Seismic Risk. What is it?
Seismic risk can be defined as the probability of losses occurring due to earthquakes within a given period of time; these losses can include human lives, social and economic disruption as well as material damage. Hence Seismic risk can be expressed by the following qualitative expression :
SEISMIC RISK = SEISMIC HAZARD * VULNERABILITY* EXPOSURESeismic Hazard may be defined as the probability of a potentially damaging earthquake effect (e.g. ground shaking, ground failure etc.) occurring at a site within a given period of time. It is commonly expressed as a relationship between the level of the seismic effect and the corresponding probability of its occurrence.
Vulnerability is the likelihood (or probability) of the occurrence of damage in a building (or population of buildings) when exposed to a particular earthquake effect. It is therefore typically represented by a relationship between the level of earthquake effect and the level of damage expected to result. The latter definition (in terms of structural damage) is what is typically used by engineers, however it should be noted that vulnerability can also be expressed in terms of potential life loss or economic loss.
Exposure is a quantification of the number of people and buildings, the amount of commercial and industrial activity, the amount and type of important infrastructure and buildings concentrated in the area assessed.
A disaster occurs when a hazard such as an earthquake hits an area that is vulnerable to the effects of that hazard and has a high level of exposure. There is no evidence to suggest that the number of earthquake events is increasing worldwide, yet recent years have shown an increase in the number of earthquake related disasters. Why?
Most economic and life losses in earthquakes occur as a direct or indirect consequence of building and infrastructure collapse. Growing urbanisation with accompanied rapid increase of poorly built housing, uncontrolled use of land, overstretched services and high population densities, has increased our vulnerability to earthquake effects and therefore increased the potential for disasters. Therefore although an earthquake is a natural phenomena, the level of losses are largely dependent on human activity and hence it is misleading to use the term “natural disasters”.
Source : SECED UK
Seismic Risk Assessment. What is it and why is it carried out?
Seismic risk assessment simply means the evaluation of the current seismic risk. i.e. looking at the existing building stock and exposure under a given hazard and evaluating potential losses. Hence it is composed of three main parts:
Seismic Hazard Assessment
Seismic Vulnerability Assessment
Exposure Assessment
Seismic risk assessments are carried out for a number of reasons for a range of different clients. Firstly, seismic risk assessments form the basis for any plan of mitigation. Hence governments / development agencies, may look to carry out seismic risk assessments to identify high risk areas likely to undergo large life and economic losses during an earthquake.
Modern seismic codes have only really been enforced since the mid- 1980’s. This means that in Europe (and the rest of the world) the majority of existing buildings is not designed to resist earthquake forces. Nevertheless, most have an inherent lateral resistance, born of over-strength factors in design codes, which may be sufficient to resist their imposed seismic hazard (if this is low to moderate) with an acceptable degree of damage. The results of seismic risk assessments can be used to decide on strengthening intervention and help informed disaster management plans to be drawn. Hence, they aid these agencies decide where and how to invest effort and resources, locate important facilities and identify the priorities for investment.
Insurers and Re-insurers carry out seismic risk assessments to evaluate the likely losses to their portfolios (humans, buildings, industry) in the case of an earthquake. A damaging earthquake causes a concentrated period of claims payout, which can compromise the cash flow of insurance/re-insurance companies. However, earthquake insurance brokering can be profitable…an earthquake may not happen during the insured life of the building. Knowing the likelihood of their losses allows insurers to spread/limit their risk by insuring in different areas, selling some of their risk to re-insurers or just not insuring certain locations/categories of buildings/ specific buildings.
Very detailed seismic risk assessments are carried out for very important buildings, the consequence of which damage or non-operation has an impact on human life, key economy or the environment. Nuclear power stations, chemical plants, hospitals, key infrastructure and schools are examples of such structures.
Source : SECED UK
When are Seismic Risk Assessments carried out?
Seismic risk assessments are carried out before an earthquake happens. They look to predict the likely loss if an earthquake happens. They use predictions of earthquake ground motions to estimate probable losses. Seismic Loss Assessments are instead carried out in the aftermath of an event. Here, the ground motion is known and the damage must be evaluated through surveys to estimate the loss. Essentially they are different, however, seismic risk assessments/predictions use tools that are based on past observations of earthquakes, their effects and building behaviour in past events.
What is acceptable loss ?
The definition of “acceptable” loss varies.
In the case of buildings and infrastructure what is regarded as acceptable varies according to the owner, importance of the buildings, the consequences of their damage, their use. The degree of acceptability further varies worldwide according to social structure, expectancy, culture and according to the severity of the earthquake. Typically multiple performance criteria need to be satisfied.
For example, it is unacceptable that nuclear power stations sustain any damage whatsoever even under the largest possible earthquake. But for normal buildings acceptable behaviour may be determined by it not collapsing under a very large event (thus ensuring life-safety), by it being damaged but repairable under a rare event and by it being undamaged and fully-operational under frequent smaller earthquake events.
Furthermore, in most cases a higher level of loss is acceptable in te case of existing infrastructure than for new. Hence the level of acceptable loss is different in the case of seismic design and seismic assessment.
Earthquake Hazards
There are a number of different threats an earthquake can impose:
Ground shaking
Surface rupture
Landslides
Liquefaction
Tsunami
All of these can be regarded as earthquake hazards. When assessing a certain location it is important to consider all of these (as well as other natural and man-made hazards).
However, in this course we will however mainly be concentrating on the main earthquake hazard: ground shaking induced by seismic waves. Ground shaking imposes inertial loads and displacements on a structure. These loads/displacements form an important input to structural evaluation in the seismic risk assessment. In the following text, when the term seismic hazard is used, this is referred to ground shaking.
Background to Seismic Hazard Assessment
A seismic hazard assessment forms the first step of any seismic risk assessment.
The aim of a seismic hazard assessment is to provide the structural (and geotechnical) engineer with an estimate and description of the ground shaking that is likely to be experienced by an engineering structure/urban environment during its design life/set period of time. To be of use it must provide a description of the dynamic characteristics of the ground motion at the site.
Ground shaking is caused by seismic waves. The characteristics of these waves are dependent on the earthquake source characteristics (such as type of fault, rupture speed and size, magnitude), on the characteristics of the path (e.g. soil strata, distance from source to site) they have taken to reach the site and on the site characteristics themselves (e.g. soil conditions at site). However, the determination of the seismic sources and their seismicity in the assessed area forms the bais of any seismic hazard assessment.
Figure 1: Historical Seismicity of Turkey with major tectonic features (mapped by Erdik et al. 1999)
The Evaluation of Regional Seismicity
As a first step, all major tectonic and geological structures need to be identified in the assessed area, as well as all major faults.
This can be done by referring to up-to-date geological/fault maps. However, in order to know how seismically active these faults are data on the location and size of past earthquakes needs to be mapped onto the geological map. Instrumental earthquake data for the area can be obtained from earthquake catalogues but should be supplemented with data of historical (pre-instrumental) earthquakes. Furthermore, it may be possible to further supplement these with observations of offsets/fault movements made in geological studies.
Examples of superimposed fault and earthquake mapping is given in figures 1 to 3.
Figure 2: Historical Seismicity of Turkey with major tectonic features (mapped by Erdik et al. 1999)
Earthquake Catalogues
Earthquake catalogues consist of a list of the parameters describing an earthquake: its time of occurrence, epicentral coordinates, focal depth and magnitude (using various scales). These catalogues will typically also contain the seismograms recorded at various sites world-wide and a summary of the parameters describing these records (e. g. peak ground acceleration, strong motion duration etc). They can be obtained from numerous agencies around the world such as the International Seismological Centre (ISC) and National Earthquake Information Centre (NEIC). (Check out their web sites).
However, earthquake catalogues must be treated with some caution. The first network of seismographs was set up in the late 19th century, and during the first few decades of the 20th century there were only few seismographs around the world distributed unevenly around the globe. Furthermore, early instruments were of low sensitivity and therefore did not record all small earthquakes. As a result most earthquake catalogues span a limited number of years (maximum 100years) and can be regarded as incomplete (i.e. do not contain all earthquakes which occurred). Also there can be large uncertainties in the hypocentral locations interpreted from seismograms, with errors up to 5km and 10km in the predicted epicentral coordinates and focal depth, respectively. Larger errors have also been known to occur (Ambraseys 1988). In order to reduce errors, correlation of location coordinates with observed damage reports must be carried out.
Figure 3: Instrumental Seismicity of Turkey with major tectonic features (mapped by Erdik et al. 1999)
100 years of data on earthquakes is not much to go on when dealing with geological processes. Therefore, in order to improve our understanding of earthquakes and how often they occur it is necessary to extend the catalogue through historical studies. Researchers have been looking through historical texts and looking at geological evidence to determine when past events have occurred and how large they were. Examples of such studies are the ones for Iran, Egypt, Arabia and the Red Sea by Ambraseys and Melville (1982) and Ambraseys et al. (1994).