10th Asian Regional Conference of IAEG (2015)

Hazard mapping of earthquake-induced deep-seated catastrophic landslide using
Helicopter-borne electromagnetic (HEM) data
Atsuko NONOMURA(1), Shuichi HASEGAWA(1) and Kazuya KAGAMIHARA(2)(1) Faculty of Engineering, Kagawa University,
E-mail: nonomura@eng.kagawa-u.ac.jp(2) NIPPON ENGINEERING CONSULTANTS, 3-23-1 Komagome, Toshima-ku, Tokyo, JAPAN
Abstract
Asia has been the site of several giant earthquakes within a few decades. Although it
is impossible to stop large-scale landslides that might be induced by the next earthquake,the damage can be mitigated by determining landslide-susceptible areas and estimating thehazard beforehand. Large number of articles is proposing many different methods oflandslide-susceptibility estimation. Topographical approaches are commonly used.
Topographical features can be used for roughly estimating distribution of large-scalelandslide, but these are not directly related to the subsurface geological conditions. On theother hand, geophysical surveys are useful techniques for estimation of regional bedrock
conditions. One of the geophysical parameters, resistivity can be available to estimate
subsurface features, although resistivity tends to be affected by several compositional
characteristics of bedrocks, such as lithology, mineralogy, and water content.
The aim of this study is to develop an algorithm to predict the susceptible slopes toearthquake induced large-scale landslide using Helicopter-borne electromagnetic (HEM)
data. The algorithm was developed and applied to slopes along roads to estimate thesituation of catastrophic deep-seated landslide after huge earthquake.
Keywords: resistivity, deep-seated catastrophic landslide, disaster risk management
1.

scale shallow landslide. Deep-seated large-scalelandslides are few (less than 10%), but the damage ofeach landslide was serious. Deep-seated landslide
occurred on the surfaces of rupture or at loosenedrock mass.
In geological studies, several geophysical methods
are used to investigate the subsurface geologicalcondition. Airborne geophysical surveys are usefultechniques for estimation of regional bedrock
conditions. Those can be completed much faster thanground-based soundings, without topographicallylimitation even in the steep slopes. Since resistivity
tends to be affected by several compositional
characteristics of bedrocks, it needs to be estimatedthe distribution of loosened zones from the resistivity
data.
To identify slopes susceptible to earthquake-induced
deep-seated catastrophic landslide, both looseness ofrock mass and topographical effect of amplifying
seismic wave needs to be considered. Hasegawa et al.
(2015) proposed the index of susceptibility forearthquake-induced
deep-seatedcatastrophic
landslides (ISEDCL) by combining looseness of rockmass and topographic effect of earthquake induced

Introduction

Asia has been the site of several giant earthquakes
within a decade, the 1999 Chi-Chi earthquake in
Taiwan, the 2005 Kashmir earthquake in Pakistan, the2008 Iwate-Miyagi Nairiku earthquake in Japan, the2008 Wenchuan earthquake in China and the 2011
Tohoku earthquake (Shou and Wang 2003; Chigira
and Yagi 2006; Sato et al. 2007; Wang et al., 2003;
Chigira et al. 2010; Maeda et al., 2011). In
mountainous areas, landslides induced by earthquakes
cause substantial damage to the lives, infrastructure,environment, and economies. Although it is
impossible to completely stop large-scale landslides
that might be induced by the next earthquake, thedamage can be mitigated by determining
landslide-susceptible areas, estimating the hazard,
and preparing it beforehand.
Huge earthquakes have triggered several thousandlandslides. Most landslides were small and middle

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10th Asian Regional Conference of IAEG (2015)

landslide. Looseness of rock mass is estimated byusing resistivity and topographical effect ofamplifying seismic wave is quantified by using
Digital Elevation Model.
In this study, the ISEDCL is applied to slopes
along the crucial national road for emergent
transportation just after the huge Nankai Earthquake.
ISEDCL was proposed as an index for quantifying
looseness of rock mass by using resistivity data.
Since the resistivity is influenced by several factors,
such as mineralogy, looseness, lithology, and watercontent, the usefulness need to be investigated under
several different geological condition.

condition show higher resistivity than sound bedrocks
in the same lithology. At the same time, resistivity
tends to be affected by several other compositional
characteristics of bedrocks, such as lithology,
mineralogy, and particle size. In order to investigate
the distribution of the loosened zones and exclude
lithological impact from the resistivity data,“ruggedness of resistivity” was proposed (Nonomura
et al., 2012). The index is available to differentiate
fractured bedrock from sound bedrock using shallow
layer (140 kHz returns data for depths of 5–30 m). In
this study, we investigate the availability of thismethod by applying it to the study area with different
lithological condition.

2. Data and method
2.1 Resistivity
Resistivity data have been commonly used todetermine landslide susceptibility, where main factorsof landslides are the presence of water, production ofclay minerals, and the existence of a slip surface,which are indicated as lower resistivity areas(Lebourg et al., 2010). Airborne electromagnetic
survey data provide information about the distributionof subsurface resistivity (Huang and Fraser, 1996;
Beard, 2000). The method is based on the
propagation of electromagnetic fields, which induce
currents in the subsurface. The system we used hadtransmitting and receiving coils with frequencies of
140 kHz, 31 kHz, 6.9 kHz, 3.3 kHz, 1.5 kHz, and 340
Hz inside a cylindrical container that was suspended
below the helicopter by 30-m-long wires andmaintained at a height of about 35 m above theground during data recording.
The high frequencies record data from shallow
layers. In order to predict the distribution of loosenedrock mass, “ruggedness of resistivity” index wasdeveloped (Nonomura et al., 2012). The unsaturated
fractured bedrocks would have relatively higher
resistivity (Sass, 2006). Loosened beds situated above
the water table in highly fractured and permeable

Mikabu Belt

2.2 Index of susceptibility for earthquake-induced
deep-seated catastrophic landslides (ISEDCL)
Deep-seated catastrophic landslides tend to occur atloosened zones with topo-graphical effect ofamplifying seismic wave. The topographic effects onearth-quake ground motion have been well
documented. It has been noted that slope and averagecurvature are very important parameters forquantifying the
topographical potentialofearthquake-induced landslide. Uchida et al. (2004)
quantified the topographical effect on earthquakeinduced shallow landslide susceptibility and proposeda parameter, which is called ‘F-value’ to estimate thelandslide susceptibility by extensively studying
landslide damage in the Rokko mountain range(granitic terrain) after the Hanshin-Awaji Earthquake(Kobe earthquake) of 1995 (Equation 1).
F= 0.075×Slope-8.9×Meancurvature+
0.0056×peak ground accerelation-3.2 (1)The applicability of F-value was shown in the 2008
Chuetsu-oki earthquake (Hasegawa et al., 2009). In
this research, F-value was utilized to estimate the
distribution of topographical effect of amplifying
seismic wave. If F-value is larger than zero, there issome possibility to amplify seismic wave and occur
shallow landslide.
Index of susceptibility for earthquake-induced
deep-seated catastrophic land-slides (ISEDCL) is
defined by multiplying F-value and ruggedness ofresistivity. Ruggedness of resistivity is a parameterfor estimating looseness of rock mass and F-value isused to quantify the possibility to amplifying seismic
wave.

Ryoke Belt
Sanbagawa Belt
Chichibu Belt
Shimanto Belt

3.

Study area

The study area is located at Shikoku mountains
located between 33°50'26”N and 33°37'37”N latitudeand between 135°38'00”E and 135°47'25”E longitude(Fig. 1). The Shikoku Island is located in the western

Fig. 1 The geology of Shikoku (after Geological
Survey Enterprises Association Shikoku HP). The
blue dot line shows the study area.

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10th Asian Regional Conference of IAEG (2015)

4. Result
4.1 Resistivity distribution
The resistivity distribution of 140 kHz
electromagnetic wave shows strong contrast (Fig.2).
Higher resistivity is found in Sanbagawa Belt withpelitic schist and in northern part of Chichibu Belt
with chert. Lower resistivity is found in Mikabu Belt
with green-schist and northern part of Chichibu beltwith pebbly mudstone and green stones. The
distribution of resistivity is influenced by thelithology.
In previous research (Nonomura et al., 2012),
ISEDCL was developed as an index for quantifying
looseness of rock mass for estimating loosened zones
with less influence of lithology; higher resistivity
zones correspond to loosened zones and lower
resistivity zones correspond to less loosened zones
within the radial distance. The test site was a small
area under one type of lithology, alternation ofsandstone and mudstone. In order to develop
versatility of the ISEDCL, the capacity needs to be
examined under several geological conditions.

Sanbagawa Belt(pelitic schist)

Mikabu Belt (green schist and green stone)
ChiChibu Belt(chert, pebbly mudstone,
green stone, limestone)
Resistivity (Ωm)

Fig. 2 Resistivity distribution in the three different
geological belts.

4.2 Ruggedness of resistivity
In order to localize distribution of resistivity, theconcept of openness is applied to resistivity data,
instead of to DEM, and the distribution of relatively
higher resistivity is estimated. It is called “ruggednessof resistivity”. To calculate ruggedness of resistivity,
the zenith angle and the nadir angle from the point of
nterest is calculated within a radial distance (L) along
all eight azimuthal directions.
The radial distance L is one of key factors to define
the range of area for calculating relative distributionof resistivity. In order to operationally estimate thepart of Japan, and it can be roughly divided into fivegeological zones: Ryoke Belt, Sanbagawa Belt,
Mikabu Belt, Chichibu Belt, and Shimanto Belt. The
study area is located over Sanbagawa Belt, Mikabu
Belt and Chichibu Belt. The Sanbagawa Belt ismainly composed of pelitic schist. Mikabu Belt ismainly composed of green schist and green stone.
Chichibu Belt is composed of chert, siliceous
mudstone, acid tuff, and limestone.

Ruggednessof resistivity

Fig. 3

Ruggednessof resistivity

Ruggednessof resistivity

Ruggedness of resistivity with different radial distance; 50m, 100m 150m

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10th Asian Regional Conference of IAEG (2015)

I-Tunnel

Fig. 4 ISEDCL estimated using the ruggedness ofresistivity with 50 m radial distance andtopographic effect of amplifying seismic wave,
with peak ground acceleration (PGA) 400gal.
ISEDCL is categorized into four levels
looseness for deep-seated landslide susceptibility,
appropriate radial distance is discussed.
Fig.3 show the ruggedness of resistivity withdifferent L; L=50m, 100m, 150m. With longer radial
distance, the ruggedness of resistivity shows more
broad view of relative resistivity distribution. If theloosened is estimated with zones or some interval,
longer radial distance is useful. If the looseness isestimated at each slope, localized relative distributionof resistivity is useful to identify the loosened slopes.
In this study, looseness is estimated by using L=50 m
in the slopes along the national road.

Fig. 5 Photos at I-Tunnel and slopes above the tunnel(a) Itagino tunnel (b) Rock mass with open crack
above the tunnel (c) Toppled and fractured rock mass
above the tunnel

4.3 Validating ISEDCL in the field survey
ISEDCL is estimated using the ruggedness ofresistivity with 50 m radial distance and topographic
effect of amplifying seismic wave, F-value with peakground acceleration (PGA) 400gal. The areas are
categorized into four levels; very high hazard, highhazard, moderate, very low (Fig. 4). In theconcentrated very high hazard zones along the road,the debris might damage the road and the road
networks might be cut off.
In order to validate the estimated ISEDCL, the rockmass condition and the topographies were observedin the field (Fig. 5). Surrounding the I-Tunnel show
the very high ISEDCL values. On the slopes above
the tunnel, the rock mass fractured and loosened withmany open cracks. Loosened rock masses bulge out
with open cracks with 20 cm width. In this area, it
was reported that the rock mass is gravitationally
deformed and loosened due to toppling. Under strong
rainfall, road section of Shikoku Regional
Development Bureau office, which works on disasterrisk management, close road to escape damage due tolandslide.

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10th Asian Regional Conference of IAEG (2015)

The loss of the road network causes serious second
damages. These earthquake-induced landslide hazard
maps are useful for estimating damages and planningmeasures for future great earthquake and for planningbypass road to mitigate the damage as possible as wecan.
5.

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Conclusions

Earthquake-induced landslide hazard maps areuseful for estimating damages of the future greatearthquake and planning measures to mitigate thedamage as possible as. In this study, a method wasdeveloped to predict the susceptible slopes toearthquake induced large-scale landslide using
Helicopter-borne electromagnetic (HEM) data andDigital Elevation Model (DEM). The method wasapplied to slopes along national roads which are
expected to be emergency route when the great
Nankai trough earthquake will occur.
The high frequencies record data of HEM indicate
the geological information from shallow layers. In
order to predict the distribution of loosened rock mass,“ruggedness of resistivity” index was developed(Nonomura et al., 2012). The unsaturated fractured
bedrocks would have relatively higher resistivity(Sass, 2006). Loosened beds situated above the watertable in highly fractured and permeable condition
show higher resistivity than sound bedrocks in thesame lithology. At the same time, resistivity tends tobe affected by several other compositional
characteristics of bedrocks, such as lithology,
mineralogy, and particle size. In order to focus on thelooseness, “ruggedness of resistivity” was proposedin the study area with several types of lithology.
Slopes above the I-Tunnel are estimated to be highly
susceptible to landslide (high value of ISEDCL). By
Shikoku Regional Development Bureau office, whichworks on disaster risk management, these slopes havebeen regarded to be hazardous slopes where the roadwill be closed to escape damage due to landslide
under strong rainfall. ISEDCL is useful to estimate
deep-seated landslide susceptibility.
Acknowledgements
The authors are grateful to the financial support
by Shikoku Regional Development Bureau office forthis research.
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