ARTICLES

Sci. Rep. Kanazawa Univ.
Vol. 61, pp. 39-47, 2017

Landslide displacement detection around
Hakusan volcano, central Japan, from InSAR analysis of
ALOS-2/PALSAR-2 data
Yuya KITOH 1, * and Yoshihiro HIRAMATSU 2
1

Graduate School of Natural Science and Technology, Kanazawa University,
Kakuma, Kanazawa, 920-1192, Japan
2
School of Natural System, Institute of Science and Engineering, Kanazawa University,
Kakuma, Kanazawa, 920-1192, Japan(Received December 12, 2017 and accepted in revised form February 5, 2018)

Abstract

InSAR analysis using ALOS-2/PALSAR-2 data was conducted to detect landslides around
Hakusan volcano, where we observed landslide movements in three areas. In the Jinnosukedani area, wedetected landslide movement with maximum annual displacement of 200 mm, which is consistent with that of
GNSS observations on the ground surface. In the Yunotani area, we detected landslide movement withmaximum annual displacement of 200 mm. The landslide area detected by the InSAR analysis is almost
coincident with a previous study of InSAR analysis, although the remarkable landslide area with annual
displacement of 100–200 mm is narrower than that reported by the previous study. In the Sennindani area, alarge slope collapse can be captured by InSAR analysis as a low coherence area. InSAR analysis around
Hakusan volcano provides results that are consistent with those of the ground-based monitoring in the field,
indicating that InSAR analysis is a useful landslide-monitoring tool.

Keywords.

Interferometric synthetic aperture radar, Jinnosukedani area, Yunotani area, Sennindani area,
Landslide movement, Slope collapse, GNSS observation

1

Introduction

Landslides, which are an important factor ofgeomorphic development because they alter topography, are often induced suddenly by heavy rains,
earthquakes, and volcanic eruptions. However, somelandslides are unrelated to those events and continue
at a constant rate for long durations.
Landslide movements have been monitored using
GNSS (Global Navigation Satellite System) and other
instruments (e.g. Ueno and Nakazato, 2012). Most
methods used to monitor landslides measure theamount of displacement at a selected locality. For thatreason, it is difficult to ascertain the spatial
distribution of landslides. Synthetic Aperture Radar———————————————————————*Corresponding author E-mail: kitoh@live.jp(SAR) irradiates microwaves from sensors mounted
on satellites or aircraft to the Earth’s surface and thenreceives the backward-scattered waves (Hanssen,2001). Interferometric synthetic aperture radar(InSAR) is a method that estimates ground movementfrom some SAR data (Massonnet et al., 1998;
Bürgmann et al., 2000). Although InSAR analysis wasdeveloped initially as a tool to detect surface changes
caused by earthquakes and volcanic activities(Massonnet et al., 1993, 1995; Tobita et al., 2001;
Ozawa, 2006), its application to other fields has
advanced in recent years. Several InSAR-analysisbased studies of landslides have been reported (Une etal., 2008; Sato et al., 2012; Nishiguchi, 2017). Around
Hakusan volcano, Michinaka and Hiramatsu (2010)

40

Landslide displacement detection around Hakusan volcano from InSAR analysis of ALOS-2/PALSAR-2 datareported landslide displacement using ALOS dataobtained by the satellite predecessor.
Several SAR satellites are available for InSAR
analysis. In May 2014, the Japan Aerospace
Exploration Agency (JAXA) launched ALOS-2,
which is equipped with SAR of the L band sensor(PALSAR-2). The ALOS-2 produces higher resolution data than past satellites. We expect that analysisof ALOS-2 data can clarify features of landslides
better than earlier studies.
This study estimates the area and the annual
displacement of landslides around Hakusan volcano
from InSAR analysis. By comparing landslides
detected using InSAR analysis with groundmonitoring results (Kanazawa River National
Highway Office, 2015), we evaluate its consistency.

2

Study area and Data set

2.1

Study area

Hakusan volcano, located on the prefectural border ofIshikawa and Gifu in central Japan, is at the center ofthe study areas (Figure 1). Its surrounding area
includes the Tedori group of the Mesozoic Cretaceous,which is covered by Hakusan volcanic ejecta/deposits(Kaseno, 1993). We overlaid a shaded-relief map withthe landslide map compiled by National ResearchInstitute for Earth Science and Disaster Resilience(NIED) (NIED, 2001). Many topographical features
around Hakusan volcano are related to landslide
movements (Figure 1b).

2.2

Data set

We used SAR images taken on October 8, 2014 andJuly 15, 2015 acquired by Phased Array L-band
Synthetic Aperture Radar-2 (PALSAR-2; 23.8 cm
wavelength) of ALOS-2 (Advanced Land Observing
Satellite-2) (Table 1).
All data used for analyses were obtained in highresolution mode, imaged obliquely downward to theright in ascending (northward) orbit. The pass/frame,
which represents the observation area, is 127/710. The
satellite advancing direction from the north (the offnadir azimuth) (𝛼) is 350.5° at the observation time.

Figure 1. (a) Location and topography in the study
area. The white rectangle shows the study area.
(b) Shaded relief map of the study area from LiDAR(Light Detection and Ranging) data provided by the
Geospatial Information Authority of Japan (GSI).
Areas enclosed by black lines are landslide areas
reported by the National Research Institute for Earth
Science and Disaster Resilience (NIED). Yellow
broken line represents prefectural boundary.

The incidence angle of the microwave at the scene
center ( 𝜃 ) is 42.9°. The perpendicular baseline
distance between the satellites at the time of data
capture is 215 m (Table 1).
We used GNSS data observed by the Kanazawa
River National Highway Office of the Hokuriku
Regional Development Bureau of the Ministry ofLand, Infrastructure and Transport in the
Jinnosukedani area in October 2014 and October 2015.

Table 1. InSAR analysis information.

Master observation dateSlave observation date

Perpendicular baseline (m)October 8, 2014

July 15, 2015

215

Yuya KITOH and Yoshihiro HIRAMATSU

3

Methods

For InSAR analysis, the software used for InSAR
analysis was RINC ver. 0.31 (Ozawa, 2014). Phase
differences obtained using InSAR analysis areaffected not only by ground deformation that isapparent from two datasets but also by satellite orbit
conditions, topographic conditions, and atmosphericconditions. JAXA and the Geospatial Information
Authority of Japan (GSI) provide precise satellite
orbit data and 10 m DEM, respectively. We used thesedata to correct the respective conditions of a satellite
orbit, topography, and atmosphere. Furthermore, weestimated the phase change correlated with the terrain(Ozawa, 2014) and removed this phase change as theeffect of the atmosphere. For the calculating the phasechange, it is important to reduce the noise attributableto reflection from multiple objects on the groundsurface. We applied so-called multi-look processing(Hanssen, 2001), averaging neighboring pixel data to
reduce noise. Ozawa (2014) reported that the numberof looks depended on a research subject. In this study,
we focus landslide movement with the area of several
hundred meters. Previous studies often applied 5–10
look processes for landslide movement (e.g. Sato,2016). For this study, we processed five-lookprocesses in the range and azimuth directions. The
spatial resolution of the ground motion diagram of theanalysis is about 10 m × 10 m. The interference image
filter using the method of Baran et al. (2003) wasapplied to remove the residual noise. We calculated
the displacement along the SAR line of sight (LoS)
direction (DLoS) from the phase value using software(SNAPHU; Chen and Zebker, 2002). This procedure,
called phase unwrapping, is formulated as
𝐷$%& = 𝜆Δ𝜙/4𝜋,(1)
where 𝜆 and Δ𝜙 represent the wavelength and thephase difference, respectively. We estimated thelandslide displacement from DLoS using a vectortransformed formula (Hanssen, 2001) as
𝐷$%& = 𝐷. cos 𝜃 − sin(𝜃) 𝐷7 cos 𝛼 − 3𝜋/2 +
	
  	
  	
  	
  𝐷< sin(𝛼 − 3𝜋/2) ,(2)
where 𝐷. , 	
  𝐷7 and 𝐷< 	
   denote ground deformation
components in the up, north, and east directions,
respectively, 𝜃 stands for the incidence angle, and 𝛼
signifies the off-nadir azimuth.
We convert the LoS displacement to the along-slope
displacement (𝐷=>%?< ) as
𝐷=>%?< = 𝐷$%& / 𝑛=>%?< ∙ 𝑛$B&
(3)
where 𝑛=>%?< is the unit vector of the slope direction(positive in downslope) and 𝑛$%& is the unit vector

41

along the LoS direction orientated positively in thesatellite-to-ground direction. The both vectors can be
written respectively in the East-North-Up reference
system as
	
  	
  	
  	
  	
  	
  𝑛=>%?< = sin 𝐴= cos 𝛿 , cos 𝐴= cos 𝛿 , − sin 𝛿 , (4)
	
  	
  	
  	
  	
  𝑛$%& = sin 𝐴$ sin 𝜃 , − cos 𝐴$ sin 𝜃 , − cos 𝜃 , (5)where 𝐴= is the slope azimuth angle, 𝛿 is the slope
angle, and 𝐴$ is the LoS azimuth angle (AL = alpha+90). In this study, 𝐴> = 80.5°	
   and 𝜃 = 42.9° ,leading 𝑛$B& = 0.67, −0.11, −0.73	
   . We convertthe 	
  𝐷=>%?< to the horizontal displacement (𝐷O ) as
𝐷O = 𝐷=>%?< ∙ cos 𝛽.
(6)4Results of InSAR analysis

We present results of the InSAR analysis around
Hakusan volcano in Figure 2a. The flight direction of
the SAR satellite is southeast to northwest. The
irradiation direction of the radar is southwest tonortheast. The displacement is expressed as a changeof the distance along the LoS (line-of-sight) direction.
Negative and positive values signify approaching or
moving away from the satellite. Shadow areas against
beam directions are masked because the coherence
was reduced in the areas (Figure 2b). We overlaid our
results on the landslide map by NIED (2001) (Figure2b). In the previous study using ALOS-PALSAR data,
there were color changes that might be related to theatmospheric effect due to vapor. In other words, thosecolor changes might be unrelated to ground movement.
However, in this analysis, such color changes are nothardly seen. This can be interpreted that ALOS-2 dataclarify features of landslides better than ALOS data.
Our results detect displacements of approx. 70 mm
approaching the satellite in two areas about 2–3 km
southeast of the Hakusan summit. We also observe
remarkable color change in an area about 5 km
northeast of the Hakusan summit. In this area, werecognize that the coherence is lower than in thesurrounding area. The former corresponds to landslideareas, named as the Jinnosukedani area (Kanazawa
River National Highway Office, 2011) and the
Yunotani area (Ishikawa Forest Office, 2013). The
latter is the Sennindani area, where a large sector
collapse occurred in May 2015 (Ishikawa Forest
Office, 2015). During the analyzed period, no
volcanic activity, large earthquake, or heavy rain
occurred. Therefore, we consider that the observed
displacement and color change reflect landslide
movement.

42

Landslide displacement detection around Hakusan volcano from InSAR analysis of ALOS-2/PALSAR-2 data

Figure 2. Results of InSAR analysis using PALSAR-2 data around Hakusan volcano overlapped with thelandslide area map by NIED (2001). (a) Change in distances between the satellite and the ground estimatedfrom InSAR analysis. Shadow areas against beam directions are masked. (b) Map of coherence.

and the slope azimuth ( 𝐴𝑠 ) of 270°. The annual
displacement was calculated proportionally from thedisplacement for nine months from October 2014 to
July 2015. With these values, we estimate the annualhorizontal displacement of 200 mm at maximum on
the middle ridge block and the annual horizontal
displacement of 100 mm at maximum on the left banklarge block (Figure 7a).
Figure 4 presents variation of the elevation and

Flight
Direc,onFigure 3 presents a close-up view of the result of
the InSAR analysis around the Jinnosukedani area. In
the center of Figure 3, landslide displacements in therange of 400 m × 1000 m are apparent on the upper
part of a ridge extending northeast–southwest, called
the middle ridge block. Another landslide displacement is apparent in the range of 300 m × 400 m on thesoutheast slope of another ridge, called the left banklarge block. Both displacements are shown asapproaching the satellite. The amounts of thedisplacement in the middle ridge block and in the leftbank block are 75 mm and 60 mm (Figure 3),
respectively. The area of the landslide around the
Jinnosukedani area observed in this study isconcordant with previously reported landslide
topography (Kanazawa River National Highway
Office, 2015). Based on the direction of the slope
where the landslide is observed, we infer that thedisplacement approaching the satellite represents
horizontal movement in the dip direction of the slope
and uplift of the slope and that the displacement
moving away from the satellite represents the slope
subsidence. The horizontal displacement in thelandslide direction was calculated from the displacement shown in Figure 3 by using vector conversion
with the slope angle (𝛿) and the slope azimuth angle(𝐴= ) reported by Michinaka and Hiramatsu (2010).
The middle ridge block has the slope angle (𝛿 ) of
20.4° and the slope azimuth angle (𝐴= ) of 223°. The
left bank large block has the slope angle (𝛿) of 28.2°

the middle
ridge block

Beam
on
Direc,

the left banklarge block-75

0
Amount of displacement (mm)75

Figure 3. Displacement estimated from InSAR
analysis in the Jinnosukedani area. Shadow areas
against beam directions are masked.

Yuya KITOH and Yoshihiro HIRAMATSU

displacements on the left bank large block along the
X–Y line, as depicted in Figure 3. The amounts ofhorizontal and vertical displacements in Figures 4c
and 4d are estimated by converting the change in the
distance between the satellite and the ground surface
to the values in the slope direction. From Figure 4aand 4b, the change in the distance between the satellite
and the ground surface is approximately equal to -50mm at the distance from 0 m (the lower end of theslope) to 100 m, increases gradually with distance, andreaches 0 m at the upper end of the slope. Generally,landslides cause vertically upward movement at thelower part of a slope and vertically downward
movement at the upper part of the slope. Therefore,
for a slope facing the satellite, one can expect that boththe horizontal and vertical displacements decrease
from the lower end to the upper end of the slope. The
estimated horizontal and vertical displacements
satisfy these features well. The horizontal displacement decreased from 100 mm to 0 mm with distance(elevation) (Fig. 4c). The vertical displacement
decreased from 80 mm to 0 mm with distance(elevation) (Figure 4d).
Figure 5 presents results obtained around the
Yunotani area. Near the center of Figure 5, one canobserve a displacement of 50 mm approaching thesatellite in an area of 1000 m × 1000 m. In this area, alarge displacement zone, up to 75 mm, is apparent,with spatial scale of 300 m × 500 m.
Figure 6 shows the change in the distances between
the satellite and the ground surface and coherence
around the Sennindani area. Two remarkable areas ofcolor changes are visible on the northern and southernparts in Figure 6, with the reduction of coherence. For
InSAR analysis, it is difficult to perform phaseunwrapping if the displacement exceeds a half
wavelength of the radar, meaning that it is difficult to
detect the ground deformation quantitatively. In
Figure 6b, one can recognize that the coherence values
are less than 0.1 in the two areas. Therefore, we cannot
estimate the displacement correctly in the two areas,
although we show the displacement in these areas in
Figure 6a as a result of the InSAR analysis. In otherwords, it is inferred that there might be some large
topographic change that InSAR analysis cannot detectcorrectly in these areas. The sizes of the northernand southern areas with low coherence are 1000 m ×1000 m and 500 m × 500 m, respectively. Both areas
are consistent with the landslide area shown in the
Hakusan landslide map (NIED, 2001).

43

Figure 4. (a) Elevation, (b) displacement between
satellite and ground, (c) horizontal displacement,and (d) vertical displacement along the X-Y line
(Figure 3) in the Jinnosukedani area.

Figure 5. Displacement estimated from InSAR
analysis in the Yunotani area. Shadow areas against
beam directions are masked.

44Landslide displacement detection around Hakusan volcano from InSAR analysis of ALOS-2/PALSAR-2 data

5

Figure 6. (a) Phase change and (b) coherence of
InSAR analysis in the Sennindani area. Shadow
areas against beam directions are masked.

Discussion

Figure 7a presents results of the GNSS observations
on October 2014 and October 2015 (Kanazawa River
National Highway Office, 2015), together with our
results obtained from InSAR analysis, in the
Jinnosukedani area. The GNSS observations showthat the horizontal displacements vary from 10 to 220
mm with the measurement locations. Large horizontal
displacements from GNSS observations are observedat points on the large displacement areas from InSAR
analysis. To compare both results directly, we
converted displacements obtained from InSAR
analysis into horizontal displacements along themoving direction of the ground estimated from GNSS
observations. Horizontal displacements from GNSS
observations correlate well with those from the InSAR
analysis (Figure 7b). The coefficient of determination
is 0.98.
We compare the area and the amount of thelandslide in the Jinnosukedani and the Yunotani areas
obtained in this study with those reported by

Figure 7. Comparison of displacement estimated from GNSS observations with displacement estimated fromInSAR analysis. (a) Gray circles show locations of GNSS observation sites. Displacement estimated from InSAR
is converted to horizontal displacement in the direction of the displacement from GNSS observation during
October 2014 – October 2015. (b) Plot of the annual horizontal displacement estimated from InSAR analysis
versus that from GNSS observation. The coefficient of determination of the plots is 0.98.

Yuya KITOH and Yoshihiro HIRAMATSU

Michinaka and Hiramatsu (2010) in Figure 8. In the
Yunotani area, the maximum landslide displacementin Figure 5 is 75 mm, which is approaching thesatellite. The horizontal displacement in the landslide
direction and the annual displacement were calculated
as the same way as for the Jinnosukedani area. The
slope angle (𝛿) and the slope azimuth angle (𝐴= ) are
19.1° and 258°, respectively (Michinaka andHiramatsu, 2010). The areas of the landslide of thetwo studies are approximately consistent. In the
Jinnosukedani area, the area where the annual
displacement of the landslide of about 100 mm or lessdetected in this study is almost consistent with thatreported by Michinaka and Hiramatsu (2010). In the
Yunotani area, the area of the annual displacement of
about 100 mm or less is also consistent between thetwo studies. However, the remarkable landslide areawith annual displacement of about 100–200 mm ofthis study is narrower than in the previous study. We
detect remarkable landslide displacement only in thelower part of the slope. The difference in resolution
between the PALSAR data used in the previous study
and the PALSAR-2 data used for this study mightcause the differences apparent in the landslide areas.
Another possible source of the difference in thelandslide area is seasonal change of landslides. In
general, landslides move greatly during the rainyseason and during the snowmelt period, and often
show periodicity (e.g. Terzaghi, 1950). The analysis
period used by Michinaka and Hiramatsu (2010) was
June–October, which corresponds to the snow melting
period, through the rainy season, to autumn, whenlandslide movement is most active. In other words, theannual displacement found their study might be
excessive estimates, resulting in a wider landslide area.
However, the analytical period of the present study islong (October–July) and does not fully include thesnow melting period and the subsequent period, whichsuggests that the estimated annual landslide
displacement might be less than the actual value.
Therefore, the estimated annual displacement in thepresent study is likely to be less than that reported byMichinaka and Hiramatsu (2010). The cause of thedifference will be specified with other new data infuture works.
In the Sennindani area, a large sector collapse
occurred on the northern side of the area in May 2015.
The Ishikawa Forest Office (2015) reported that thecollapse area was 300 m × 600 m (Figure 6). In Figure
6b, for the analysis area as a whole, the value of
coherence is 0.3 or greater. This implies that we can45

Figure 8. Comparison of landslide areas in this study
with those reported by Michinaka and Hiramatsu(2010). Solid orange polygons show areas ofdisplacement about 100 mm/year or less, and dashedones Michinaka and Hiramatsu (2001). Solid redpolygons show the areas of displacement of 100–200mm, and dashed ones Michinaka and Hiramatsu(2001).

detect landslide movement from InSAR analysis in
the Yunotani area. However, the coherence is less than
0.1 in the area of 1000 m × 1000 m on the northernpart of Figure 6b. As described previously, lowcoherence might indicate greater displacement than ahalf wavelength of the radar. In fact, a remarkable
color change is recognized in the area of 1000 m ×1000 m (Figure 6a), which corresponds to low
coherence and which includes the collapse area. It isnoteworthy that this area is much larger than thecollapse area. If this low coherence area reflects
landslide movement, then we suggest that the collapse
is only a superficial phenomenon. We present thepossibility that the landslide occurred in a wider areaof the ridge in the northern part of the Sennindani area.
Furthermore, Figure 6 suggests the possibility of
another landslide in the southern part of the area,
where no collapse has been reported, because aremarkable color change and lower coherence areapparent there. Successive monitoring is necessary to
clarify features of the landslide in the southern part.

6

Conclusions

From InSAR analysis using ALOS-2/PALSAR-2 data,
we detected landslide movements in three areas

46

Landslide displacement detection around Hakusan volcano from InSAR analysis of ALOS-2/PALSAR-2 data

(Jinnosukedani area, Yunotani area, and Sennindaniarea) around Hakusan volcano. The total landslidearea in the Jinnosukedani and the Yunotani areas isalmost consistent with those reported previously,suggesting continuous landslides there. However, in
the Jinnosukedani area, we detected the large
landslide movement in a part of the middle ridge blockthat has not been described in Michinaka andHiramatsu (2010). In the Yunotani area, the area ofdisplacement found in this study is narrower than that
presented in Michinaka and Hiramatsu (2010). The
possibility exists of seasonal fluctuation of landslides
in these areas, although the data resolutions of the twostudies differ. In the Sennindani area, we recognize
two low-coherence areas, which implies thepossibility of large landslide displacement. One islocated in the northern part of the Sennindani areawith area of 1000 m × 1000 m, including a collapse
area of 300 m × 600 m. The other landslide area, 500
m × 500 m, is located in the southern part in the
Sennindani area. These areas are consistent with thelandslide area reported previously. There are no
remarkable color changes caused by noises in thisstudy, while remarkable color changes caused bynoises might be found in Michinaka and Hiramatsu(2010). Therefore, we consider that ALOS-2 data canclarify features of landslides better than ALOS data.
The landslide displacement detected by InSAR
analysis in the Jinnosukedani area is consistent with
the GNSS observations, demonstrating theavailability of InSAR analysis as a landslide
monitoring tool around Hakusan volcano.

Acknowledgements
This study was supported by the Earthquake ResearchInstitute The University of Tokyo cooperative
research program. The PALSAR-2 data are shared by
PIXEL and are provided by the Japan Aerospace
Exploration Agency (JAXA) through a collaborative
research agreement between JAXA and theEarthquake Research Institute (ERI), The Universityof Tokyo. PALSAR-2 data are owned by the Ministryof Economy, Trade and Industry (METI) and JAXA.
We used RINC ver. 0.31 (Ozawa, 2014) for InSAR
analysis. The GNSS data are provided by the
Kanazawa River National Highway Office of the
Hokuriku Regional Development Bureau of the
Ministry of Land, Infrastructure and Transport.
Constructive comments from two anonymous
reviewers are useful to improve the manuscript.

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