Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, Japan

FAULT RUPTURE AND ASSOCIATED GEO-HAZARDSOF FUKUSHIMA-PREFECTURE HAMADOORIEARTHQUAKE
Zaheer Abbas KAZMI1, Kazuo KONAGAI2, Cigdem TETIK3, Jonathan BRAY4 andAshley STREIG5
1

Graduate Student, Department of Civil Engineering, University of Tokyo,
Tokyo, Japan, kazmi@iis.u-tokyo.ac.jp
2
Professor, Department of Civil Engineering, University of Tokyo,Tokyo, Japan, konagai@iis.u-tokyo.ac.jp
3
Geological Engineer, Prime Ministry Disaster and Emergency Management Presidency
Turkey, cigdemtetik@hotmail.com
4Professor, Department of Civil and Environmental Engineering, University of California,
Berkeley, USA, bray@ce.berkeley.edu
5Graduate Student, Department of Geological Sciences, University of Oregon,Oregon, USA, streig@uoregon.edu
ABSTRACT: On April 11th, 2011, Iwaki region of Fukushima prefecture was jolted by
Fukushima-Prefecture Hamadoori Earthquake. Surface ruptures were observed alongcausative Idosawa and Yunotake faults. In addition to numerous small slope failures, acoherent landslide and damage to Tabito Junior High School, bisected by Idosawa Fault,
were observed. A precise digital elevation model of the coherent landslide was obtained
through the LiDAR surveys. Repeated measurements of perimeters of Tabito Junior HighSchool have shown that ground undergoes a slow and steady/continual deformation.
Key Words: Hamadoori Earthquake, Idosawa and Yunotake Faults, landslide,gymnasium building, swimming pool, ground deformation

INTRODUCTIONOn March 11, 2011, the 2011 Off the Pacific Coast of Tōhoku Earthquake, also known as the Great
East-Japan Earthquake, of magnitude 9.0 jolted the east coast of Japan. The earthquake triggered
extremely destructive tsunami waves of up to 38.9 meters (128 ft), in some cases traveling up to 10 km(6 miles) inland. According to the United States Geological Survey (USGS), Japan had experienced
over 900 aftershocks since the March 11th earthquake with about 60 being over 6.0 M and three over
7.0 M.
One month after the Great East Japan Earthquake, the Iwaki region of Fukushima Prefecture wasjolted by a series of intense earthquakes. The largest one of them was the Fukushima-Prefecture
Hamadoori Earthquake (Hamadoori Earthquake hereafter), a strongest inland event after the 2011 Off
the Pacific Coast of Tōhoku Earthquake, of Mj7.0 (Mw6.6), that occurred at 17:16 JST (08:16 UTC)289

on Monday, April 11th, 2011 (JMA, 2011). In the first 24 hours, the Hamadoori Earthquake wasfollowed by eight aftershocks of magnitude greater than 4.5, including three with magnitude greaterthan 5.5 and hypocenters shallower than or equal to 20 km (Table 1).
Table 1 According to NIED records; main shock of the Hamadoori Earthquake and following
aftershocks above M4.5 in the first 24 hours (11-12 April 2011)
Date
April 11thApril 12thTime (JST)
17:56
20:42
22:05
00:43
00:57
02:21
05:03
07:03
14:07

Magnitude
7.0
5.9
4.7
4.5
5.0
4.5
4.5
4.5
6.4

Depth (km)
6.4
10.5
11.2
10.8
10.5
20.0
10.0
20.0
15.0

The focal mechanism of this earthquake was a normal-fault type with the tensional axis
oriented in west-southwest to east-northeast direction and was a shallow crustal earthquake.
This area had been seismically inactive until the Great East Japan Earthquake, but frequent
seismic events of the normal fault type have been reported since then. In the Hamadoori
Earthquake, surface fault ruptures appeared along the recognized traces of Idosawa andYunotake Faults (Figure 1) over a distance of at least 11 km in southeastern Fukushima
Prefecture (Ishiyama et al., 2011). Though major hazards were found mostly along Yunotake
Fault, Idosawa Fault was considered to be the most responsible for the April 11th Hamadoori
Earthquake. Actually fault dislocations were in general larger along Idosawa Fault (most ofthe observation ranging from 0.8 to 1.5 meters) than those along Yunotake Fault (Figure 2).
Where present in shallow bedrock, the fault rupture was relatively distinct and linear;
however, where present in unconsolidated alluvium the rupture is characterized by a fold
scarp and hanging-wall cracking.
A big earthquake can cause large strains as well as intense shakes to be localized near the activatedseismic fault, where large scale mass movements such as landslides, rock falls and debris flows would
occur. Justifying the conclusions derived from the past studies, Hamadoori Earthquake wasresponsible for some landslides, rock falls and other geo-hazards, damaging roads, buildings anddrainage structures. One of the major concerns was that the disturbed soil masses would move again in
the upcoming rainy seasons and/or in possible aftershocks that might occur in rapid succession.
Understanding this threat, the authors have been frequently surveying the areas along Idosawa andYunotake Faults since the occurrence of Hamadoori Earthquake (May 18th, June 7th, June 23rd andAugust 20th). Other than slope failures and failures in the areas difficult to access, a coherent landslideand building structures of Tabito Junior High School, bisected by Idosawa Fault, were found along thecausative faults (Figure 1). Much of the time of authors’ survey was devoted to gather every small signof continual mass movement and to attain the current configuration of the landform that might be used
as reference in future to evaluate soil mass movements that might occur in frequent aftershocks and/orheavy rains. The above mentioned two geo-hazards along with other observations along both the faults
are discussed in detail in the following sections. It is also noted that there was a conjugate fault of
Yunotake Fault appeared near the coherent landslide (Figure 1). Although no clear trace was foundclose to coherent landslide, the conjugate fault might have skimmed this area and could have been one
possible cause for this landslide.

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5 kmN
Yunotake Fault
Landslide

Idosawa Fault

Tabito JHS

Epicenter ofHamadoori Eq.
Conjugate FaultFig.1 The area struck by Hamadoori Earthquake andthe resulting geo-hazards. The solid blue lines show
the causative Idosawa and Yunotake Faults

Fig. 2 Fault dislocations along Idosawa
Fault. The road meandering around themountain was bisected at a number oflocations (36o59’41.5”, 140o41’12.8”(upper photo) 36o58’44.4”, 140o41’42.4”(lower photo)).

LANDSLIDE
The landslide, mentioned above, was one of the most catastrophic failures caused by this earthquake.
The soil mass moved coherently down the slope and blocked local Highway No. 14, which connects
the area with a major freeway, the Joban Expressway. The landslide was triggered at the contact
between highly weathered tuff and mudstone. Additionally, tall and steep road cuts were present
adjacent to the landslide in (Figure 3, 4 and 5), which suggest that the road construction resulted insignificant removal of lateral support. One person, whose vehicle happened to pass by, was found deadin the landslide mass.
Knowing the importance of that road, a new/temporary road was planned to be constructed overthe landslide mass. It was, therefore, important to monitor the gradual movement of the landslide mass,given the current landform configuration as reference. Large scale tension cracks, indicating thepresence of deep seated slip surfaces, were observed along the top of the landslide mass whichindicated a possibility that the landslide mass would move, especially during the rainy season. For this,the authors conducted a LiDAR survey on May 18th to obtain a precise digital elevation model (DEMhereafter). The obtained DEM (Figure 6) covered only an upper middle to toe parts of the landslide
mass because we had a narrow view of the landslide mass through thick vegetation covering it. Lateron it turned out that one more image from an air-born LiDAR was obtained by Aero Asahi Co. for theupper half of the landslide mass, and the image was combined with what obtained from our fieldsurvey (Figure 7).

291

Fig. 3 Contour map of the area before thelandslide was triggered. (Figure provided by
Fukushima Prefecture).

Fig. 4 An aerial view of the landslide mass (Photoby Kyodo News). The coherent landslide mass
flushed the road passing by its toe.

Fig.5 A closed view of the landslide mass
(37o00’00.3”, 140o47’14.5”)

Fig.6 Landform of the lower half of the landslide
mass obtained by ground LiDAR survey

AB

Section A ~ A’
B’A’

Section B ~ B’
Fig. 7 Precise Digital Elevation Model (DEM) of the whole landslide area obtained by combining theground and air-born LiDAR survey data292

After the authors’ LiDAR survey, the landscape was dramatically changed for the construction oftemporary road (Figure 8). Earth cut and fill operations were done in such a way that the road would
be constructed directly on the landslide mass without deeply cutting it, thus the greater toe part of thelandslide mass would remain as it is for better stabilization of the soil mass. However, a careful andcontinuous eye is to be kept on it because of the disturbed landslide mass as well as the fill for thetemporary road is not completely consolidated (Figure 9) and there surely exists a deep-seated slip
surface, as can be seen from the presence of large tensional cracks along the top of landslide mass
(Figure 10).

Fig. 8 Panoramic view of the landslide mass, re-profiled to an apparent stable configuration for thepurpose of road construction.

Fig. 9 Embankment of the road constructed on the landslide mass (left photo). Small scale cracks wereobserved near the base of embankment even during construction (right photo).

Fig. 10 Huge tension cracks present along the top of the landslide, showing the presence of deep
seated slip planes.

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A natural slope immediately right to the current landslide reportedly started to move slowlyimmediately after the road was constructed across this area. Therefore the slope was intensively tiedback with pre-stressed anchors. During the above mentioned earthquake, this slope was also disturbed
significantly, being dragged by the adjacent major landslide mass. During the last survey on June 23,
the positions of ground anchors were also measured to monitor gradual slippage of this slope (Figure11).

4 652915
1211837141317

1916

18

101Fig. 11 A disturbed natural slope adjacent to the coherent landslide mass. The numbers for the groundanchors were marked for the reference to be used in future.

TABITO JUNIOR HIGH SCHOOL
On the southern extension of Idosawa Fault, there is Tabito Junior High School whose gymnasiumbuilding and swimming pool, immediately next to each other, were located exactly above Idosawa
Fault. As shown in Figure 12, the structures were deformed and tilted, about 3o westward, tounserviceable condition and large scale open cracks were observed along their peripheries. Though thedamage to these structures had nothing to do with major landslides along Idosawa Fault, temporalmonitoring of their deformed shape can give an idea of a possible continuous deformation buildup insoils near the fault.

Fig. 12 Idosawa Fault passing under the gymnasium building (left photo) and swimming pool (rightphoto) of Tabito Junior High School, tilting and deforming the structures (36o57’52.2”, 140o42’10.0”)
Perimeters of both the structures were laser scanned on June 7th, 24th and August 20th to observe
the continual ground deformation. Swimming pool was small and visible enough to be scanned from asingle scanner position while for the gymnasium building, four scanner positions were required to scan
the complete periphery. Although a single reference point (top of an antenna) was also measured fromall the positions, the resolution of the tilt-meter was not so accurate in shooting the reference point at294

large dip angles to figure out the deformations all along the periphery of the gymnasium building.
Therefore, the front and rear faces (AB and CD in figure 13, respectively) of the gymnasium building,perpendicular to the fault line, are considered only.
Assuming that the hanging wall has not moved, elevations of all other points are determinedsetting the hanging wall elevation as a reference. On June 7th, only front face of the gymnasiumbuilding was measured. While comparing it with that measured on June 24th, it is found that thefootwall side has sunk down by about 3~4cm (Figure 13a).
A107310

June 2011
August 2011

BC107280

CD-5
77360

77365

Ea77370

A0.0a
June 7, 2011
June 24, 2011
August 19, 2011

st5-W-510

es-1 015tA

June 7, 2011
June 24, 2011
August 20, 2011

-0.2

Elevation (m)Elevation (m)

-0.2

0.0

00-0.4-0.6

ou77355

East - West (m)a105-S77350

Bth77345

Aor77340

DN

1072700 .2
0 .0- 0 .2
-0 .4
-0 .6
-0 .8
-1 .0
-1 5
-1 0thon107290ElevatiNorth - South (m)107300-0.4-0.6-0.8-0.8

B-1.0-1.0-202

468

101214

1618

B-1.2

20

05

Distance (m)b0.10.0bC0.0

June 24, 2011
August 20, 2011

-0.1

15C20

25

June 7, 2011June 24, 2011
August 20, 2011

-0.2

-0.2

Elevation (m)Elevation (m)10Distance (m)

-0.3
-0.4
-0.5
-0.6

-0.4

-0.6

-0.8

-0.7

D-0.8
-20246810

1214

16180

Distance (m)Fig. 13 Continual deformation of the groundsupporting gymnasium building of Tabito JuniorHigh School from June 2011 to August 2011 a)face AB b) face CD

D-1.0
510

152025Distance (m)Fig. 14 Continual deformation of the groundsupporting swimming pool of Tabito Junior HighSchool from June 2011 to August 2011 a) faceAB b) face CD295

Similarly, comparing the measurements of June 24th and August 20th, the front face does not show
any change but the rear face shows that foot wall area has deformed by 2~3cm (Figure 13a and b).
This suggests that ground had been undergoing a continuous/continual deformation; as one of teachersat this junior high school noticed that some cracks were getting wider slowly but steadily.
The swimming pool was deformed considerably by the fault dislocation (Figure 14). Following
the same way as for the gymnasium building, change in elevation of the faces perpendicular to thefault line (face AB and CD, marked in Figure 14) was determined. It is quite clear from Figure 14 thatthe footwall side has sunk down by almost the same amount as it was observed for gymnasiumbuilding.

CONCLUSIONS
A big earthquake can cause large strains as well as intense shakes to be localized near the activatedseismic fault, where large scale mass movements such as landslides, rock falls and debris flows wouldoccur. Justifying the conclusions derived from the past studies, Hamadoori Earthquake wasresponsible for some landslides, rock falls and other geo-hazards, damaging roads, buildings anddrainage structures.
Once a soil mass gets disturbed, it is very likely that it moves again with a slight increase ofdriving force, which may come from frequent aftershocks, changes in pore water pressure and water
infiltration during the rainy seasons.
Though there is an immense need to rehabilitate/reconstruct structures and other facilities
damaged by the slope failures, it’s quite important to ensure that the soil masses are stable enough to
support these facilities. Otherwise we shall have to suffer from repeated failures and economical loss.
A precise DEM of the landslide mass discussed in this article can be of great importance to monitor its
slow and steady deformation. By measuring perimeters of the gymnasium building and swimming
pool of Tabito Junior High School, it was found that the ground still undergoes a slow and steadydeformation.

ACKNOWLEDGEMENT
This paper summarizes some of the outcomes of the Research Project “Detection of Lagrangian
displacements of terrains along an active fault and their implementation for land conservation”, No23246087, Ministry of Education, Culture, Sports, Science and Technology. Major part of this paper istaken from paper number 6-036, presented by the authors in the 31st symposium on Earthquake
Engineering, JSCE, 2011.
REFERENCES
United States Geological Survey (USGS), 2011.
http://www.usgs.gov/
Japan Meteorological Agency, 2011, A strong aftershock of ‘The 2011 off the Pacific coast of Tohoku
Earthquake’ occurred Monday 11 April
http://www.jma.go.jp/jma/en/News/20110411_aftershock.html
Ishiyama, T., et al., 2011. “The surface earthquake fault of the 11th April 2011 earthquake in Hamadori
Fukushima Prefecture,” Outreach information from the University of Tokyo, April 20;
http://outreach.eri.u-tokyo.ac.jp/eqvolc/201103_tohoku/eng/fukushimahamadoori2/

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