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Many areas in the world are prone to seismic activity - ie. earthquakes. Densely populated cities such as Tokyo, San Francisco and Istanbul are in major earthquake zones and architects and civil engineers have to take this into account when designing buildings and structures.

The destruction of buildings during an earthquake, is, not unsurprisingly, due mainly to the earth's shaking transferring into the structure and causing it to wobble. If the quake's energy is not dissipated then it can lead to forces in the building's load-bearing paths which exceed their structural strength, resulting in a collapse.

There are therefore three ways to reduce the impact of an earthquake on a structure. The first is to prevent the quake's energy from being transferred into the building, the second is to introduce ways in which the energy can be dissipated and the third is to use materials and construction techniques with sufficient structural integrity. In any practical solution all three options will be used in combination.

ISOLATION
Buildings can be constructed upon a base of isolators, which are nothing more than a large set of shock absorbers often consisting of alternate layers of rubber and steel bonded together around a central core. These reduce the earthquake loads felt by the building and its occupants.

DISSIPATION
Tall structures, such as skyscrapers, are often less prone to damage than shorter ones, as they are naturally more flexible. When a quake strikes a tall building the energy will be dissipated by the flexing of the structure, which although disturbing for occupants will usually minimize the possibility of collapse.

Whatever the height of the building the energy mustn't be allowed to concentrate in a particular area - each support element needs to be able to transfer and dissipate the energy, and with clever design the energy can be transferred back to the ground.

STRENGTH

The strength of a structure will depend on the materials used and the design of the support structure. In line with the flexibility considerations it is best to use flexible materials as they can withstand more distortion without breaking. Steel and wood are better in this regard than concrete or masonry. Interconnecting support elements and support trusses, such as those used in the TransAmerica pyramid in San Francisco may be used to improve integrity.

Any solution is never 100% safe against earthquakes, but by testing models of designs on shake tables and monitoring the effects of minor quakes on existing buildings, design improvements can be made which will help improve safety. Of particular importance is the nature of the ground around the building, as this can modify the characteristics of the quake and create problems if the seismic frequency is close to the resonant frequency of the building. If measurements suggest that this could be a factor, then additional structural elements may be needed to alter the building's characteristics.

Finally, designers should be ensure that services such as electricity, water and particularly gas are designed in such a way as to minimize the probability of causing fire or additional disruption. This may employ similar techniques to those used on buildings or incorporate cut off features.

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