January 17, 2012 | 3
In the morning of January 17, 1995 a strong earthquake hit the Japanese city of Kobe and despite the modern city infrastructure was assumed earthquake-proof more than 6.000 people were killed, 26.800 injured, 46.000 buildings destroyed or damaged and more than 300.000 people were made homeless.
The Kobe earthquake lasted for 14 to 20 seconds and reached a magnitude of 7.2 after Richter (7 according to the Japanese intensity scale – shindo, the maximal possible value), the strongest earthquake in western Japan since 1923. The devastating earthquake was not directly connected to the nearby subduction zone of the Philippine Sea Plate, but generated along a local fault system formed by the intersection of the Nojima fault with the Suma fault (see also KOKETSU et al. 1998). The epicentre was located in the Akashi strait only 20 kilometres to the west of Kobe, the hypocenter was situated in a depth of just 10 kilometres. Such a shallow earthquake was not expected to occur in Kobe and the vicinity to the city amplified the disastrous effects of the shakes.
The earthquake damaged seriously the traditional buildings of the residential areas in the western and eastern part of the city; people were surprised asleep in their homes and killed or injured by the collapsing houses. The expressway of Hanshin, opened in 1962, was build according to the earthquake-resistant construction directives of the time; however it collapsed partially during the earthquake as the horizontal shear movements pulverized the concrete of the supporting pillars.
The harbour of Kobe, one of the most important in the world and situated on an artificial island, was severely damaged. The vibrations of the earthquake liquefied the soil and groundwater was pressed out from the pores, fissures opened in the ground and mud inundated parts of the harbour. The basement of many buildings became instable in this water-sand mixture and parts of the harbour slipped into the sea.
Based on the observations of the collapse of the Hanshin expressway and the devastation of the harbour the Japanese government improved the already severe guidelines for anti-seismic buildings.
A sort of anti-seismic construction method was already known in ancient Japan as many old pagodas show characteristics that can minimize the dangerous oscillations of a building caused by an earthquake. In a pagoda the central column, supporting the weight of the entire building, is made of a single log. Like a shaft of bamboo (the largest species can reach a height of 30m) this central column can swing in all directions and adsorbs most of the kinetic energy during an earthquake.
The various floors of the pagoda can move independently and are connected to the inner central column by a complicated construction made of wood, acting like a spring or shock absorber it will also reduce dangerous horizontal movements.
The first modern principles of anti-seismic buildings were introduced at the beginning of the 20th century in Tokyo. The earthquake-proof “Imperial Hotel” was commissioned in 1915 and inaugurated in 1923 as a luxury hotel for foreigners in imperial Japan. It was planned by the American architect Frank Lloyd Wright (1867-1959), who visited Japan for the first time in 1905.
The site for the hotel seemed unfavourable for such a large building, located in the seismic zone of Tokyo on a 2,4m thick organic soil resting on 20m of soft alluvial sediments.
On such an unstable ground engineers normally choose a deep basement, trying to reach stable rocks or “anchoring” the building in the underground. Surprisingly Wright planned a very shallow basement with just 2-3m depth. He argued that a deep foundation would transfer the oscillations of an earthquake from the ground to the building; however the alluvial mud of the construction site should adsorb the seismic energy and the hotel could float on the sediment like “a battleship floats on water.”
The hotel had several ulterior features designed to minimize the destructive effects of an earthquake:
- The pool in front of the entrance was not only a decorative element, but provided water to fight fire. This element in fact saved the hotel from the firestorm raging after the 1923 earthquake.
- Supplementary reinforced structures on the outside of the building provided extra support for the floors.
- The walls were not constructed simply with bricks, but with a sort of innovative sandwich technology: reinforced concrete between an external and internal layer of bricks.
- The walls of the lower floors were thicker and stronger than the walls on the upper floors, with small openings and few windows.
- A light copper roof would not oscillate as strong as a massive roof, reducing the danger of collapse of the entire building.
- Supplementary “seismic separation joints”, made of ductile lead, were inserted into the walls of the building; during an earthquake the single segments and floors should be able to move independently.
- Pipes were not encased in the concrete, but could swing in cavities hidden in the walls.
- Wright avoided unnecessary decorative elements on the outside of the hotel, which during an earthquake can fall down and kill people.
September 1, 1923 Tokyo was hit by a massive earthquake with a magnitude of 8.3 after Richter, 5.000 buildings collapsed and thousands were heavily damaged.
Wright anxiously awaited information on his hotel and two weeks later he received a telegram from the Japanese businessman Baron Kihachiro Okura:
“Hotel stands undamaged as monument to your genius – Congratulations”
Wright passed this telegram to journalists and helped so to perpetuate a legend that his hotel was completely unaffected by the earthquake, maybe even the only building still standing in Tokyo!
In reality the building was damaged; the central section slumped, several floors bulged and four pieces of stonework fell to the ground – however the building was still standing.
Ironically the building’s only main flaw was the shallow basement. During the earthquake the basement sunk by 0,6m into the liquefied mud and in the subsequent years continued slowly to sink into the ground. The damage and instability of the entire construction finally resulted in the necessity to demolish the entire hotel in 1968.
However many of the anti-seismic features introduced by Wright are still used today and hopefully many new inventions will minimize the deadly effects of future earthquakes.
KOKETSU, K.; YOSHIDA, S. & HIGASHIHARA, H. (1998): A fault model of the 1995 Kobe earthquake derived from the GPS data on the Akashi Kaikyo Bridge and other datasets. Earth Planets Space, 50: 803-811
WALKER, B. (1982): Earthquake. Planet Earth. Time Life Books: 154