Makoto Watabe Professor of Environment and Information, Keio University, 'L-25-5 Tokiwadai Itabashi-Ku, Tokyo, Japan

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1 Kobe Earthquake Makoto Watabe Professor of Environment and Information, Keio University, 'L-25-5 Tokiwadai Itabashi-Ku, Tokyo, Japan Abstract This paper is about the Kobe earthquake in Japan which happened early in the morning on January 17,1995. First, the total damage due to the earthquake is described, followed by an account of the damage done to reinforced concrete (R.C.), steel and wooden structures and other damage caused by the ground motions following the main part of the Kobe earthquake. 1. Introduction In the early hours (05.45) on 17 January 1995, the biggest earthquake this century in western Japan happened. Japan is famous for earthquake ground motion damage, occurring, on average, every 2 years, mainly in the northern or eastern part of Japan like Hokkaido. Usually less than 100 people are killed. However during the Kobe earthquake, about 6400 people were killed. 8.5% of these died after the earthquake, and more than 300 people committed suicide. Total (direct and indirect) damage caused by the Kobe earthquake, was about 12 Trillion Yens (within an area of 120km) with a bill of 10,000 Japanese yen. 2.Why earthquakes happen As you may know, the radius of the earth is about 6300 km, and less than 1% of it, 60 to 30 km, is our ground. The second section, the mantle, moves according to changes in temperatures, along with the ground above. This causes a fault and subsequently an earthquake. Around Japan, the Pacific and Philippine plates (ground), move 3cm to the north each year and the Soviet plate moves less than 1 cm to the south each year, so on average, every 2 years, in Japan, there is earthquake damage.

780 Earthquake Resistant Engineering Structures The Izu Peninsula, near Tokyo, was, over 200 million years ago, one of the islands of the Philippines, but it recently touched the main Japanese island.

2 780 Earthquake Resistant Engineering Structures The Izu Peninsula, near Tokyo, was, over 200 million years ago, one of the islands of the Philippines, but it recently touched the main Japanese island. There are two types of earthquake. One is the nearfieldearthquake and the second, most common in Japan, is a sub-ducting plate earthquake. One or two nearfielddamage earthquakes usually occur once every century in Japan, and the Kobe earthquake was due to nearfieldearthquake types. This type happens in the same place in Japan only once every 2000 years. 3. Kobe earthquake ground motions and reinforced concrete structure buildings. Fig. 1 shows the Kobe earthquake motions records, recorded at Kobe Ocean Meteorological Agency. The peak acceleration was not as large as that of the Northridge earthquake, an American earthquake that happened just 1 year before the Kobe earthquake, and which killed about 60 people, or the Kushiro earthquake in Hokkaido, Japan. The largest accelerations of the Kobe earthquake were about 0.8G and then it slowed, with the strong motions lasting less than 10 seconds. Vertical movement was minimal and not as exaggerated as that of other earthquakes. Fig.2 shows the Kobe earthquake fault which was 160cmrightlateral in horizontal, 130cm reverse fault, in vertical ways, moved. Within about 100 metres from the fault, there was limited damage to buildings, mostly to the walls. People near the fault say that the houses moved due to the earthquake, but did not shake, so almost no damage was done to their houses. Kobe No. 1 and No. 2 buildings can be seen in Fig. 3. The tall building on the left moved 20cm to the north and the 6* floor of the building on therightwas completely broken, unlike the 5* floor which was made of steel reinforced concrete. Soft storeys were broken completely as shown in Fig 4 and 5. Where thefirststorey was a car park, it became the soft storey and these buildings were completely destroyed by the Kobe earthquake. In general, concrete is strong for withstanding compressions and steel bars are strong for resisting tensions, whilst hoops are effective against both tension and compression. Up until 1971, the Japanese building code required the minimum distance between hoops to be less than 30 cm. Since then 10cm has became the longest distance between hoops in Japanese buildings. As a result of the Kobe earthquake, columns with hoop intervals of 30cm were completely broken whilst hoops 10cm apart were not completely broken. 4. Steel buildings damage Beam to column connections were broken and buildings could not be used until repaired. In steel buildings 3S types of structures were completely broken and brittle. 3S means, sizes of more than 5cm thickness and 50cm width, shapes of a

3 Earthquake Resistant Engineering Structures 781 closed type like pipes, not open like H type, and high speed, double length in 1 second. Ashiyahama apartment houses were broken in this way (Fig 9), and columns of steel pipes for trains were broken in a brittle way even though they had steel columns. 5. Damage to wooden buildings. So many wooden buildings were broken completely. In the case of wooden buildings, the following conditions resulted in total destruction. 1. Heavy roofs made with clay designed to withstand strong winds not earthquake motions. 2. Wooden columns were already broken by termites (white ants) even before this earthquake. 3. Wooden buildings did not satisfy the recent building code for earthquake motions (it was changed since 1981). The result of these conditions was fatal to old people as 50% of those killed were more than 60 years old 6. Conclusions The buildings of Toumon Street, within 1km west of Kobe station and buildings around Toumon Street, were not greatly damaged, except one or two buildings. A spokesman for the Architectural Society announced that the reason was: "the name of this building is "Gessekai" (moon world), so ground motions in the moon were much smaller than in the earth". This was not the true reason but a joke. The real reason was found by FEM (finite element method). This showed that the Subway is not always bad for buildings, as up and down the subway about 30 degrees to the south, only 3 shock waves could pass per second. Atfirstwe thought, religion might be a factor involved, as many Islamic and Buddhist temples were quite safe even with a softfirststorey, whilst many Christian churches and Japanese shrines (for instance the Doita shrine infig.12) were heavily damaged. Buildings and Torii (Japanese shrine entrance) were frequently safe by the subway entrances, which are 20 meters wide and 500 meters long.

4 782 Earthquake Resistant Engineering Structures Fig. 1: Motions recorded at Kobe Fig. 2: Fault of Kobe Earthquake Fig 3: No 1 and 2 Kobe buildings Fig 4: Soft first storey building Fig 5: Broken soft storey Fig 6: Distance between hoops were 30cm

5 Earthquake Resistant Engineering Structures 783 Fig. 7: Distance between hoops was 10cm Fig. 8: Beam to column connection damage Fig. 9: Brittle steel column was broken Fig. 10: Wood building with lightweight roof Fig 11: Safety of Torii (shrine entrance) Fig 12: Damaged Ikuta shrine

Title. Author(s)ISHII, K.; KIKUCHI, M.; SHIRAI, K. Issue Date Doc URL. Type. Note. File Information HIGASHI-HONGANJI HAKODATE BETSUIN

Title. Author(s)ISHII, K.; KIKUCHI, M.; SHIRAI, K. Issue Date Doc URL. Type. Note. File Information HIGASHI-HONGANJI HAKODATE BETSUIN Title STRUCTURAL CHARACTERISTICS OF A HISTORICAL REINFORCE HIGASHI-HONGANJI HAKODATE BETSUIN Author(s)ISHII, K.; KIKUCHI, M.; SHIRAI, K. Issue Date 2013-09-12 Doc URL http://hdl.handle.net/2115/54344 Type