10th Asian Regional Conference of IAEG (2015)

An investigation of a submerged landslide by using an innovative leisure-use sonar
Shintaro YAMASAKI
Kitami Institute of Technology, Japan
E-mail: yamasaki@mail.kitami-it.ac.jp
Abstract
There are many subaqueous topographies related to geohazards around the world.
However, investigations for them are fewer than on land due to their difficulty. Recently, I
used a fish finder designed for leisure use and other low-cost equipment to conduct anunderwater acoustic survey. This paper introduced our investigation of a submergedlandslide in Lake Ashinoko, Kanagawa, Japan. By our developed method, I created abathymetric map and obtained side-scan images showing features of the landslide mass
exposed on the lake floor, and found that the main landslide body is located on 300-450 m
offshore at 30 m deep. The landslide body had trees standing on the surface, suggests it
traveled on the lake floor without turbulence. The method is helpful to consider processesof submerged landslide.
Keywords: submerged landslide, submarine landslide, lake, fish finder, side-scan sonar
1.

Introduction

Oki (1993) suggested the trees were carried bylandslides with their diving investigation and theresults of 14C dating for trees. Because Lake
Ashinoko is a dammed lake by a collapse of volcano
in about 3000 years ago (Nagai and Takahashi, 2008).
These trees aged around 1050, 1600 and 2100 years
ago, those are quite younger than 3000 years ago. Oki(1993) made distribution map of trees on the lake
floor by a traditional fish finder, but did not
determine the shape and extent of landslides.
By the way, marine acoustic technologies have
conventionally been employed to investigate seafloor
topography. Obtaining shape of them are easy byusing narrow and high frequency acoustic beam thatare used for scientific research. However, thesesurveys are usually quite expensive. Recently, someleisure-use new fish finders have high frequency
acoustic beam systems and side-scan sonar systems tofind fish reefs, and such systems can couple therecorded sounding data with location data obtainedby the global navigation satellite system (GNSS). I
developed a method employing the fish finder toinvestigate subaqueous topography (Yamasaki andKamai, 2015). In this paper, I present it thorough theinvestigation of submerged landslide in Lake
Ashinoko.

There are many subaqueous topographies that
related to natural hazards around the world. For
example, submarine landslides, volcanos and active
faultshave
causeddeadlydisasters.
Inearthquake-prone areas such as Japan, historical
records has documented the occurrence of sudden
landslides in coastal areas that have swept residences
and other infrastructure into the sea or lakes. For
example, in the 1792 Unzen Mayuyama landslide, avolume of 3.4 × 108 m3 swept through Shimabara
killing about 10,000 people. The landslide reached
the sea and caused tsunamis that killed another 5000
people in and around Ariake Bay (Tsuji and Hino,1993). In 2000s, Hayashi and his research groupbegan to investigate 10th century-submerged ruins inLake Biwa, Japan, by conducting dives and acoustic
marine surveys. They inferred that these ruins werecarried into the lake by landslides induced by theliquefaction of lakeside sediments (Hayashi et al.,2012). The aim of acoustic marine surveys is not only
to conduct archeological research but also to estimate
the vulnerability of areas to future hazards,
particularly coastal areas. However, few studies haveinvestigated subaqueous topographies related togeohazards because of the difficulties in surveying
such areas.
In this study, we examined submerged landslidein Lake Ashinoko. In the lake, trees standing on a partof lake floor had been eyewitnessed for long time.

2.

MethodThe application of acoustic reflections is aconventional method to obtain data on water depth,

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Fig. 1 (A) Schematic diagram of the fish-finder
system and (B) photo of the boat equipped withthe transducers. Further details are suggested in thetext.

Fig. 2 (A) aerial photo of Lake Ashinoko and (B)Topographic and bathymetric map of the area
shown in (A) (contour interval on land and underwater, 1 m). Numbers in axes are the distance(meter) of the control point. The square indicatesthe boundary of the side-scan sonar scanning area
(Fig. 3). Red dashed line enclosed area is "highreflection area" observed by side-scan sonar. Two
black lines indicates the locations of the 455 kHz
acoustic profile (Fig. 4). The bathymetric map wasgenerated from data obtained from this study. The
topography on land was obtained from 5-m DEM
data of the Geological Survey Institute of Japan.

underwater topography, and bottom materials. A
single-beam echo sounder (or single beam sonar) isused to measure depth and detect objects just beneatha ship; fish finders employed for leisure use are anexample. Multi-beam sonars can measure multiple
depth data simultaneously and have been used on
scientific research vessels, but those systems are
expensive and too heavy to be used on small boats,such as the inflatable raft that I used for my survey ofshallow nearshore waters. Modern fish finders are
small enough to be used on smaller boats. Somenewer fish finders also employ GNSS to couple
recorded depths with location information. Someleisure-use fish finders employ a side-scan sonar
system, which enables them to obtain a wider imageof the seafloor.
For the investigation in Lake Ashinoko, I used
HDS-5 Gen2 fish finder (Lowrance, US), which wascapable of obtaining consecutive location information
to within 3 m horizontally through the use of a GNSSsystem. The transducer on the fish finder was a B60(Airmar Technology Corporation, France) with anassigned frequency of 200 kHz. The minimum
recordable depth interval was 1 cm. The data obtainedby this transducer were used to create a bathymetricmap because the fish finder has a narrower measuring

interval than the side-scan sonar. I also used aside-scan sonar system (StructureScan LSS-HD,Lowrance) to record consecutive sonar and side-scanimages and depth information on an SD card. The
side-scan sonar system had a 25-cm-long transducer
with an assigned frequency of 455 kHz. Both systems
were powered with a 12-V lead acid battery. The total
weight of both systems, including the battery, wasabout 10 kg, so I was able to place the equipment on asmall boat that could navigate the shallow waters of2

10th Asian Regional Conference of IAEG (2015)

corrected map, I corrected the coordinates of point
data with Japanese rectangular plain system IX, forwhich the origin point is E139˚50’00”, N36˚00’00”.
Follow-up underwater camera observation wasconducted after the acoustic investigations to confirm
the findings.
3.

Results and Discussions

In this paper, I show an investigation case for thenorth part of Lake Ashinoko with obtained data.
Generated bathymetric map (Fig. 2) shows acharacteristic ca. 400-m-long ridge from the center ofbay to the deeper part of lake. The steep slope islocated from water surface to the depth of 20 m
(725-705 m in Fig. 2). The ridge connects the foot of
the slope and the ridge has a rise at its deeper part.
Synthetic side-scan image (Fig. 3) in the squared
area in Fig. 2 suggests the rise is high acoustic
reflection area (brighter color) suggesting thepresence of high dense material exposed on the lake
floor. The shape of “high acoustic reflection area”
corresponds to the topography of the deeper part ofridge. In addition, smaller high reflection areas(bright color area) are separately distributed onshallower area than the rise, and they are scarce on
deeper area. Low acoustic reflection area (darkercolor) that widely covered on the lake flower is lose
lacustrine sediment. Hence, “high acoustic reflectionarea” is other origin, like the sediment on land.
Vertical acoustic profiles (Fig. 4) shows rises
with high acoustic reflection and stick like structures
standing on the rises. These stick like structures weretrees were observed by an underwater camera (Fig.
4C). The trees on the lake floor has been reported byOki (1993). A number of trees were tilted onshore(Fig. 4B). It is reasonable that the trees are carried
from land, because trees were rare on the lacustrine
sediment and on deeper part of the lake. At least they
are not the trees before the lake advent.
Oki (1993) already suggested the trees on thelake floor caused by past landslides. The scatteredFig. 3 Synthetic side-scan sonar images indicatedin Fig. 2. Red dashed line enclosed area is "highreflection area".
the study area (Fig. 1).
The bathymetric survey was conducted in shallow
water (<30 m) over an area of 1000 m × 700 m atintervals of 30 m. The side-scan survey wasconducted over a smaller part of this area (400 m ×500 m) along 11 measurement lines. Side-scan
images were captured directly from the display of thefish finder.
I extracted consecutive location information withcoordinates and depths with SonarTRX software(Leraand Engineering, US), a sea-bottom mapping
software application, from the data obtained by thefish finder. I then used the mapping software Surfer
12 (Golden Software, US) to construct a contoured
bathymetric map. Interpolation was done by thekriging method. Then, to generate a geometricallyFig. 4 (A and B) Vertical acoustic profiles of 455 kHz shown in Fig. 2 and 3. Red arrows indicates treesstanding on the lake floor. (C) A trunk of tree observed by underwater camera.

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10th Asian Regional Conference of IAEG (2015)high acoustic reflection areas under water are
considered as scattered masses by a submergedlandslide.
Since I do not have any information of the pastlake water level, I cannot discuss where the landslide
occurred. However, the bathymetric map give someclues. As the map showed, there is steep slope fromthe water surface to the depth of 20 m, it is thecommon feature around the lake. The landslide
formed ridge is located on the foot of the steep slope.
The steep slope has probably formed with erosion byrising water level. Also the landslide probably
occurred in a certain past age. With rising water level,
ground water level also rises. That situation makes
preferable condition landslides occur.
The research group of Kihara Institute for
Biological Research and Kanagawa Hot spring andGeological Research Institute took samples of trees
around the “high acoustic reflection area” for 14C
dating and obtained chronological value of 1600
years ago (Oki, 1993). That value is quite younger
than the age of lake (3000 years ago: Nagai andTakahashi, 2008). As Oki (1993) proposed, somestrong earthquake might have triggered the landslide.
The bathymetric map and the side-scan image
shows also clues of the submerged landsliding
process. The large high acoustic reflection area is apart of large landslide mass. The trees on the mass
may have tilted to onshore by water pressure. The
landslide travelled at least ca. 300 m sliding parallelto the lake floor without turbulence.
4.

Acknowledgements
I developed the equipment with the assistance of
the Technical Division and Engineering Center andthe Kussharo Lake Training Institute of the Kitami
Institute of Technology, and the Toya Lakeside
Training Institute of Hokkaido University. Masahiro
Hiramatsu of the Kitami Institute of Technology wasparticularly helpful. The investigation in Lake
Ashinoko was done with Prof. Yuki Matsushi ofKyoto University, Prof. Kyoko Kataoka of Niigata
University and Prof. Naofumi Yamaguchi of Ibaraki
University. This work was supported by aGrant-in-Aid for Scientific Research (No. 26560187)
and General Collaborative Research of Kyoto
University Disaster Prevention Research Institute (No.
26G-09). I thank all of those who helped with theresearch.
ReferencesHayashi, H., Kamai, T., Haraguchi, T. (2012):
Submerged
VillagesinLakeBiwa(Jishin-de-shizunda-kotei-no-mura), p. 137. (inJapanese)
Nagai, M., Takahashi, M.(2008): Geology anderuptive history of Hakone volcano, central Japan.
Res. Rep. Kanagawa prefect. Mus. Nat. Hist., Vol.
13, pp.25-42. (in Japanese with English abstract)Oki, Y. (1993): Fossil ceder trees of Lake Ashi asfossils of large earthquakes in southern Kanto
District, Journal of Geography (Chigaku-Zassi), Vol.
102(4), pp.437-444. (in Japanese)
Tsuji, Y., Hino, T. (1993): Damage and inundation
height of the 1792 Shimabara landslide tsunami
along the coast of Kumamoto Prefecture, Bull.
Earthq. Res. Inst., Univ. Tokyo, 68, pp. 91–176. (InJapanese with English abstract)
Yamasaki, S., Kamai, T., 2015. A novel method ofsurveying submerged landslide ruins: Case study of
the Nebukawa landslide in Japan. Engineering
Geology, 186, pp. 28-33.

Conclusions

I investigated a characteristic subaqueous
topography of Lake Ashinoko by using a recent
fish-finder, made bathymetric map, and obtained
side-scan images and vertical acoustic profile images.
I clarified the topography was formed by a pastlandslide, and discussed the landsliding processes
and timing of the occurrences. The subaqueous
investigation by using fish finders is useful forfinding past subaqueous hazard events.

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