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Possible hazards from earthquakes can be classified as follows:

1. Ground Motion

Shaking of the ground caused by the passage of seismic waves, especially surface waves, near the epicenter of the earthquake are responsible for the most damage during an earthquake and is thus a primary effect of an earthquake. The intensity of ground shaking depends on:

 

Local geologic conditions in the area. In general, loose unconsolidated sediment is subject to more intense shaking than solid bedrock.

 

Size of the Earthquake. In general, the larger the earthquake, the more intense is the shaking and the duration of the shaking.

 

Distance from the Epicenter. Shaking is most severe near the epicenter and drops off away from the epicenter. The distance factor depends on the type of material underlying the area. There are, however, strange exceptions. For example, the 1985 Mexico City Earthquake (magnitude 8.1) had an epicenter on the coast of Mexico, more than 350 km to the south, yet damage in Mexico City was substantial because Mexico City is built on soft unconsolidated sediments that fill a former lake.

 

Damage to structures from shaking depends on the type of construction. Concrete and masonry structures are brittle and thus more susceptible to damage wood and steel structures are more flexible and thus less susceptible to damage.

2. Faulting and Ground Rupture 

Ground rupture generally occurs only along the fault zone that moves during the earthquake, and are thus a primary effect. Thus structures that are built across fault zones may collapse, whereas structures built adjacent to, but not crossing the fault may survive.

3. Aftershocks

These are smaller earthquakes that occur after a main earthquake, and in most cases there are many of these (1260 were measured after the 1964 Alaskan Earthquake). Aftershocks occur because the main earthquake changes the stress pattern in areas around the epicenter, and the crust must adjust to these changes. Aftershocks are very dangerous because they cause further collapse of structures damaged by the main shock. Aftershocks are a secondary effect of earthquakes.

4. Fire

Fire is a secondary effect of earthquakes. Because power lines may be knocked down and because natural gas lines may rupture due to an earthquake, fires are often started closely following an earthquake. The problem is compounded if water lines are also broken during the earthquake since there will not be a supply of water to extinguish the fires once they have started. In the 1906 earthquake in San Francisco more than 90% of the damage to buildings was caused by fire.
 

5. Landslides

In mountainous regions subjected to earthquakes ground shaking may trigger landslides, rock and debris falls, rock and debris slides, slumps, and debris avalanches. These are secondary effects.

6. Liquefaction

Liquefaction is a processes that occurs in water-saturated unconsolidated sediment due to shaking. In areas underlain by such material, the ground shaking causes the grains to lose grain to grain contact, and thus the material tends to flow. Liquefaction, because it is a direct result of ground shaking, is a primary effect. You can demonstrate this process to yourself next time your go the beach. Stand on the sand just after an incoming wave has passed. The sand will easily support your weight and you will not sink very deeply into the sand if you stand still. But, if you start to shake your body while standing on this wet sand, you will notice that the sand begins to flow as a result of liquefaction, and your feet will sink deeper into the sand.

7. Changes in Ground Level

A secondary or tertiary effect that is caused by faulting. Earthquakes may cause both uplift and subsidence of the land surface. During the 1964 Alaskan Earthquake, some areas were uplifted up to 11.5 meters, while other areas subsided up to 2.3 meters.

Tsunami (image: constructionweekonline)

8. Tsunami

Tsunami a secondary effect that are giant ocean waves that can rapidly travel across oceans, as will be discussed in more detail later. Earthquakes that occur beneath sea level and along coastal areas can generate tsunami, which can cause damage thousands of kilometers away on the other side of the ocean.
 

9. Flooding

Flooding is a secondary effect that may occur due to rupture of human made dams and levees, due to tsunami, and as a result of ground subsidence after an earthquake.

For the designers or owners of individual buildings, or for urban planners or city authorities, the issue is how likely a specific site is to experience earthquake forces of a certain severity. Building design codes adopt one of two alternative procedures for specifying the geographical distribution of design loads:
(1) seismic zonation or
(2) contour mapping of expected ground motion.
 

Most national codes of practice use the seismic zonation concept. The country (or region) covered by the code is divided into a small number (usually no more than four or five) of separate source zones, within each of which the lateral loading requirement for earthquake-resistant design is constant, and is specified by a zone coefficient. The zone coefficient relates to the expected peak ground acceleration within a predefined return period, but this information does not need to be known by the designer. The Turkish seismic zonation map (Figure 1) is a typical example. In this code the zone coefficients are 0.1, 0.06, 0.04 and 0.2 for Zones 1, 2, 3 and 4 respectively, corresponding roughly to the peak ground acceleration (as a proportion of the gravitational acceleration g) with a 10% probability of exceedance in 50 years. These coefficients are converted into a response spectrum for design using further coefficients for local soil type and building importance. Similarly, Figure 2 shows seismic zones of Pakistan.

The advantage of this method for specifying design loads is its simplicity for designers. The zones, although defined from knowledge of regional seismicity, are not given a formal definition in terms of expected ground motion. Their significance derives from the use of the zone coefficient in the formulae in the accompanying code, so they have a semi-legal character, like district boundaries.

However, the approach also has disadvantages. One disadvantage is that the seismic zonation is coarse, and is unable to take into account the effects of local features such as fault zones. Another is that only a single parameter is defined, whereas it is now accepted that at least two independently varying parameters are needed to take adequate account of the variations in regional seismicity. These two disadvantages are overcome through the use of contour maps such as those accompanying the 2000 International Building Code. The code specifies that the design loading should be that associated with the maximum credible earthquake (MCE) at the site. Contour maps of the entire United States indicate the values of two key design parameters to be used to construct the design ground motion response spectrum at that site: the spectral acceleration values at 0.2 s and 1.0 s periods. The value of these parameters is derived from the US Geological Survey’s hazard maps which are contour maps of the 0.2 and 1.0 s spectral accelerations with a 10% probability of exceedance in 50 years, but with modifications for some parts of the United States to take account of the effects of known local faulting on design loads, and with variations for different classes of site defined by soil conditions. Figure 3 shows, for example, the MCE ground motion map of a small part of Western United States for the 0.2 s horizontal spectral acceleration (% of g), for Site Class B. Maps such as these represent a considerable step forward in defining appropriate design load coefficients and are likely to become the standard approach for future codes in other countries.

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.

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.

2. 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.

3. Landslides

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

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.

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.