide areas can be roughly
divided into three areas, from a geological perspective, and from the occupying position of their sliding
surface.
1) Oshiba district: No obvious scarp is noted.
The basement rock consists of Funatsu granite rocks
and cataclastic granite rocks penetrated by andesite
dykes and has, overall, been subject to hydrothermal
alterations. Boring core observations and data from
borehole inclinometers indicate that the sliding surface likely occurred along a westward-tilted clay
seam formed by regional hydrothermal alteration.
2) Kita-mametani and Ohmametani districts: Obvious scarp is noted, with landslide surface shapes ofthe bottleneck type. The Nirehara formation is distributed as dip slopes on the higher levels of the Funatsu
granite rocks, while the volcanic rocks of the Iwaine
formation are distributed in a cap rock shape on theuppermost layer. The surface layer is covered by colluvium from large-scale landslides and collapses in
the past. Overall, the Iwaine formation has been altered to a green color and the Nirehara formation,
composed of sedimentary rocks of sandstone bed andGraben, which cuts through the Hida denatured plutonic rocks; it extends northeast to southwest and isbordered by a fault system. The Neogene-Miocene
Iwaine formation (andesitic lava and homogeneousash breccia) of the green tuff layers is distributed on
the northern to western side of the survey site. Furthermore, the Nirehara formation (sand stones andconglomerate stones), formed between the end of the
Paleogene period and the beginning of the Neogene
period, is located between the Funatsu granitic rocks
and the Iwaine formation, covering the Funatsu granitic rocks (Nozawa et al., 1981).
2.2 Summary of unstable slopes such as landslides
A geological map and a geological feature diagram prepared based on a geological reconnaissanceand existing geological literature, as well as boring
survey results, are shown in Figs. 1 and 2. There is a
cataclastic section in the Funatsu granitic rock area,
believed to have been caused by structural movements that occurred prior to the formation of the Toga
Graben. There are solid masses in this section of cataclastic granitic rocks, but there are more significant
hair cracks along the fault of the Toga Graben, andthere is a stronger tendency for hydrothermal alterations to occur in association with multiple dykes. Funatsu granitic rocks, which have significant cracks,
likely cause a regional slope failure by bedrock deterioration due to hydrothermal alteration from deep
underground.
There is a distinct, northeastern striking fault on
the northwestern slope (near-vertical dipping), at theborder between the Funatsu granitic rocks and the
Iwaine formation, that comprises the western edge of
the Toga Graben. Many white viscous layers haveformed in the fault and cracks from hydrothermal alterations and these layers are the cause of landslides.
Generally, igneous rocks have higher magnetic susceptibility than sedimentary rocks (5 × 10-6 to 2 × 10-2
SI for igneous rocks); the magnetic susceptibility ofandesitic rocks and felsic granitic rocks is 9 × 10-5 to
1 × 10-2 SI and 5 × 10-6 to 5 × 10-3 SI, respectively(Society of Exploration Geophysicists of Japan,1999). While magnetic susceptibility remains within acertain range for homogeneous rocks, it drops significantly below this range when the magnetic minerals
have degraded or altered due to the effects of alteration.
57

T. SUGIMOTO, K. KAWASAKI, H. SAKAI

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58

Use of magnetic surveying in landslide analysis at the boundary between the granite region and the green tuff region in southwestern Toyama Prefecture

Geologic province Crushing degree

Average(SI) M aximum(SI) M inimum(SI) Number of data

Average㻝㻚㻞㻱㻙㻜㻠 㻠㻚㻝㻱㻙㻜㻠 㻞㻚㻢㻱㻙㻜㻡㻞㻤㻜䠘30%
Funatsu㻝㻚㻞㻱㻙㻜㻠
㻠㻚㻝㻱㻙㻜㻠
㻞㻚㻢㻱㻙㻜㻡
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Geologic province
Crushing degree
Average(SI)M aximum(SI)M inimum(SI)
Number of data
30% < Average
granite㻝㻚㻞㻱㻙㻜㻠 㻠㻚㻝㻱㻙㻜㻠 㻞㻚㻢㻱㻙㻜㻡㻜㻞㻤㻜
non-crushing
䠘30%
Funatu㻝㻚㻞㻱㻙㻜㻠 㻠㻚㻝㻱㻙㻜㻠 㻞㻚㻢㻱㻙㻜㻡㻜
Average 30% < 㻝㻚㻠㻱㻙㻜㻠㻝㻚㻣㻱㻙㻜㻟 㻞㻚㻢㻱㻙㻜㻡㻤㻣㻟
graniteCataclasite
䠘30%
non-crushing㻝㻚㻟㻱㻙㻜㻠 㻝㻚㻣㻱㻙㻜㻟 㻞㻚㻢㻱㻙㻜㻡㻣㻞㻥
Funatsu
30% < Average 㻝㻚㻡㻱㻙㻜㻠㻠㻚㻜㻱㻙㻜㻠
㻞㻚㻢㻱㻙㻜㻡
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Andesite
30% < Average 㻝㻚㻥㻱㻙㻜㻟㻞㻚㻞㻱㻙㻜㻟
㻤㻚㻡㻱㻙㻜㻟
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non-crushing
Andesite30% <㻝㻚㻥㻱㻙㻜㻟
㻤㻚㻜㻱㻙㻜㻟
㻝㻚㻣㻱㻙㻜㻠
Total Average㻟㻚㻜㻱㻙㻜㻠
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㻞㻚㻢㻱㻙㻜㻡
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Altered
Propertyclassfication
scarcely clay seam
0: unaltereda little clay seam
1: weakly altered
2: moderate altered moderately clay seammuch clay seam
3: strongly altered
Geologic province Crushing degree
Average

Funatsu
granite㻝㻚㻟㻱㻙㻜㻟

㻡㻚㻣㻱㻙㻜㻟

㻝㻚㻞㻱㻙㻜㻠

30% <Average㻝㻚㻟㻱㻙㻜㻟
㻞㻚㻣㻱㻙㻜㻠

㻡㻚㻣㻱㻙㻜㻟
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Total Average

Average

Funatsu
granitenon-crushing

Cataclasite
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graniteAverage

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㸦G㸧Magnetic susceptibility of No.4 core sample

㸦E㸧Magnetic susceptibility of No.2 core sample

Fig. 5 Magnetic susceptibility of the boring cores.
7DEOH
similar material, hasalso been altered. The sliding
surfaces were formed primarily within the Nirehara
formation, with the andesitic rocks of the Iwaine formation comprising the majority of the landslide moving bed. The geological profile is shown in Fig. 2 (a).
3) Toga and Iwabuchi districts: The landslide
structure is of the cap lock type, with the Iwaine formation distributed with the Nirehara formation, and ispartially trapped on
the top level of the Funatsu gran7DEOH
ite rocks. Many andesite dykes penetrate the Funatsu
granite rocks, with clay zones due to hydrothermal alterations often confirmed in the vicinity of the boundary with the Iwaine formation. Landslide moving
clumps are thickly distributed in the lower section ofslopes. Sliding surfaces are formed in the vicinity ofthe boundary between the Funatsu granite rocks andthe Iwaine formation.
3. RESEARCH METHODS
3.1 Airborne magnetic survey
An airborne magnetic survey was conducted using a Scintrex CS2, a cesium magnetometer with asensor towed by a helicopter. This CS2 magnetometer,with a sensitivity of 0.01 nT, was held at a height of
30 m above the ground during the flight survey. The
survey lines were planned mainly in an east-west direction and arranged at intervals of 100 m. The lengthof each survey line was less than 3.5 km and the total
59

T. SUGIMOTO, K. KAWASAKI, H. SAKAI

length was approximately 186.6 km. At each survey
line, the magnetic force was measured every 1.75 m.
Data that could be clearly identified to represent steel
towers, buildings, and related structures were deleted
from the raw data. Furthermore, no altimetrical compensations were performed, since implementing altimetrical compensations in the research zone in themountainous area smoothens the localized magnetic
alterations at shallow levels resulting in magnetic abnormalities with unclear underground structures. Several corrections, such as diurnal correction and removal of the IGRF-7, were made to the observed dataof the airborne magnetic survey. Sugimoto et al.
(2013) used the data after correcting the effect of theearth’s magnetic field by IGRF data. However, the residual magnetic anomalies that are used to determine
the underground magnetic structure are sometimes
difficult to determine using this method. The RTP
method was considered to be an effective method forsolving this problem (e.g., Bhattacharyya, 1965;
Makino, 1993). Details of the RTP method are provided by Baranov (1957) and Baranov and Naudy(1964). In the RTP method, an observed magnetic
anomaly is transformed into a magnetic anomaly thatwouldresult had the area been surveyed at the magnetic pole (Nabighian et al., 2005). In this study, the
RTP method has been applied to residual magnetic
anomaly data after IGRF correction to study the relationship between the analyzed magnetic anomalies
and the geology and/or geographical features in relation to the landslide. Before applying the RTP method, the magnetic anomaly data was subjected to the
Kriging method for space interpolation. The earth’s
current magnetic field direction of the study area
(E137° 02’, N36° 27’, elevation 500 m), which is thetransformation parameter for the RTP analysis, is a
declination (D) of -7.4° and an inclination (I) of
50.4°. Finally, a magnetic anomaly map of the Ohmametani area was created to compare the variation inmagnetic force traversing the landslide region to thegeological cross section.

hydrothermally altered zone. The rock types and thealteration grading were examined through all thecores. The magnetic susceptibility was subsequently
measured every 5 cm on the core, using a Bartington
MS-2K magnetometer. Further, microscopic and xray powder diffraction analyses were conducted on
samples from the clayey seam of core No. 4 to examine the relationship between the minerals and/or thealteration degrees of the clay and the mechanism oflandslide.
We conducted two kinds of X-ray analysis using:1) a whole-rock indefinite orienting method; and 2)
an elutriation and orienting reaction method (ethylene
glycol treatment). The airborne magnetic anomalies
after IGRF and RTP corrections were then comparedwith both the geology and the data obtained from theboring cores.
4. RESULTS
4.1 Airborne Magnetic Survey Results
Figure 3 shows an IGRF residual magnetic
anomaly map of the study area acquired by the airborne magnetic survey, and Fig. 4 shows the RTP distribution map. In Figs. 3 and 4, the strong magnetic
field is shown in a warm color and the weak magnetic
field is shown in a cold color. Distributions of thepositive and negative anomalies are more obvious
with RTP transformation. High magnetic anomaly
zones are observed at the eastern to southeastern partsof the Iwaine formation as well as in a southwesterly
direction from Kita-mamedani in the Funatsu granitic
rock area (Fig. 4). Conversely, low magnetic anomalies are significant from northeast to southwest in the
study area. Intense low magnetic anomalies are also
observed in the southwesterly directions from Oshiba
to the left bank of the Toga River. Linear magnetic
anomalies with low and high values are observed
clearly in the Kita-mamedani to Iwabuchi area.
The distributions of the magnetic anomalies arecompared with the main landslide area and the other
districts:
1. Oshiba: The bedrock in the Oshiba landslidearea consists of Funatsu granitic rocks. No clear scarp
is found in the study area. RTP transformation showsthat low magnetic anomalies are observed from Oshiba to the Mametani dam (in a NE-SW direction), particularly in the middle of the slope. The upper slope isa high magnetic anomaly.

3.2 Study of the boring core
Four boring cores were drilled in the study area.
Cores No. 1 (length: 30 m) and No. 2 (length: 50 m)
were taken at the northern sites in the fresh Funatsu
granite, and cores No. 3 (length: 120 m) and No. 4(length: 90 m) were taken at the Oshiba sites in the60

Use of magnetic surveying in landslide analysis at the boundary between the granite region and the green tuff region in southwestern Toyama Prefecture

2. Kita-mametani and Ohmametani: Low magnetic anomaly is observed in the entire area of thelandslide with the IGRF-corrected data. After RTP
transformation, a clear low magnetic anomaly isfound over the top of the hillside around the Ohmametani district and at the top of the Kita-mamedani district.
Figure 2(b) shows the magnetic field intensity
for the A-A’ survey line in the Ohmametani district.
The magnetic field intensity for this survey line is as
low as -70 to -30 nT between the 0 m and 1300 m
point on the distance table in the Funatsu granitic areas. The magnetic field intensity continuously decreases between the 1300 m and 2070 m points on the
distance table, and the lowest value of -120 nT is observed at the 1800 m point in the detritus area. The
magnetic field intensity then gradually increases between the 2070 m and 2500 m points on the distance
table, and the highest value of 420 nT is observed atthe 2420 m point within the Iwaine formation.
3. Toga and Iwabuchi: The magnetic anomaly atthe bottom of the slope in the Toga-Iwabuchi districtis detected as a strong low magnetic anomaly using
RTP transformation. A strong low magnetic anomaly
extends in a NE-SW direction along the fault. Conversely, the magnetic anomaly at the top of the slope
is shown to be a high magnetic anomaly.

surements and descriptions of cores. Cores No. 1 andNo. 2 consist of unweathered Funatsu granite andcataclastic granite with clay seams in the fracture. The
andesitic dyke with clay seams has intruded into coreNo. 2. The averaged magnetic susceptibilities at a
depth of 30 m in core No. 1 and at a depth of 50 m incore No. 2 is 1.1 × 10-3 SI and 1.2 × 10-3 SI, respectively. A high average magnetic susceptibility of 1.3–
1.7 × 10-3 SI is observed in the Funatsu granite. Conversely, the susceptibilities of the cataclastic Funatsu
granite and andesitic dyke are a low average of 2.7–
4.2 × 10-4 SI and 4.4 × 10-4 SI, respectively.
Cores No. 3 and No. 4 consist of weathered andfractured Funatsu granite, cataclastic Funatsu granite,
and an unweathered andesitic dyke with a small whiteclay seam. The fractured and fine-grained cataclasite
granite is browned by weathering. At a depth of 120
m in core No. 3 and at a depth of 90 m in core No. 4,
the low averaged magnetic susceptibilities are shown
to be 3.0× 10-4 SI and 5.7 × 10-4 SI, respectively. The
averaged susceptibilities for the weathered Funatsu
granite and the cataclastic Funatsu granite are low
values with an average of 1.2–6.6 × 10-4 SI and 1.4–5.0×10-4 SI, respectively and the susceptibility of andesite is 2.2 × 10-3 SI.
4.3 Microscopic observation and result of x-ray
powder diffraction
Based on microscopic observation of the five samples from the clayey area of core No. 4 (Table 1), it isclear that biotite changed to chlorite, and that feldspar

4.2 Magnetic susceptibility measurement results
and boring core descriptions of boring cores
Figure 5 shows the results of the magnetic mea-

Table 1. The minerals observed by microscope of the boring cores No.4
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61

T. SUGIMOTO, K. KAWASAKI, H. SAKAI

by hydrothermal alteration, which led to a reduction
in magnetic susceptibility of the granitoids, so thatmajor magnetic susceptibility changes are no longer
preserved. In the drill core, alteration zone No. 3,
which contains numerous clay layers, represents a slip
surface. Within this zone, the 75.5–94.0 m depth
range can be considered to be the horizon with the
deepest slip surfaces.
Core No. 4 at a depth of 85.9 m consists of Funatsu granite and cataclastic Funatsu granite, whichhave been altered along a number of cracks to clay
seams or clay beds due to hydrothermal alteration, resulting in a lowering of the magnetic susceptibility(10-4 SI). Hot water penetrates into cracks along therock and, in places where the cracks have not been altered, the magnetic susceptibility is high (10-3 SI).
On the other hand, high magnetic susceptibility is observed in cores No. 1 and No. 2 (Figs. 5(a) and (b)) in
the unweathered hard Funatsu granite in the northernpart of the study area. However, a green-gray clay
seam is found in the fracture, and low magnetic susceptibility is therefore observed in cataclastic Funatsu
granite with many normal-sized and hairline cracks.
In addition, the andesitic dykes of core No. 2 are seen
to be green-gray, and have therefore been hydrothermally altered by the presence of clay, and have low
magnetic susceptibility. These conditions are consistent with the alteration classification of this core. In
contrast, the andesitic dykes of core No. 4 have highmagnetic susceptibility in fresh black rock. In thisway, the level of magnetic susceptibility reflects thedegree of alteration and types of minerals in the Funatsu granitic and the andesitic dyke rocks.

changed to calcite. Calcite, chlorite, and sericite were
determined for all five samples using x-ray diffraction(Table 2).
5. DISCUSSION
5.1 Relationship between magnetic minerals(classification, degree of alteration) andmagnetic susceptibility
Magnetic anomaly depends on the magnetic susceptibility and natural remanent magnetization (NRM)of magnetic rock. Magnetic susceptibility is the ratio
of induced magnetization that appears in response to
the external magnetic field.
And magnetic susceptibility can be changed with theconcentration, composition, and grain size of themagnetic minerals present in the materials (Sakai etal., 2001; Tarling et al., 1993). The remanent magnetization in igneous rock is primary thermal remanent
magnetization, which is acquired in the cooling process during the formation of igneous rocks. Furthermore, subsequent secondary magnetization, such as
viscous remanent magnetization, could be acquiredsince rocks have formed. Sugimoto et al. (2013)
found that induced magnetization was very muchstronger than residual magnetization in this study
area, resulting in a major source of the magnetic
anomaly. Hence, the effect on magnetic susceptibility
due to alteration and the type of mineral was investigated. Based on a combination of the major alteredminerals, the Oshiba district is likely affected by regional propylite mineralization under neutral hot water at between 100 and 300°C (Utada, 1990; Sugimoto
et al., 2013), and the magnetic minerals in granodiorite were clearly altered to weaker magnetic minerals.
As shown in Fig. 5(d), the 56.3–79.0 m section ofcore No. 4 is a strongly altered zone with a core of
low magnetic susceptibility (10-4 SI).
Both the Funatsu granite and cataclastic Funatsu
granite have been altered along numerous cracks toclay or clay seams by hydrothermal processes, resulting in lower magnetic susceptibility. As a result of observation of core No. 3, strengths and weaknesses
were found in the alteration classification. In the andesitic dyke, the degree of alteration is relatively low
overall, with andesite preserving the magnetic susceptibility of the source rock. However, the magnetic
susceptibility of Funatsu granitic rocks is uniformly
low. It is believed that this entire region was affected

5.2 Comparison of magnetic pole anomaly results,
magnetic susceptibility measurement results,
and results of the boring cores
The airborne magnetic survey results are compared to the magnetic susceptibility results and theobservation results of the boring cores in the Funatsu
granitic rock area．In the northern study area, at a
depth of 30-50 m, cores No.1 and No. 2 at a depth of
30-50 m have high magnetic susceptibility (10-3 SI)
with a high averaged magnetic susceptibility of 1.1 −1.2×10-3 SI in fresh granite. The pole magnetic anomaly in this area is a low magnetic anomaly of -40 nT.
In contrast, cores No. 3 and No. 4 at a depth of 12090 m, consisting predominantly of Funatsu granite
62

Use of magnetic surveying in landslide analysis at the boundary between the granite region and the green tuff region in southwestern Toyama Prefecturethat has been hydrothermally altered with cataclasite
facies, have low magnetic susceptibility of 3.05.7×10-4 SI, respectively. The magnetic anomaly of
the Oshiba region is remarkably low at -90 nT. Therefore, the magnetic pole anomalies are clearly concordant with the magnetic susceptibilities of the shallow
cores.
An airborne magnetic survey measures the integrated value of the magnetic field by the constituentsof each depth. The magnetic field is attenuated in inverse proportion to the cube of the distance from thetarget material. Therefore, magnetic anomaly by airborne magnetic survey reflects the distribution ofmagnetization substances that are distributed more
greatly in the shallow part. Sugimoto and Sakai(2013) reported the validity of the magnetic survey inthe tunnel that is covered with soil of up to 150 m inthis study area.
Locations of low magnetic anomaly are zones
where rocks and colluvium are distributed predominately in the shallow subsurface. Therefore, using
aeromagnetic survey data is suitable for identifying
locations with unstable slopes, which have a risk oflandslides.
Conversely, if strongly magnetized rocks are
predominately present at the shallow subsurface, theywould blind out any colluvium layers at the subsurface.

the susceptibilities of the Funatsu granite and thecataclastic granite, indicating that cataclasis does notchange the susceptibility.
Hence, the intensity of the magnetic anomaly reflects the degree of alteration of the Funatsu granitic
rocks. Low magnetic susceptibility is likely producedby hydrothermal alteration, weathering, and/or grain
size reduction caused by disruption due to landslides.
Therefore, the strength of the magnetic anomaly reflects the occurrence and history of landslides and thedegree of deterioration of the Funatsu granitic rocks.
The magnetic anomaly is also compared with theregional geology of the A-A' cross section shown in
Fig. 2(a), (b). The thicker the deposition layer of colluvium is, the weaker the magnetic field observed.
The observed smallest magnetic field is found at the
maximum thickness of the deposition. The Iwaine
formation region consists of ash breccia curd and andesite, causing a strong magnetic field. In this area,
there is a positive correlation between the magnetic
field and the layer thickness, and there is a correlation
with the layer thickness and the layer changes in thelandslide colluvium. Ikeda et al. (1973) reported thatthe magnetic field changes with the geological structure. Here, we have shown a positive relationship between the magnetic field and geological structure in
igneous-rock-distributed areas.
5.4 Relationship between landslides and otherregions and the magnetic anomaly distribution
The airborne magnetic survey results are compared to the geological structures of the major landslide areas and other regions:
1. Oshiba: The landslide area clearly appears as
a low magnetic anomaly area by RTP transformation.
The Funatsu granitic rocks are obviously altered by
hot water from deep underground to the surface, andespecially significant alteration by hot water as well
as extremely low magnetic anomaly was found at the
middle of the slope. A hydrothermal alteration zone in
the bedrock is found after RTP transformation. Moreover, two Igw locations show very different magnetic
strengths. Based on geological reconnaissance as well
as boring survey results, the layer thickness of Igw
reached a maximum of 100 m near the bottom, whereas the Igw thickness was thinner in the upper part, reflecting the influence of the high magnetic anomalyof the Funatsu granite.

5.3 Relationship between geology and magnetic
anomaly distribution
As shown by the magnetic anomaly map in Fig.
4, the boundary between the low and high magnetic
anomaly values is located in a line passing from Kitamamedani to the Iwabuchi district, where Funatsu
granitic rocks are on the northwest side of the boundary and early Neogene volcanic rocks are on thesoutheast side. This boundary corresponds to a normal
fault trending north-northeast of the eastside moving
downward (Figs. 1 and 2); the linear magnetic anomalies are concordant with the trend of the inferred fault
location in the estimation of faults and the geologicalboundary. The fresh Funatsu granitic rocks in thenorthern study area show high magnetic susceptibility
as observed in the core samples (Fig. 5). Conversely,
the Funatsu granitic rocks of the Oshiba district have
low magnetic susceptibility due to hydrothermal alteration. However, there is no significant difference in63

T. SUGIMOTO, K. KAWASAKI, H. SAKAI

2. Kita-mamedani and Ohmametani: The extremely low magnetic anomaly area from the middle
to the upper area of the landslide correlates with thelayer thickness of the landslide colluvium. In addition, a thin depositional area of the landslide colluvium on the lower slope soil corresponds to the places
where the Funatsu granite shows high magnetic
anomaly. The thick detritus from the landslide collapse is concordant with the low magnetic anomaly
found by RTP transformation. In addition, the thin detritus area in the lower part of the slope shows highmagnetic anomaly, which is likely caused by thestrongly magnetized Funatsu granitic basement.
3. Toga and Iwabuchi: The middle and upperparts of the slope show high magnetic anomaly. Thisregion is consistent with the unaltered Iwaine formation. However, the lower slope of the landslide areashows a significant low magnetic anomaly, because
the edge of the low-susceptibility Nirehara formation
and Funatsu granitic rocks has been hydrothermallyaltered by the dyke and thick colluvium is distributed.
4. Other low magnetic anomaly area: Visual observation during tunnel construction indicates that
low magnetic anomalies on the left-bank side of theToga River are likely to have originated in the cataclastic Funatsu granitic and Funatsu granitic basementthat contained numerous cracks, which were commonly altered by hydrothermal processes to clay
seams and veins. Where alteration pervades the coreof the rock, white clay is common and the rocks show
low magnetic anomalies (Sugimoto and Sakai, 2013).
The Funatsu granite zone of the Toga region ischaracterized by many small faults. The dip of thesefaults is the dip slopes with a NW trend, causing oneof the most important predisposing factors for landslides on the right-bank side. In contrast, on the leftbank side, the dips of the faults are opposite slopes
and a slope failure region is therefore likely to occur
in the surface layer portion undergoing deterioration.
In addition, intense low magnetic anomalies areobserved within the Oshiba to Kita-mamedani area.
This district, which consists of well-developed fractures and faults and contains numerous clay veins
formed by hydrothermal alteration, is more likely toundergo slope failure. Hence, this area has become athin-ridged terrain with deep erosion valleys.

6. Summary
An airborne magnetic survey has been conducted
for a landslide area located at the boundary between
the Hida belt and the green tuff region in southwestern Toyama Prefecture. In addition, boring cores taken from the region were subjected to susceptibility
measurements and x-ray diffraction. RTP transformation clearly defines the magnetic anomaly distributions, and the results are compared with the geological structure of the unstable slope, i.e., the landslides.
The following results were obtained:
1) The high and low magnetic anomaly distributions are shown more clearly by RTP transformation
than by solely using the IGRF residual magnetic
anomaly. The area where igneous rocks are distributed
shows high magnetic anomaly, and the hydrothermally altered zones and/or areas where there are colluvial
deposits of landslides have low magnetic anomaly.
This linear magnetic anomaly is clearly observed insuch areas and the anomaly is concordant with the locations of the fault and geological boundary.
2) It was confirmed that the high and low magnetic anomalies of the Funatsu granitic rock distribution area are controlled by changes in layer thicknessof landslide collapse material and extent of weathering and hydrothermal alteration along cracks andfaults, as supported by data from boring core investigations. And magnetic anomalies observed by an airborne magnetic survey correlate with the distributionof remanent magnetization in the shallow part. In addition, the surface layer covered by thick colluvium
deposited by collapse and landslides shows extremely
low magnetic anomaly. The observed magnetic field
intensities correlate with the geological structures of
the Iwaine layer distribution and with the colluvium
layer thickness in the landslide area. These results indicate that magnetic anomaly is useful for determining the predisposition of an area to a major landslide.
Aeromagnetic surveys could identify the distributionof collapsed soil sediments in the Neogene as well as
the areas of Funatsu granitic rock with a high potential for landslides. Low aeromagnetic intensities identified zones where the colluvium layer in landslide areas, as well as hydrothermally altered and weatheredrocks, occurred predominately in the shallow subsurface. This method can be used to efficiently screen
large areas for potentially unstable slopes that may
lead to the occurrence of landslides.
64Use of magnetic surveying in landslide analysis at the boundary between the granite region and the green tuff region in southwestern Toyama Prefecture3) The results of boring core investigations led tothe development of an alteration classification for different rock units. In hydrothermally altered Funatsu
granitic rocks, the magnetic susceptibility has decreased throughout the entire core demonstrating theeffects of hydrothermal alteration. However, in areas
where hot water had not penetrated the entire core,strong magnetism without alteration was preserved.
The degree of hydrothermal alteration cannot be determined by core observations alone.
The degree of alteration with depth might easily
by determined by measurement of the magnetic
anomaly of drill cores at each depth. Magnetic surveyof drill cores is effective because it would provide information about the correlation between the geology,
lithology, and magnetism.
As future works, various topographical effects aswell as variations in magnetic anomaly with depth
should be considered. Further research is also suggested in order to test the applicability of such surveys to complex geological areas and sedimentary
distribution areas with weak magnetism.

in an area of weak magnetic anomaly and its interpretation. Japan Soc. Eng. Geol. vol. 50, no.1, pp. 2-15.
Hasegawa K (2009). Example of applying aeromagnetic
survey technique to low magnetic anomaly area.
JAEA-Research vol. 54, pp. 1-51.
Hayashi K, Wakai A, Tanaka N and Abe S (2011). Risk
evaluation method for earthquake-triggered landslide
enhanced by cooperation of topographical-geologicalanalysis and finite element analysis, Japan. Journal of
the Japan Landslide Society vol. 48, no. 1, pp. 1-11.
Higaki D and Abe S (2011). Landslide investigation basedon geomorphological/geological reconnaissance survey.
Ikeda K, Matsuo S, Mochida U, Tsuchiya T, and Hasegawa
K (1973). Airborne magnetic survey applied for civil
construction purpose. Journal of the Japan Society ofGeology vol. 14, no. 1/2, pp. 21-26．
Japan Association of Rivers (1997). Ministry of Construction of Japan, Technical Criteria for River Works:
Practical Guide for Investigation.
MacLeod IN, Jones K, and Dai TF (1993). 3-D analytic signal in the interpretation of total magnetic field data at
low magnetic latitudes. Explor Geophys vol. 24

Acknowledgments(3/4):679-688.
Nabighian M, Grauch V, Hansen R, LaFehr T, Li Y, Peirce J,

The authors gratefully thank: Dr. Nozaki for his
many helpful suggestions with landslide analysis and
Mr. Kawato of Nippon Engineering Consultants Co.,
Ltd. for his cooperation in the data analysis of the airborne magnetic survey.

Phillips J, and Ruder M (2005). The historical development of the magnetic method in exploration. Geophysics vol. 70, no. 6, pp. 33ND-61ND.
Nozawa T, Sakamoto T, Kanou, T, and Inagaki T (1981).
Geology of the Shirokimine region, Geological Surveyof Japan, pp. 8-61.

REFERENCES

Okuma S and Kanaya H (1990). Rock magnetism and magnetic survey. Chishtsu News, no. 428, pp. 20-26.
Okuma S, Nakatsuka T, Morijiri R, Makino M, Uchida T,

Baranov V (1957). A new method for interpretation of aeromagnetic maps: Pseudo-gravimetric anomalies. Geo-

Ogawa Y, Ikakura S, and Matsushita N (2001).

physics vol. 22, no. 2, pp. 359-382.

Preliminary results of a high-resolution aeromagnetic
sur- vey over Usu Volcano, Hokkaido, Japan. Bull.

Baranov V, Naudy H (1964). Numerical calculation of theGeol. Surv. Japan vol. 52, no. 4/5, pp. 149-154.

formula of reduction to the magnetic pole. Geophysics

Public Works Research Institute (2008). Manual for the ex-vol. 29 (1), pp. 67-79.

traction of streams that may occur in the deep collapse.

Bhattacharyya B K (1965). Two-dimensional harmonic

Public Works Research Institute materials.

analysis as a tool for magnetic interpretation. Geophys-

Sakai H, Nomura S, Horii M, Araki S, Kashiwaya K, Tani Y,

ics vol. 30, pp. 829-857.
Hasbaatar B, Muranaka, R, Maruyama K, Hanaoka M

Shibata Y, Kawai T, Kravchinsky V, Peck J, and King J(2009). A study of causative factors for landslides in-(2001). Magnetic susceptibility studies on surface sedi-

duced by the Mid-Niigata Prefecture Earthquake ofments of Lake Baikal and Lake Biwa. Geologiya i
Geofizika vol. 42, no. 1, pp. 339-347.

2004, Japan. Journal of the Japan Landslide Society

Shiosaka K, Nishikawa H, Fujii H, Kudo K, and Kondo T

vol. 45, no. 6, pp. 428-434.

(2010). A Study on the inflection of remote sensing in

Hasegawa K and Matsuoka T (2009). Aeromagnetic survey

65

T. SUGIMOTO, K. KAWASAKI, H. SAKAI

the landslide investigation. Journal of Japan Society of

Mountain stream extraction manual for potentially

Civil Engineering G vol. 66, no. 3, pp. 85-95.

deep-seated landslide occurrence (draft) vol. 4155.
Tanahashi M, Morikawa T, Kusakabe K, Yoshikawa H,

Society of Exploration Geophysicists of Japan (1999). Geo-

Tanaka K, Inokuchi T, and Takeuchi M (1997). Air-

physical exploration Handbook, pp. 475-515.
Sugimoto T, Nozaki T, and Sakai H. (2013). Applicability

borne electromagnetic investigation in the Bandai Vol-of helicopter magnetic prospecting to the landslide

cano area: The relation between the 1888 debris ava-

area on the rim area of Toga-Graben. Journal of the Ja-lanche and low-resistivity zone. Bandai Volcano, pp.

pan Society of Erosion Control Engineering vol. 66,

1-11.
Toyama Prefecture (1992).1:100 000 Toyama geologicalno. 1, pp. 23-29.

map instructions. Naigai Map Inc., pp. 159-168.

Sugimoto T and Sakai H (May 2013). Availability of themagnetic survey to investigate the geology of tunnel

Uchida T, Nakano Y, Akiyama K, Tamura K, Kasai M, andsite in the granite hydrothermal alteration zone. Journal

Suzuki R (2010). The role of LiDAR data on evalua-of the Japan Society of Civil Engineering F1 vol. 69,

tion of shallow landslide susceptibility and detection of

no. 3, pp. 11-19.

mass-rock creeping slope. Japanese Geomorphological
Union vol. 31, no. 4, pp. 383-402.

Suzuki R, Uchida T, and Tamura K (2009). A method of ageophysical structure survey for identifying areas

Uchida T, Yokoyama O, Suzuki R, Tamura K, Ishizuka Tprone to a deep-seated landslide occurrence. Civil En-(2011). New method for assessing deep catastrophic

gineering Journal, no. 51-7, pp. 8-18.

landslide susceptibility. International Journal of Erosion Control Engineering vol. 4, no. 2, pp. 32-42.

Tarling D H and Hrouda F (1993). The magnetic anisotropy

Utada M (1990). Hydrothermal alteration related to igneousof rocks (Chapman and Hall), pp. 217.

activity in cretaceous and neogene formation in Japan.

Tamura K, Uchida T, Suzuki T, Terada H, Kurihara J

Mining Geol., spec. issue 8, pp. 67-83.

(2008). Public Works Research Institute materials,

66