New Horizons in Structural Design April, 2008

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08FACT　NIKKEN SEKKEI　　　　構造デザインの最前線Earthquakes and Safety MeasuresIn structural design, it is important to consider protection against large forces that might impact a building in the event of an earth-quake or a typhoon, as well as the everyday forces that act on the building, including its own weight. Particularly in Japan, which fre-quently experiences earthquakes, it is important to have a strong understanding of earthquake phenomena. Here, we explain about the science behind earthquakes and the concept of aseismic design.The Mechanism of EarthquakesOur planet is not a uniform sphere but rather a globe composed of elements that differ in nature—the center core, the mantle, and the surface crust. The hot mantle experiences convection slowly at high temperatures, and this forces the crust on the surface to move incrementally. In fact, the surface of the Earth is divided into dozens of plates that shift relative to the convection of the mantle. In other words, the planet is not covered with a shell of crust like an egg but with plates floating on the mantle that move several centimeters each year. Consequently, frictional stress accumulates at the bound-aries at which these plates come into contact with each other. When the accumulated frictional stress exceeds a critical value, the crust undergoes violent displacement to release this stress, trigger-ing an earthquake.This process frequently causes earthquakes around the boundar-ies at which the plates abut each other. The map on page 4 shows the global distribution of earthquakes. Around the Pacific Ocean, the Pacific Plate makes contact with another plate, producing an "earthquake nest." Near the Japanese archipelago, the Pacific Plate and the Philippine Sea Plate descend beneath the continental plate. This complex structure creates very severe seismic environments.Earthquakes occurring in this region are roughly classified into two types. A plate-boundary earthquake is triggered by two plates that cause a displacement through which they release frictional stress. The Great Kanto Earthquake, as well as the Nankai and the Tonankai Earthquakes are examples of this type. The second type is known as an inland epicentral earthquake, and results from a release of accumulated stress on a plate surface. This type is repre-sented by the Great Hanshin-Awaji Earthquake and the more recent Chuetsu-oki Earthquake, which occurred in Niigata Prefecture in July 2007. The plate-boundary earthquake, which has a wider focal region and a larger magnitude (typically of around 8) causes widespread heavy damage to cities. The motion induced by this type of earth-quake persists for long periods. Particularly on plains, waves con-tinue to propagate for a long time (long-period ground motion) due to the effect of the sedimentary layer. When such an earthquake occurs, high-rise buildings undergo considerable swaying. There-fore, we need to check inhabitability in the event of long-lasting sway, as well as the functional status of facilities and equipment. It is also important to examine earthquake safety by properly identify-ing the characteristics of a given building.On the other hand, inland epicentral earthquakes generally have a magnitude of around 7. Although the damage caused in a limited area can be extensive, the scope of the damaged area is less wide-spread. In the Great Hanshin-Awaji Earthquake of 1995, ground motion with a seismic intensity of 7 hit the Sannomiya area in Kobe. This type of earthquake, with its smaller focal region, causes ground motion that is limited in scope. The principal motion lasts for a short period, which is typical of epicentral earthquakes. In such cases, devastating forces act on buildings for a short time. Intermediate-現在、政府の地震調査研究推進本部においては、主要な98断層帯について活動度を評価しています。日本は世界でも有数の地震国であり、そこに建てられる建造物には高い耐震性能が求められます。The Headquarters for Earthquake Research Promotion contin-ues to evaluate 98 major fault zones. Earthquakes are fre-quently experienced in Japan and buildings must be designed with high earthquake-resistant features.（出典：内閣府中央防災専門調査会ホームページ）Based on Central Disaster Management Council, Cabinet Office, Government of Japan website.活断層の分布Distribution of active fault zones

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