10th Asian Regional Conference of IAEG (2015)

Clarifying Geological Structure of Deep Catastrophic Landslide Using Airborne
Electromagnetic Survey
Hiromitsu ISSHIKI (1), Atsuhiko KINOSHITA (2), Teruyoshi TAKAHARA (2),Tadanori ISHIZUKA (2), Katsushi KAWATO (3) and Minoru OKUMURA (3)(1) Oyo Corporation, Japan E-mail:isshiki-hiromitu@oyonet.oyo.co.jp(2) Public Works Research Institute, Japan (3) Nippon Engineer Consultants Co., Ltd, Japan
Abstract
The airborne electromagnetic (AEM) survey is useful to estimate the resistivity
structure of the underground by measuring electromagnetic inductions for a short period. In
this study, we target the deep catastrophic landslides (DCL) which had been triggered by
Iwate-Miyagi Inland Earthquake in 2008 at the mountain slopes of Mt. Kurikoma. Thefluidized materials ran down and attacked a small spa hotel and resulted in a lot of loss ofhuman life. In considering mitigation measures, it is important to predict the slope wherethe DCL occurs. In this study, we aimed to estimate DCLs risk of slopes. First, weconducted a field survey; we confirmed the distribution of geological conditions andcollapse section. Soft altered tuff breccia has spread on the whole slopes of the survey area,
and is covered by andesite cap rock at the upper part of slopes. It was estimated DCL hasoccurred at the boundary of andesite and altered tuff breccia. Secondly, we conducted AEM
survey over a wide range including the landslide. While the specific resistivity of theuncollapsed zone is 400Ω-m or more, that of collapsed zone is 100-200Ω-m below andshows low resistivity value. Lastly, as a result of the comparison between the results of thefield survey and that of the AEM survey, a spatial distribution of specific resistivity almost
correspond to the underground geologic settings, such as andesite cap rock. By thedistribution of high-resistivity zone, andesite and collapsed andesite layer could be
estimated. In this survey area, the collapse occurs in the andesite cap rock, thehigh-resistivity zone is equivalent to the slope at risk of DCLs. In conclusion, in order toevaluate a risk of DCL, it is necessary to take into account underground information. TheAEM survey can be one of the practical methods applicable for the evaluation of a risk ofDCL.
Keywords: deep catastrophic landslide, airborne electromagnetic survey, resistivity,

1.

Introductionan active volcano in the north-eastern Japan. Field
survey was conducted in and around the DCLs where
the airborne electromagnetic survey, hereinafter
called AEM survey, was also executed. In order toevaluate applicability of the AEM survey, the authors
compare the results of the field survey with that of theAEM survey in this report.

Prediction of deep catastrophic landslide-prone(DCL-prone) slope is critical to make mitigation
measures. In order to evaluate a risk of DCL, it isnecessary to know underground information. About
the extracting method of the DCLs risk of slopes,
many studies have been accomplished until now(Sasahara et al., 2014; Chigira et al., 2012;
Yokoyama et al.; 2012). These studies were based on
an analysis technique that focuses on the topographyof the ground surface. There are few studies focusedon the subsurface structure which has influenced
outbreak of the DCL.
In this study, the authors target DCLs which had
been triggered by Iwate-Miyagi Inland Earthquake in2008 in mountain slopes of Mt. Kurikoma, which is2.

Location

Survey area is the headwaters of the Dozou of theleft hand tributary of Sanhazama river of KitakamiRiver system. DCLs were induced in the headwaters
slope by the Iwate-Miyagi Inland Earthquake on June
14th, 2008. The fluidized materials ran down andattacked a small spa hotel "Komanoyu Onsen" andresulted in a lot of loss of human life (Fig.1).

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Survey areaFig. 1 Location of the survey area

Ground speed 30-80km/h
Transmissionof the primarymagnetic field

Electromagnetic sensor Bird
Reception of secondary magnetic field

Terrain clearance 30-60m

Occurrence of secondary
magnetic field

Fig. 2 The schematic view of the airborne electromagnetic survey method
3.

Investigative method

value that varies depending on the type of rock, water
saturation, porosity, and clay minerals. Generally,
resistivity is high in fresh hard rock, and decreases
with the degree of weathering (Archie, 1942;
Takakura, 2009).
Fig. 2 shows the schematic view of the airborne
electromagnetic survey method with actual survey
situation. With this system, it is possible to explore
about 100 to 150 m depth and measure three
dimensional resistivity distributions by navigating ahelicopter in a lenticular pattern.
In this study, AEM survey was conducted aroundMt. Kurikoma in August 2009.

3.1 Geological distribution and structure
We performed geological survey to clarify thegeological distribution, structures and weathering
state of rocks of the entire landslide area. Geologicalsurvey was uniformly carried out not only within thelandslide but also back slope. We observed theoutcrops along the route, and measured the fault
direction, weathering conditions and geologicalstructure. The field survey was conducted to measure
the resistivity of the outcrop rock using a simple ratio
resistivity measuring device.

4.

3.2 AEM survey
AEM survey obtains the ground resistivity of awide area by measuring the intensity of a secondary
magnetic field induced in the ground by a primarymagnetic field transmitted at a transmitting coil.
Since the lower the frequency of the electromagnetic
waves, the deeper they penetrate into the ground, it ispossible to survey a wide range of depths by using
multiple frequencies simultaneously. Resistivity
obtained by airborne electromagnetic survey is a

Result and Discussion

The geological setting of the survey area iscomposed of volcanic ejecta of Mt. Kurikoma. Tuffbreccia, altered tuff breccia, autobrecciated lava, andandesite are layered and tephra is on the top of thelayers (Fig.3). The dimension the DCL was about
400m in length, about 300m in width and about 15 to20 m in thickness. The volume of the landslide wasestimated to be about 890,000 m3.

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10th Asian Regional Conference of IAEG (2015)

volcanic ashes
Andesite
Autobrecciated lava

Altered tuff breccia

Fig. 3 Circumstances surrounding the collapse scarpof Dozou valley

Table 1 Relationship between geology and resistivity
Fig. 5 Cross longitudinal profile of specificresistivity along the section line A-A’

Fig. 6 Cross longitudinal profile of specificresistivity along the section lines B-B’ and C-C’
Fig. 4 Spatial distribution of specific resistivity ofthe shallowest layer of the survey area
The sliding mass is divided into the two blocks.
The first one is the upper block near the head scarp
and the second is the lower block which had alreadyrun down as a mudflow and hardly remains on theslope. The weakly altered tuff breccia with kaolin
clay is exposed on the slope surface lower than theupper block.
Fig. 4 shows spatial distribution of specificresistivity of the shallowest layer of the survey area.
Fig. 5 shows cross longitudinal profile of specificresistivity along the section line A-A’ in Fig. 4
which is a middle line of the DCL in 2008. Fig. 6
shows the sections B-B’ and C-C’ where nolandslide has occurred so far. As shown in Fig. 4
the field survey infers that the sliding mass of theDCL in 2008 seemed to be composed of theandesite cap rock, and weakly altered tuff breccia.
As shown in cross sectional profile of the A-A’
section line, the distribution of the andesite caprock seems to correspond almost to a higher
specific resistivity zone (over 400Ω･m) , that ishigher than 200Ω･m while the distribution of theunderlying altered tuff breccia is roughly shown asa lower specific resistivity zone. On the other hand,a section line B-B’ crosses over an ancient DCLscar. As the cross sectional profile of the section

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10th Asian Regional Conference of IAEG (2015)
line B-B’, no higher specific resistivity zone isfound in the upper part of the slope whereas there isa thin layer with higher specific resistivity on thelower part of the slope. It seems to correspond to
the fact that there is no andesite cap rock but
andesite talus deposits because the cap rock seemed
to have collapsed and deposited on the lower part ofthe slope at the time of the ancient DCL. While thespecific resistivity of the uncollapsed zone is400Ω ･ m or more, that of collapsed zone is100-200Ω ･ m below and shows low resistivity
value.
Using simple resistivity measuring instrument,
the resistivity of outcrops was measured. The hard
andesite lava was high resistivity, altered andesite
was low resistivity, and especially spring part isvery low. Trends in the local resistivity byresistivity measurement of outcrops wereharmonious with the aerial electromagneticresistivity.
5.

Chigira, M. and C Y Tsou, and Matsushi, Y. andHiraishi, N. and Matuzawa, M. (2012): Sitecharacteristics of deep-seated landslides induced
by Typhoon 12–Analytical results by using
high-resolution DEMs before and after the events–, Characterization, prediction, and managementof deep-seated catastrophic landslides, pp. 24-34.
Daimaru, H., Murakami, W., Tada, Y., Okamoto,T., Sammori, T., Esaka, F. (2011):
Geomorphological and geological background of
the Llandslides induced by the Iwate-Miyagi
Nairiku Earthquake in 2008 in the upper part of
the Ichihasama River watershed, northeast Japan,
Journal of the japan Landslide Society, Vol. 48,No.3, pp.23-36.
Fujinawa, A., Fujita, K., Takahashi, M. (2001):
Development History of Kurikoma Volcano,Northeast Japan, The Volcanological Society ofJapan, Vol. 46, No.5, pp. 269-284.
Sasahara, K. and Hiura, H. and Doshida, S. (2014):
Geomorphological characteristics of the collapsed
slopes due to Typhoon No. 14, 2005 at theupstream of the Monobe river, Kochi, Japan,
Journal of Erosion Control Engineering, Vol. 66,
No. 6, pp.40-49.
Takakura, S (2009): Influence of pore-watersalinity and temperature on resistivity ofclay-bearing rocks, The Society of Exploration
Geophysicists of Japan, Vol. 62, No. 4, pp.
385-396.
Yokoyama, O. and Uchida, T. and Nakano, Y. andIshiduka, T. and Kasai, M. and Suzuki, R (2012):
Clarifying surface shape of mass rock creep
revealed by using airborne LiDAR, Journal ofErosion Control Engineering, Vol. 64, No. 6, pp.
13-24.

Conclusion

As a result of the comparison between theresults of the field survey and that of the AEM
survey, a spatial distribution of specific resistivity
almost correspond to the underground geologic
settings, such as andesite cap rock. In order toevaluate a risk of DCL, it is necessary to take into
account underground information. The AEM
survey can be one of the practical methods
applicable for the evaluation of a risk of DCL.

References
Archie, G. E.（1942）：The electrical resistivity log
as an aid in determining some reservoir
characteristics, Trans. A. I. M. E., 146, 54-67.

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