（３）斎藤大樹（独立行政法人 建築研究所）「LESSONS OF RECENT GIGANTIC EARTHQUAKEDISASTERS IN JAPAN」
LESSONS OF RECENT GIGANTIC EARTHQUAKE DISASTERS IN JAPANTaiki SAITO1
1Chief Research Engineer, International Institute of Seismology and Earthquake Engineering,
Building Research Institute, Tsukuba Japan, tsaito@kenken.go.jp

1. Introduction
In the last 100 years, there are three gigantic earthquake disasters in Japan which caused
tremendous loss of human life: the 1923 Great Kanto Earthquake Disaster (hereinafterreferred to as 1923 Kanto Earthquake), the 1995 Great Hanshin-Awaji Earthquake Disaster(hereinafter referred to as 1995 Kobe Earthquake) and the 2011 Great East Japan
Earthquake Disaster (hereinafter referred to as 2011 Tohoku Earthquake). Figures 1 showsthe epicenters and Table 1 summarizes the disasters.

1923 Kanto Earthquake

1995 Kobe Earthquake2011 Tohoku Earthquake

Figure 1 Epicenters of earthquakes (figures are cited from Wikipedia)
Table 1 Three gigantic earthquake disasters in Japan after 1900

DateTime
Magnitude
Death & missing
Maincauseofdeath
Majorbuilding
damage

1923
GreatKanto
Earthquake
1923.09.01
11:58
7.9
Around 105,000
Fire 85%-Wooden houses
Brickbuildings
adopting westernstyle

1995 Great Hanshin
Awaji Earthquake
1995.01.17
05:46
7.2
6,434
Building Collapse 75%
Fire 12% (*1)
- Old type buildings
designedbefore
1981- RC buildings withSoft first story*1 from statistics of Kobe city
*2 from Japanese National Police Agency (as of 22 Dec. 2011)

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2011 Great East Japan
Earthquake
2011.3.11
14:46
9.0
19,312 (*2)
Tsunami 92% (*2)
-Minor damage forshaking
Tsunami damage

The cause of loss is different in each disaster. Roughly, it can be categorized as “Fire (1923
Kanto Earthquake)”, “Shaking (1995 Kobe Earthquake)” and “Tsunami (2011 TohokuEarthquake)”. Even Japan has learnt severe lessons from earthquake disasters and has
improved knowledge and technology for countermeasures, the 2011 Tohoku Earthquake
reveals the fact that our effort is not still enough. This paper summarizes lessons of eachdisaster and efforts which have been taken, especially focusing on the structural design ofbuildings.
2. Earthquake intensity
Figure 2 shows the sizes of earthquake fault including the 1960 Chile Earthquake which isknown as the largest earthquake in the world.

Figure 2 Size of earthquake fault
The 2011 Tohoku Earthquake is known at the largest earthquake in the history of Japan
from the size of magnitude. However, comparing other two earthquakes, the epicenter wasrelatively far from the land. In general, intensity of earthquake ground shaking is getting104

reduced as the distance from the epicenter increases (as shown in Figure 3). This could be thereason that building damage due to the 2011 Tohoku Earthquake was relatively minor.
However, the maximum accelerations of earthquake ground motions recorded at the 2011
Tohoku Earthquake are quite large as shown in Figure 4. Question still remains why thebuilding damage was minor even earthquake ground motion has such a high acceleration
value.

Figure 3 Intensity of earthquake

Figure 4 List of large earthquake ground motions at the 2011 Tohoku Earthquake
Figure 5 shows the comparison of pseudo velocity spectrums between the 1995 Kobe
Earthquake and the 2011 Tohoku Earthquake. The 1995 Kobe Earthquake occurred justbeneath the city of Kobe and produced intensive shaking with high frequency around 1 to 2
Hz as shown in the spectrum. On the other hand, it is seen that the power of the 2011 Tohoku
Earthquake in this frequency range is relatively small. Generally wooden houses and low rise
reinforced concrete buildings are affected by the shaking with the frequency component
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around 1 to 2 Hz; this could be the explanation of minor building damage at the 2011 Tohoku
Earthquake.

Figure 5 Pseudo velocity spectrums with 5% damping
3. Seismic capacity of buildings
3-1. 1923 Kanto Earthquake --- from brick to reinforced concrete
There was no seismic design code in Japan before the 1923 Kanto Earthquake. As that time,Japanese Government decided to introduce Western culture and technology to catch up
Western countries. Fire resistance was the main concern in urban areas since the mostbuildings were constructed by wood. Therefore, Government recommended buildings made ofbrick instead of wood.
At the 1923 Kanto Earthquake, brick buildings were severely damaged. The Ginza Brick
Street, the symbol of Western style town, destroyed completely. Ryou-unkaku, the highest
brick tower in Japan with 12 stories, also the landmark of Asakusa district, has collapsed. Onthe other hand, reinforced concrete buildings showed excellent performance both against fireand earthquake shaking. Reinforced concrete has jumped to the leading role in building thecity of Japan. The first seismic design code was enacted next year of the 1923 Kanto
Earthquake.
There are unique structures became popular after the 1923 Kanto Earthquake. One is
the steel encased concrete structures (so called SRC) and another one is the reinforced
concrete shear walls.
3-2. 1995 Kobe Earthquake --- start promotion of seismic retrofit
Japanese seismic design code has been revised many times based on the lessons ofearthquake disasters. The 1968 Tokachi Earthquake caused severe damage to reinforced
106concrete buildings. The Architectural Institute of Japan (AIJ) revised design guideline ofreinforced concrete buildings in 1971, requiring more stirrups to increase shear resistanceand ductility of buildings.
The biggest revision of seismic design code was made in 1981 after the 1978 Off Miyagi
Earthquake. The code adopted two level design procedures as shown in Figure 6; one is theserviceability limit design for moderate earthquakes with a base shear capacity of 20 percentof total weight of a building, another one is the safety limit design for strong earthquake witha base shear capacity of 100 percent of total weight of a building.
C0 = 0.2 moderate earthquake
1st Level
Seismic force

Elastic analysis

Building must be elastic based onallowable stress design.

Wight W

F = C0 W

Shear resistance
C0 = 1.0 strong earthquake

C0 W

2nd Level

Nonlinear analysis

Building capacity must be checked
to prevent collapse.

Figure 6 Concept of the two level design procedure at 1981 revision
The effect of this revision was proved at the 1995 Kobe Earthquake. As shown in Figure 7,
the ratio of collapse buildings designed after 1981 was very small. The minor revision wasmade after the earthquake to prevent collapse of soft first story buildings. After theearthquake, Japanese Government issued a law to promote seismic retrofit of existingbuildings design before 1981, providing subsidy and other benefits.

Collapse of buildings
After 1981
3%

76%

21%

1971～1981

Before 1971

Figure 7 Building damage at the 1995 Kobe Earthquake
3-3. 2011 Tohoku Earthquake --- need to consider Tsunami force in building design
Tsunami induced by the 2011 Tohoku Earthquake struck the Pacific coast of eastern Japan
including prefectures of Iwate, Miyagi, Fukushima, Ibaraki and Chiba. In some areas,tsunami height reached over 10 m and washed away the houses and buildings in the areas.
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The earthquake shaking was also strong in wide area of Japan; however, the damage ofbuildings due to shaking was limited. Extensive liquefaction of sandy soil occurred in Kantoarea.
In tsunami disaster site, overturning was observed in 4-story or lower reinforced concretebuildings with relatively small size of openings as shown in Figure 8. In all overturnedbuildings, the maximum inundation depth exceeded their height. Most of the overturnedbuildings are of mat foundation. In some overturned buildings on pile foundation, piles werepulled out from the ground.

Figure 8 Overturning of a reinforced concrete warehouse in Onagawa city
Consideration of the tsunami effect on buildings is not mandatory for the structuraldesign and there is no regulation to define the tsunami force in the Building Standard Law inJapan. However, the Cabinet office, government of Japan released official guideline on thestructural design of buildings for vertical evacuation from tsunamis in 2005. This guideline
provides the simplified method to calculate tsunami wave pressure affecting a building using
design water depth (see Figure 9). The guideline was revised in 2011 to change the run-up
height of tsunami using the parameter α which ranges from 1.5 to 3.0 depending on thedistance from the coast and the existence of shielding fence of tsunami.

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building2005 guidelinezDesign wave pressureqzq z = ρ g (3h − z )3hDesign water depth: h3ρgh2011 guidelineq z = ρ g (αh − z )α ρgh(α =1.5 – 3.0)

Figure 9 Guideline on the structural design of tsunami evacuation buildings
4. Conclusions
Tsunami has attacked Tohoku regions repeatedly. However, people forgot such lessons andstarted living again in dangerous areas near the ocean. A gigantic earthquake like the 2011
Tohoku Earthquake is supposed to occur every 1,000 years. The 1995 Kobe Earthquake is also
expected to occur every 2,000 years. The return period of the gigantic earthquake is too large
for human to keep awareness of disaster prevention. Therefore, it is important to change
regulations or make the new ones reflecting the lessons as soon as possible. Also, sharing such
experience with other countries is important.
Acknowledgement
The author would like to express his deepest condolences to those who lost their families andthose who are suffering from the disaster. Alos, he would like to express his heartfelt
appreciation to people from around the world for their warm support.

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