10th Asian Regional Conference of IAEG (2015)

Seismic measurement in the mudstone landslide area
Issei DOI(1), Sumio MATSUURA(1), Tatsuya SHIBASAKI(1)and Hikaru OSAWA(1)(1) Disaster Prevention Research Institute, Kyoto University, Japan
E-mail: doi.issei.5e@kyoto-u.ac.jp
Abstract
In the southeastern part of Hokkaido, Japan, landslides in sedimentary layers(especially mudstones, alternate layers of mud and sand in the Cretaceous and in theTertiary) are often observed along the coastline. The surface layer is completely weathered
due to cold temperature under which water in it exists as ice during winter. Ocean waves
erode the bottom side of the shore in the situation where the landslide areas in this region
are facing the Pacific. In these circumstances, the stability of the slopes along the coastline
is decreasing. Owing to the tectonic setting that the Pacific Plate is subducting beneath thesoutheastern part of Hokkaido, large inter-plate and intra-slab earthquakes frequently occur.
For example, ten earthquakes with magnitude larger than 7 are recorded there during recent~20 years. Strong motion may be generated due to the non-linear behavior of the soft soils
and the effect of the pore pressure in them, and in turn, unstable slope may slide or collapse
due to the strong motion. It is important to grasp how the soils shake in response to thestrong seismic motion. We installed a seismometer on the slope in the ~20 m distance fromthe coastline, in the landslide at Akkeshi, which is one of the coastal towns in thesoutheastern part of Hokkaido. The natural frequency of the seismometer is 2 Hz. The
seismic signals are continuously recorded every 0.01 seconds with the resolution 24 bit. The
observation started on Sep. 24, 2014, and continues until now. So far, M5 earthquakes
which occurred in the southeastern part of Hokkaido were recorded. We will elucidate
amplification mechanism and non-linear effect in the soil by analyzing obtained seismic
waveforms.
Keywords: coastal landslide, earthquake, mudstone
1.

Introduction

inter-plate and intra-slab earthquakes frequently occur
and have been actually occurred. For example, tenearthquakes with magnitude larger than 7 arerecorded there during recent ~20 years. Therefore, it
is a suitable place to understand the landslide
mechanism depending on the various causes such as
rainfall, sea shore erosion, and earthquake strong
motions.

In recent days, global warming is causing various
kinds of serious problems. Landslide accompaniedwith seashore erosion due to sea surface elevation isimportant especially for conservation of national land.
In order to understand the process of the landslides,
we have to grasp their characteristics depending on
the causes such as rainfall, cut-off at the foot side,and earthquakes.
In the southeastern part of Hokkaido, Japan,landslides in sedimentary layers (especiallymudstones, alternate layers of mud and sand in theCretaceous and in the Tertiary) are often observed
along the coastline (Yamaguchi et al., 1975). Since
the water under the ground surface freeze due to aterrible coldness, the surface layer is completelyweathered. Ocean waves erode the foot side of theshore because some of the landslide areas in thisregion are facing the Pacific. Resultantly, the slopes
along the coastline are becoming unstable. With thetectonic setting that the Pacific Plate is subducting
beneath the southeastern part of Hokkaido, huge2.

Observations

We selected a small scale (50 m length), coastal
landslide at Akkeshi, which is one of the coastal
towns in the southeastern part of Hokkaido, because itis easier to make a sensor network in order to grasp
whole behavior of the landslide blocks. The locationis shown in Figure 1. The Pacific Plate is subducting
beneath the North American Plate there, generating
M8 class inter-plate and intra plate earthquakes.
Photo 1 shows the satellite image of the landslide,
while Photo 2 its view from the bottom side. Thelandslide block is facing the sea (Akkeshi Bay) sothat the bottom side of the landslide is eroded by1

10th Asian Regional Conference of IAEG (2015)

ocean waves. The geology is completely weatheredmudstone rich alternation with sandstone. The rocks
near the ground surface are broken into almost soil
properties due to the phenomena of freezing andthawing. Therefore, instability of the slope is brought
to generate slope deformation as shown in Photo 3.
In this landslide, Matsuura et al. (2015) alreadyinstalled an extensometer across the scarp to monitor
the deformation of the slope. They also settled apotentiometer and a meteorological station in thelandslide block. In this study, we installed aseismometer on the slope in the landslide block (~20m distance from the coastline, denoted by yellow
diamonds in Photos 1 and 2), to estimate the responseof the slope to the seismic shaking. The natural
frequency of the seismometer is 2 Hz. The seismic
signals are continuously recorded every 0.01 seconds
with the resolution 24 bit. The observation started on
Sep. 24, 2014, and continues until now. Photo 4shows the seismometers we settled in the landslide.

Figure 1: Map showing the location of Akkeshi,
Shibecha-S (diamonds), and two epicenters (stars)
analyzed in this study.

Photo 1: Aerial photo (from Google Map) of thelandslide where we installed a seismometer.
Yellow diamonds denote the locations wherethe instruments are settled. A dotted line and anarrow show the landslide blocks and thedirection of the ocean waves, respectively.

Photo 3: Slope deformation in the landslide where wesettled a seismometer.

Photo 2: Landslide view from the bottom side. Yellow
diamonds denote the locations where theinstruments are settled.

Photo 4: A seismometer which is settled on the slope.

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10th Asian Regional Conference of IAEG (2015)3.

Seismic waveforms due to a large earthquake

spectrums show that the frequency components weredifferent. The seismic waveforms at landslide area
contained more of high frequency (> 7 Hz)
components.

In Figure 2, we drew the seismograms for the M
4.6 event which occurred in the Pacific slab 89 kmsoutheastward at depth of 48 km on Nov. 5, 2014.
For comparison, the seismograms at Akkeshi station
operated by Japan Meteorological Agency (JMA) andSouth-Shibecha (Shibecha-S) by National ResearchInstitute for Earth Science and Disaster Prevention(NIED) are shown together. The seismometers aresettled on the ground surface at Akkeshi (JMA) but
buried on the bedrock at Shibecha-S.
In order to understand the frequency
characteristics of the seismic waves, we visually
picked direct S-times and calculated the spectrums
for the waveforms from one second before to six
second after the S-times. The results are displayed in
the lower panel of Figure 2.
The peak ground velocity (PGV) at the landslidearea reached 0.11 cm/s. This value is 4-5 times larger
than that at Shibecha-S. It is suggested that the effectof amplification from the bedrock to the groundsurface is large, even considering the difference inepicentral distance at landslide area and at
Shibecha-S station.
On the other hand, PGV at landslide area wascomparable to those at Akkeshi (JMA). However, the4.

Strong motion and landslide displacements

We show extensometer data from Sep. 24, 2014
to Feb. 7, 2015 in Figure 3 to compare with thestrong motion which the landslide experiences. This
landslide become active in the winter season(Matsuura et al., 2015), and had displacement of
more than 8 mm from December, 2014 to January,
2015.
The landslide tended to move accompanied with
rainfall, however, not with earthquake shaking. In
fact, a M5.5 earthquake occurred on Jan. 9, 2015,
with the epicentral distance about 100 km. The PGVat a landslide area exceeded at least 0.33 cm/s, but
unfortunately, the waveform records were saturated
due to high-gain recording. The day when thisearthquake occurred is pointed by an arrow in Figure
3. No displacement was recorded accompanied withthis earthquake. We interpreted that landslides arestable under the shaking as large as 0.33 cm/s andthat much larger shaking would be necessary to makethe soil plastic to cause landslide moving.

Figure 2: Seismic waveform data and spectrums for S-wave parts. The left, central, and right panels show thewaveforms and spectrums for updown, north-south, and east-west components, respectively. The colors show
the stations: blue (landslide area), red (Akkeshi, JMA), and green (Shibecha-S, NIED), respectively. The
distances from the landslide to Akkeshi (JMA) and Shibecha-S were 3 km and 23 km, respectively.

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10th Asian Regional Conference of IAEG (2015)
146

144

Displacement (mm)

142

140

M5.5138

136

120

15/0
2/2
15/0
120

15/0
120/1

22/0

3/1
14/1
220

20
14/11

3
14/1
120

20

14/10/1/0

4/2
44/09
20
1/23

M4.6

134

Figure 3: Extensometer data with large earthquakes (arrows).

5.

Conclusions and future planspressure respond at that time?

We installed a seismometer in the coastal
landslide area in the southeastern part of Hokkaido.
One intra-slab event recorded by this seismograph
was analyzed to understand the amplification effect.
Comparison with the seismic waveform records at thepermanent stations around revealed that there was 4-5
times amplification in PGV due to the existence ofcompletely weathered mudstone, and that highfrequency components were more largely amplified
in the landslide area. So far, there was no landslide
displacement in accompany with earthquake shaking,though the slope experienced the strong motion aslarge as at least 0.33 cm/s.
In the future, we should confirm that theamplification effect is the same or not for events withdifferent back-azimuths. Moreover, we will measure
a pore pressure in order to elucidate the ground water
response to the seismic waves. Then, the answers to
the following questions should be given. How dolarge seismic waves make the soil plastic andgenerate landslide displacement? How does the pore

Acknowledgements
The authors are grateful to JMA for cataloging theearthquake list. Seismic waveform data obtained by
JMA and Hi-net (NIED) were used in this study. We
referred to aerial photo data from Google Map site.
We used General Mapping Tool (Wessel and Smith,1991) for drawing figures.
References
Matsuura, S., Shibasaki, T., Osawa, H., Sato, H., Doi,I. (2015): Displacement properties of a costal
landslide during a winter season, Proceedings of
10th Asian Regional Conference of IAEG, abstract
accepted.
Wessel, P., & Smith, W.H.F. (1991). Free software
helps map and display data, EOS Trans., 72, 441.
Yamaguchi, S., Sato, H., Matsuda, T., Suda, Y.
(1975): Geological Map, 1:200,000, Nemuro,
Geological Survey of Japan, 55.

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