1. Large Ground Deformations
Large, permanent ground deformations often occur at the surface breaks associated with fault ruptures in earthquake as shown in Figure 1. Vertical and horizontal displacements of one side of the fault break relative to the other of a number of metres have occurred; where this relative movement occurs under a building catastrophic damage can result. Local deformations sufficient to cause severe damage can occur up to a few hundred metres from the fault.
|Figure 1: Large ground deformations due to earthquakes|
Fault breaks are known to occur repeatedly at the same location and it is therefore advisable not to locate buildings in the immediate vicinity of known previous fault breaks, although avoiding these does not guarantee protection from new surface faulting. It is particularly important to avoid such locations for sensitive installations such as power stations, chemical plants or major hospitals, the loss of which could be catastrophic for the whole community. For sub-surface pipes, roads and railways it may be impossible to avoid the network crossing a fault, and building to resist rupture may not be feasible. In such cases, the best protection strategy is to ensure that alternative routes are available, and that the flow of liquid or gas in the pipelines can be rapidly shut off in the event of a rupture.
|Figure 2: A building collapsed by liquefaction|
Earthquake-induced soil liquefaction has been the cause of catastrophic damage in a number of earthquakes. Certain types of soils, when they are saturated with water and then suddenly shaken by an earthquake, completely lose all shear strength, and flow like a liquid. The support to the foundations of buildings built on such soils then disappears, and they can plunge into the ground or overturn as shown in Figure 2, or be carried sideways bodily on unliquefied masses of soil. Liquefaction is most likely to occur in loose cohesionless soils, such as fine sand or silts; these are most commonly found in sea or river-deposited sediments laid down within the last few thousand years. Simple in situ soil testing using a cone penetrometer has been shown to be a good indicator of potential liquefaction susceptibility in a soil layer, and it is possible to establish magnitude and intensity thresholds below which liquefaction is not likely to occur. Clearly sites which may be subject to liquefaction should be avoided if possible for any massive structure; alternatively foundations should be designed to bear on stable soil layers below the layers that may liquefy.
|Figure 3: Landslides due to earthquakes|
Sloping ground or rock masses which are stable under normal loading can lose their stability during an earthquake causing effects ranging from a slow progressive creeping of the ground to a dramatic landslide, rockfall or flow failure as shown in Figure 3. Slope failures are particularly likely to occur when the ground is saturated following rainfall. Whether sudden or slow, such slope failures are liable to cause complete destruction of any building founded on them or in the path of the slide. Slope failures can contribute a high proportion of the losses from earthquakes in mountainous terrain. Earthquakes in mountainous terrain can also trigger rockfalls and mudflows large enough to engulf whole settlements. Landslides and lateral spreads can also cause extensive property damage.
The only effective means of protection from the landslide hazard is to avoid building on sites which may be affected. Sites on or at the top of steep slopes, or where there is evidence of recent instability, are those most obviously at risk. Known landslides can sometimes be stabilised through drainage, excavation, retaining structures or other geotechnical work, but while this may protect structures below the slide, it is unlikely to make the site safe for building. In some areas maps of previous and potential landslide areas may be available.
4. Tsunamis and Floods
|Figure 4: Tsunamis due to earthquakes|
Flooding following earthquakes may also result from seiches (oscillation of the water in enclosed bodies of water such as reservoirs) or from the failure of reservoirs or embankments. The probability of such flooding hazards is not easy to determine. They need to be acknowledged in selecting a site which is vulnerable, but the risk of damage or life-loss is probably not great enough for the site to be avoided altogether, except for very sensitive facilities.
Tsunamis are sequences of long-period sea waves generated by earthquakes, often those which occur in the sea bed as shown in Figure 4. They travel long distances at high speed, and when they reach the shore, they may under certain conditions result in huge waves a number of metres in height, which can surge well inshore. Low-lying coastal areas on the margins of the large oceans, especially the Pacific Ocean, are most vulnerable. Considerable damage can be caused by tsunamis and many coastlines such as those of North America, Japan, Hawaii, Peru and Chile are vulnerable. Some warning of the arrival of a large tsunami is usually available, enabling the vulnerable population to evacuate. Low lightweight buildings may be severely damaged by the high-velocity water impact, but more substantial structures can survive.
5. Ground Shaking Amplification
Choice of siting should also take into consideration the probable effect of the siting on the extent of ground shaking which will be experienced in an earthquake. It has frequently been observed that earthquake damage is greater in settlements sited on soft soils than in those sited on hard soil or on rock sites as shown in Figure 5. This is mainly due to amplification of the ground motions in transmission from bedrock to surface through the soil layer, but additional factors which may be involved include the destructive effect on foundations of subsidence which may have occurred on soft ground prior to the earthquake and the effect of ground deformations during the earthquake. Generally rock sites are to be preferred, and where siting on soft soil is unavoidable, provision should be made in the design of the building and the foundations for the more severe movements which will be experienced. Most building codes include provision for the effects of subsoil conditions. A full geotechnical investigation of the site is needed to consider the likely consequences of the subsoil conditions for the design of buildings.
|Figure 5: Ground Shaking Amplification|
Settlements located on deep deposits of soft soil types or compressible deposits are a special case. Such deposits can have a strongly defined natural frequency of vibration, amplifying that part of the bedrock motion which is of similar frequency, and filtering out the rest. Buildings will be affected selectively according to their own natural frequency of vibration. Such amplification will be particularly strong for distant earthquakes for which filtering of the high-frequency component of the motion has already occurred. Low-frequency components of ground motion have caused damage to medium- to high-rise buildings on a number of city sites located on deep soft soil deposits. In settlements founded on such deep alluvial deposits it may be necessary to restrict the height or mode of construction of buildings so that their natural frequency of vibration is not of the same order as that of the underlying soil deposits. Avoiding such sites altogether is rarely an option, since the pattern of urban development may have already been established.
Ground motion amplification can also occur as a result of topographical effects; in particular, buildings sited on ridges may be vulnerable. However, the extent of this effect and the factors influencing it are not yet sufficiently well understood for any clear rules to be formulated. Again, it may well not be possible, for economic reasons, to avoid building on ridges.