Landslide disasters induced by the 2011 off the Pacific Coast of Tohoku Earthquake
Daisuke HIGAKI, Tatsuki TSUCHIYA and Misa TUSHIMA(Faculty of Agriculture and Life Science, Hirosaki Uiversity, Japan)
1. Introduction
The largest scale earthquake with the epicenter approximately 130 km southeast of theOshika Peninsula of Miyagi Prefecture ,the fault rapture length about 500 km and amagnitude 9.0 (Mw) caused extreme catastrophic disasters in East Japan in March
2011. The damages by a wide-spread seismic vibration with over 6 of the Japaneseseismic scale and a Tsunami from Hokkaido to Chiba Prefecture caused about 26,000
people killed and missing as of 11 May 2011. Long-term stop of lifelines have had greatinterruption to searching missing persons, rehabilitation and reconstruction activities
and daily life of evacuating people.
The difficulty in disaster management of this earthquake is that various remedial
works must be operated at the same time as planning reconstruction of the devastation
areas along the Pacific coast suffered from Tsunami.
At the first stage, there were several information of landslide disasters in contrast tolarge earthquake. Gradually, we have found that the damages are widely distributed
over the whole East Japan, and there are several areas of concentrated occurrences insouthern part of Fukushima Prefecture, hilly residential areas of Miyagi and
Fukushima Prefectue , island areas of Matsushima coast.
We briefly introduce the situation of landslide disasters in Northeast Japan.
2. Distribution of landslide disasters
We give an outline of distribution of landslide disasters in Tōhoku region by theinvestigation team of the Tōhoku branch of the Japan Landslide Society. Some of them canbe referred on the Japan Landslide Society’s Home Page, the information site of the Great
East Japan Earthquake (Fig 1). The distribution of landslides we have already known untillate May are as given below.
1) From reading aerial photography taken by Geographical Survey Institute of Japan (takenon March 2011) from southern part of Aomori to Fukushima coast (Abe,・Higaki, 2011;Sato・Higaki,2011), slope failures over 20m length and width seems to be little.
Especially in the mountain areas of Sanriku coast
though the extensive damages by Tsunami.

many landslides have not occurred

2) In the residential areas which have been developed on the hills composed of the Pliocene
to Pleistocene sediments, slopes of earth-fill have been deformed by many cracks, steps,slides, and thus many houses were destroyed in Sendai city with 788 square kilometers inarea (Fig 2). Many of them occurred by the main shock on 11th March. After that, many
after-shocks have made further deformation. Such slope deformation also appeared insouthern part of Miyagi and Fukushima Prefecture, so large number of residents have beenevacuating.
3) On Matsushima coast area of Miyagi facing Pacific Ocean of the Neogene volcanic
deposits with mudstone/sandstone, many slope collapses occurred in the islands (Fig 3).
From interview and fieldworks, many of them occurred during the main shock and soon
Tsunami attacked the residential area.
4) In the southern part of Fukushima Prefecture that had affected by both strong main
shock of 6 in Japanese seismic scale andmany after-shocks, many landslides occurred near

Shirakawa and Iwaki city. Some landslides occurred in relation to Pleistocene volcanic ash
and welded tuff layer in hilly area in the eastern areas of Oou Mountains.
5) On the south eastern areas on Iwaki city happened magnitude 7.1 inland earthquake on
11th April , land surface deformation appeared along the Idosawa fault (Shionohira fault)
and Yunodake fault during this earthquake. Many rock slides and deep-seated landslides
occurred in the surroundings of faults to coast areas of the Pacific Ocean (Fig 4).
In this area, landslide disasters happened on the slopes consisting of the Neogene andPaleogene strata and the crystalline schist area. Since them some of slopes have been still
unstable due to after shocks and rainfall, so it is urgent to distinct movement rate and riskof them.

3. Conclusion

Many different types of mass movement (landslides) have been caused by the 2011 off thePacific Coast of Tohoku Earthquake and the after-shocks of it in wide areas in theEastern Japan. Concentrated distribution of them is 1) residential areas of earth-fill in
the hilly terrain of Sendai city, 2)the slopes of volcanic deposits in the inland area ofsouthern Fukushima, 3)Hilly coastal areas of Neogene soft rocks around Matsushima
Bay, 4) Surrounding areas of the displaced faults during the large after-shock in Iwaki
city of sandstone/madstone and schist.
Further investigation is urgently needed to distinct other risky landslides by theearthquake.

Miyagi Pref.

Matsushima

maSendai

Fukushima Pref.
Iwaki

Fig.1 Location of the epicenter(Base image: Landsat)

Fig.2 Houses destroyed by alandslide in Sendai

Fig.3ASabusawa

slope

failueinIsland(Tsunami

from left side also destroyed thehouses

Fig.4

A

deep-seatedslide

blocked a highway in Iwaki