Chapter 5
The Science of Landslides and Slope Hazards
Field of expertise: Geotechnical Engineering

Shuji Moriguchi
Summary
The Great East Japan Earthquake caused a number of ground disasters. In coastal areas, tsunami
damage and liquefaction attracted attention, and in inland areas, landslides and damage to hilly
residential areas occurred. Such ground and slope hazards occur not only in earthquakes but also
in heavy rainfall disasters, and it is important to improve laws and technologies to know the risks andpredict them in advance.
Keywords: geotechnical disaster, residential land development, numerical analysis

Introduction
The Great East Japan Earthquake caused a number of ground and slope disasters.
Deformation of the ground or slope has a significant impact on the surrounding buildings andinfrastructure, and directly leads to human suffering. Although it is impossible to prevent thesedisasters completely, it is possible to mitigate and control the damage through both structural andnon-structural measures based on prior risk assessment, and it is important to conduct research toachieve this efficiently.

1: Problems Revealed by the Great East Japan Earthquake
What happened?
In the Great East Japan Earthquake, seismic motions propagated over a wide area, causing
many ground and slope disasters. The details of these damages are summarized in the report of thejoint survey team of several academic societies (Geotechnical Edition) (Japanese GeotechnicalSociety, 2014). Particularly in the coastal areas of the Tohoku region, tsunamis overcame ordestroyed seawalls and river levees, causing the buildings and infrastructures that were protectedby them to be swept away, resulting in a large number of human casualties. In addition, these coastal
areas were pulled into the Pacific Ocean, causing land subsidence. This land subsidence caused
problems such as a decrease in the height of levees, and the seawater from the tsunami notreceding, which hindered reconstruction and led to secondary damage such as salt damage. Largescale liquefaction occurred along the coast of Tokyo Bay and around the Tone River basin in theKanto region, and the damage to reclaimed land was particularly severe. There are photos and1

videos that still exist today showing liquefaction in the coastal areas of the Tohoku region, but thetraces of this liquefaction were washed away by the tsunami, and there are many aspects about thisregion’s liquefaction that are still unclear.
The direct damage to buildings and infrastructure caused by the tsunami and liquefaction
were the focus of attention in the coastal areas, but a lot of damage also occurred in the inland areas.
Slope disasters occurred over a wide area, and damage to residential areas in hilly areas (Moriguchi,2011; Sato, et al., 2015) was also significant.
As described above, different types of ground and slope disasters occurred in the coastal
and inland areas, but since the damage in the coastal areas has been covered in many otherchapters, I will focus on the damage to the inland areas.
The damage
According to a survey by the Ministry of Land, Infrastructure, Transport, and Tourism (MLIT),
141 slope disasters, including mudslides, landslides, and steep slope landslides (cliff collapses),
have
been
confirmed
tohaveoccurred
witha
totalof13
fatalities(http://www.sabo.or.jp/saigai/110311touhoku-earthquake.htm (In Japanese)). However, this does
not include small-scale collapses, and since traces of such collapses have been washed away andcannot be confirmed in coastal areas, we can assume that more damage would have been recorded
if these types of collapses were included.
As for developed residential areas, damage occurred mostly in hilly areas built during Japan'speriod of rapid economic growth from the 1950s to the 1970s. In order to create flat land, land wascarved out from the hills that used to exist there, and filled land is mixed in to create a hilly residential
area. Sato et al. analyzed the developed residential areas in Sendai City, Miyagi Prefecture, thatwere damaged by the 2011 Great East Japan Earthquake, and reported that the incidence of grounddeformation and damage to wooden buildings was more than twice as high at embankments andcut-and-fill boundaries (the boundary between embankments and cut-and-fill land) than at cut-andfill areas. It is also reported that there is a correlation between the age of land development and thedamage rate, which is strongly related to the changes in laws and design standards for residentialareas.

2: Paradigms Destroyed by the Earthquake
In the aftermath of the Great East Japan Earthquake, the importance of prior information andforecasting of disaster risks has increased. These were not necessarily newly recognized in the wakeof the earthquake, but the earthquake raised the shared awareness of their importance andaccelerated related research.
In terms of prior information, tsunami hazard maps were of course closely watched, but thisalso marked a turning point in the recognition of the importance of landslide-related disasters. Inaddition, before the earthquake, there was little information disseminated on the details of the landin developed residential areas, and residents did not understand the necessity of knowing thisinformation. Since the earthquake, there has been an increase in realizing the importance of knowing
this information in advance.
In order to forecast disasters, it has become necessary to include and target complex
phenomena and the scale of large areas. For example, in the case of landslide disasters, there is anincreasing need for risk assessment at the practical level, not only to evaluate whether or not a slope
is stable against external forces, but also to determine how far the sediment will reach in the eventof a collapse and how much impact force it will have when the debris hits a building. We not only2

need to calculate the stability of a single slope, but also evaluate wide areas that include manyslopes.

3: A New Approach
First of all, in the maintenance of prior information, I’d like to introduce the map of residential
land development history in Sendai City. After the earthquake, the city of Sendai published a map of
the distribution of cut-and-fill soil and the date of residential land construction(http://www.city.sendai.jp/kaihatsuchose-chose/kurashi/anzen/saigaitaisaku/kanren/joho/index.html(in Japanese)). This map was developed in response to the many cases of damage to developed
areas in Sendai City. Anyone can access this information through the Internet and can easily findout where their house is located on the boundary of fill or cut-and-fill, and when the land was built.
Figure 5-1 is a cut-and-fill map of a part of Sendai City, where orange represents the fill area andblue represents the cut area. Collapses of these residential areas can occur not only due toearthquakes but also due to heavy rain. In fact, a large-scale collapse occurred in Shogun, Izumi
Ward, Sendai City, when Typhoon No. 19 hit eastern Japan in 2019. This map shows that thecollapse was caused by an embankment constructed in the 1960s, a form of embankment in whichsoil is attached to a slope.

Figure 5-1. An example of the Sendai City residential land development history map
Next, as a new approach to forecasting, I’d like to introduce a simulation method for landslide
risk assessment in a wide area. This simulation method, based on mechanics, was developed atTohoku University and was refined by our research group to enable the evaluation of a wide area.
Figure 5-2 shows the map of the simulation results as well as the actual slope disaster distributionof the 2018 Hokkaido Eastern Iburi Earthquake. The actual collapse distribution map is published on
the web by the Geographical Survey Institute of Japan, and the simulation results show the thicknessof the collapse as a contour diagram. The simulation results are generally consistent with thedistribution of slope disaster occurrence. The simulation results are generally consistent with thedistribution of slope hazards. Although the simulation is still in the verification stage, it has highpotential for future slope disaster prevention because it can take into account the effects of rainfall
as well as earthquakes.

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Figure 5-2. Hokkaido Eastern Iburi Earthquake landslide area

4. Achievements and the Future
As mentioned above, new attempts have been made to know and assess the risk of groundand landslide disasters in advance. However, this information and technology has not yet been usedeffectively in practical disaster management. In particular, there is a lack of information on the internal
structure of the ground and the properties of geomaterials as input conditions for simulation. This
needs to be improved, including through better preparation of information.

Conclusion - From the author
In addition to the information development and forecasting technologies for ground disasters
and landslides described above, various other information and technologies have been developed
and are evolving everyday. However, in order to protect human lives from ground disasters andlandslides, it is essential for individuals to become more aware of disaster prevention as well. It is
my sincere hope that a world in which people can coexist with disasters will be created through theaccurate understanding of disaster risks in advance and the development of individual disasterprevention capabilities.

References
Japanese Geotechnical Society. (2014). The Great East Japan Earthquake Joint Survey Report(Geotechnical Edition). (In Japanese).
Moriguchi, S. (2011). Vulnerability to earthquakes of areas created by recent land development.
Association of Educational Facilities, Quarterly Journal of Educational Facilities, 42, 19-22.
Sato, S., Kazama, M., Ohno, S., Mori, T., Minami, Y., & Yamaguchi, S. (2015). Damage Analysis of
Built-up Urban Areas in Sendai City Hills during the 2011 Tohoku off-the-Pacific coast Earthquake.
Proceedings of the Japan Association for Earthquake Engineering, 15(2), 97-126.
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