an approach to earthquake risk management

An approach to earthquake risk management E. M. F o u r n i e r d 'A lbe

9 bis, Passage Barrault, 75013 Paris. France

The paper formulates a conceptual basis for regarding earthquakes as a problem of social and economic risk which is part of everyday life, rather than regarding them as 'acts of God'. The main elements of the problem and the relationships between them are identified. It is argued that one should act to mitigate the impact of an earthquake rather than simply reacting to the losses it inflicts.

Key words: earthquake, risk management.

I n t r o d u c t i o n

Earthquakes are usually regarded as essentially unmanage- able phenomena. They strike suddenly, without warning, and bring to an instantaneous halt the normal activities of the stricken community. The material losses they cause often represent many years of labour by the community as a whole. And yet, they are rare events, too rare to be considered as part of every day life, like fires or car accidents.

Traditionally, they have been regarded as 'acts of God', unpredictable and inevitable. All that could be done was to rescue, relieve and assist the survivors of a disaster to re- establish a normal life in the stricken area as soon as possible.

Only in the past 50 years has it been realized that it is technically possible to build in such a way as to reduce or prevent damage and loss of life in earthquakes. The science of earthquake engineering has advanced rapidly during the last 20 years and may do so even more rapidly in the near future. However, much of the new knowledge and tech- nique is still not applied in practice as widely as it should be, sometimes for lack of the necessary financial or human resources but more often because of a lack, at the mana- gerial or political level, of awareness of the nature of earthquake risk and of what can be done to reduce it.

From the point of view of public policy, investment in seismological and engineering research or in earthquake- resistant construction must be justified not only in scientific and technical, but in social and economic terms. Costs and benefits must be analysed and acceptable risks defined. In other words, there must be a coherent policy of earthquake risk management. The present paper is an attempt to identify the main elements of this problem and to discuss the relations between them.

Definitions

Consider the case of a building damaged by an earthquake which caused ground motion of intensity i at the site of

the building. In economic terms, we define the loss suffered as a result of this damage, as follows:

Loss = Cost of restoring the building to its state just before the earthquake, or of demolishing it and replacing it by an identical building at the same site (1)

In order to simplify matters, we leave out of consideration any loss of human life, which is difficult, not to say impossible, to express in economic terms; also, secondary losses due to interruption of function, which must nevertheless be taken into account in any complete analysis.

Another useful concept is that of specific loss, defined as follows:

cost of restoration or replacement Specific loss = (2)

value of building

In accordance with the definitions adopted by UNDRO 1, we define as follows the vulnerability of the building to earthquake ground motion:

Vulnerability, V/= Specific loss in event of (to ground motion of intensity i (3) motion of intensity i)

Note that vulnerability, thus defined, cannot be expressed by a single figure but by a mathematical function or set of functions relating vulnerability to one or more parameters of ground motion. The vulnerability of a building or structure is determined by its dynamical properties only, and does not depend on whether or not the building is in fact liable to be subjected to earthquake ground motion.

A building may be vulnerable to earthquake ground motion but it will be at no risk unless there is a finite probability of such ground motion occurring at the site during the lifetime of the building. We define risk as follows:

Risk = Probability of loss in a given period of time (4)

By analogy with the notion of specific loss defined above,

0141[0296/82/03147-06/$03.00 © 1982 Butterworth & Co. (Publishers) Ltd Eng. Struct., 1982, Vol. 4, July 147

An approach to earthquake risk management." E. M. Fournier d'Albe

we may introduce the term 'specific risk', defined as follows:

Specific risk = Probability of specific loss in given period of time (5)

Expressing by V/the vulnerability of a building or structure to seismic ground motion of intensity i, and by Pi the probability of such ground motion occurring at the site during a given period of time, we may write:

/max

Specific risk = / V/• Pi " di (6) i / 0

Thus, specific risk is obtained by convolution of the probabilities of all possible intensities of ground motion with the vulnerability corresponding to each intensity. In other words:

Specific risk = (vulnerability) x (seismic hazard) (7)

The vocabulary adopted by UNDRO 1 includes the term 'element at risk', signifying any object situated in an area subject to earthquakes. If one attaches a value E to one such element, the risk to that element is:

/max

Risk to element = E I- Vi "Pi " di (8)

0

In equations (6) and (8), the seismic hazard is expressed by its probability density function Pi " di. For actual com- putation, this may be replaced by a step function, particularly if intensity is measured on a macroseismic scale, or by the probabilities of exceedance of various intensities. These details are discussed in reference 1.

Of the three factors which determine seismic risk, (value of the elements at risk, their vulnerability, and the seismic hazard), only the first two are amenable to human control. The art of earthquake risk management is to manipulate these two factors in such a way as to reduce the risk to a level acceptable to the community. However, the first need, in any attempt to define a policy of earthquake risk management, is for accurate information on the seismic hazard, its spatial distribution and its eventual variations in time.

Seismic haza rd

The concept o f seismic intensity We have defined seismic hazard as the aggregate of prob-

abilities of occurrence of seismic ground motion of different intensities at a given site and during a given period of time. We need to ask what we mean by 'intensity' of ground motion, and how is it measured.

The concept of intensity is derived from observations that, in any earthquake, the degree of damage to buildings and other structures is a function of the magnitude of the event and of position with respect to the earthquake focus. In the absence of instruments able to record strong ground accelerations with accuracy, the inspection of damage has represented the only way of assessing the severity of ground motion, and several scales of measurement have been elaborated (e.g. Rossi-Forel, Mercalli, Medvedev-Sponheuer- Karnik, etc.). The common characteristic of all such scales is that they offer a means of expressing the inensity of ground motion in terms of the observed degrees of damage suffered by characteristic buildings in certain well-defined

categories. It is implicil in all such scales tha~ the vulnerabi° lity of all buildings in a given category is the same for all types of ground motion. The inferred intensity is expressed by a single figure on each scale.

This is an obvious over-simplification and it is not sur- prising that field surveys of the effects of earthquakes often reveal patterns of damage that are difficult to interpret in terms of a single parameter of ground motion. Further- more, laboratory tests have shown that the vulnerability of a structure is usually a complex function of several parameters (e.g. peak acceleration, frequency spectrum, dura- tion of shaking, etc.) of ground motion.

After a damaging earthquake has occurred, it is custom- ary to prepare a map showing the distribution of intensity of ground motion caused by the earthouake. Given what we now know about the response of buildings and structures to ground motion, such maps can qualify as maps of intensity only if intensity is expressed in terms of ground motion parameters actually recorded by appropriate instruments. Maps on which intensity is expressed as degrees on a macroseismic scale are not, in fact, maps of intensity of ground motion but rather maps of specific loss. Similarly, maps of 'maximum expected intensity', based on statistical analysis of macroseismic intensity data on past earthquakes, must rather be considered as maps of maximum expected specific loss, or in other words as maps of one parameter of specific risk.

We will examine some of the practical implications of these arguments later in the paper.

The mapping o f seismic hazard

To illustrate the procedures followed in the preparation of seismic hazard maps, we may take as example the maps prepared for the Balkan region under the auspices of the UNDP/UNESCO project carried out in that region between 1970 and 1976. z The sequence of steps was the following (see Figure 1):

Compilation of a regional catalogue of earthquakes including both instrumental data and data from historical records going back about 2000 years; Preparation of maps of earthquake epicentres; Preparation of a seismotectonic map of the region, and maps of recent crustal movements; Preparation of maps of seismic origin zones; Derivation of a magnitude-frequency relation for each zone; Derivation of attenuation laws governing the propagation of seismic waves within the region; Preparation of regional seismic hazard maps on which hazard is expressed in terms of ground accelerations or velocities with various probabilities of occurrence.

One such map is reproduced in Figure 2. On others, ground motion is expressed in terms of peak particle acceleration and the probabilities relate to other periods of time.

The choice of parameters in this case was determined by the needs of engineers concerned with the design of earthquake-resistant structures. They were drawn on a scale of 1 : 2 500 000 and were intended to serve as a basis for the preparation of larger-scale hazard maps of each country. It is worth remembering in this connection that, given the scale of seismic phenomena and the accuracy of the data, little is to be gained by preparing seismic hazard maps on scales larger than 1 : 1 000 000, since the general level of hazard (soil conditions being uniform) does not vary significantly over distances less than about 10 kin.

148 Eng. Struct., 1982, Vol. 4, July

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Figure 1 Elements in the seismic risk method: (a) hypothetical source area; (A) known earthquakes; ( - - ) known faults; ( - - - --) inferred faults, (b) log N vs M relationship for source area shown in (a). (c) Attenuation curves. (d) Distribution over time of intensity of shaking effects at each site. (Figure copied from Algermissen, S. T. et al. 'Seismic risk evaluation of the Balkan region'. In Proc. seminar on seismic zoning maps, Vol. 2, UN ESCO, Skopje, 1976)

It is of course well known that the intensity of ground motion at any site is strongly influenced by the dynamic properties of the sub-soil at the site. These properties may vary significantly within distances of the order of tens of metres. No entirely reliable method has yet been developed whereby local subsoil effects on seismic intensity can be accurately predicted. What can be done at the present time is to make a qualitative classification of subsoil types and of their responses to seismic wave motion, based on observations of damage distribution in past earthquakes. One may then prepare seismic microzoning maps on scales of 1: 5 000 or 1:10 000, which, used in conjunction with general seismic hazard maps, are an essential tool in urban planning and design in seismic areas.

The prediction of time-variations in seismic hazard It may be argued that the above sub-title has no meaning, since seismic hazard has no objective existence in nature, being just a mental concept. In reality, there are only earthquakes, which occur at times and places determined by the interplay of stress and strain in the earth's crust. Were it feasible to measure the stress field and the mechanical properties of crustal rock at intervals of a few tens of kilo- metres and down to depths of about 30 km, it would probably not be necessarv to speak of seismic hazard but only of expected earthquakes wiaose times, locations and

An approach to earthquake risk managemen t: E. M. Fournier d'Albe

magnitudes would be known in advance. But such measure- ments are beyond the scope of our present technology and the cost of obtaining such data might well exceed the losses inflicted by the earthquakes themselves. The deterministic prediction of the occurrence of individual earthquakes, with an accuracy approaching that of astronomical, or even meteorological, predictions seems therefore beyond our reach in the foreseeable future, and any predictions couched in deterministic language should be regarded with grave suspicion.

Successful predictions have nevertheless been made on several occasions during the past six years and in at least one case have been instrumental in reducing loss of life. They have been based on the observation of geophysical, geochemical or even biological phenomena which past experience has shown often to precede the occurrence of major earthquakes. This method of prediction, essentially probabilistic in nature, can be summed up in statements of the type:

'Given a set of data relating to recent observations of suspected precursory phenomena, a comparison with data relating to similar phenomena and to earthquakes in the past indicates that the probability of an earthquake of magnitude M occurring in area A in the course of the next N days (months, years) is P.'

This seems likely to be the form that predictions scientific predictions, that is) will take for some consider-

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Figure 2 Acceleration'with 70% probability ofnot being exceeded in 200 years. (Figure copied from Algermissen, S. T. etal . 'Seismic risk evaluation of the Balkan region', In Proc. seminar on seismic zoning maps, Vol. 2, UN ESCO, Skopje, 1976)

Eng. Struct . , 1 9 8 2 , Vo l . 4 , July 1 4 9

An approach to earthquake risk management: E. 114. Fournier d'Albe

able time to come. In this form predictions may in fact be considered as statements regarding periods of enhanced seismic hazard in certain areas, and may thus become amenable to some degree of management.

Seismic vulnerability

The problem of assessing the vulnerability of buildings and structures to seismic ground motion presents itself in radi- cally different terms, depending on whether one is concerned with new or with old structures.

The evaluation and control of vulnerability, through engineering calculations and laboratory tests at the design stage, is part and parcel of earthquake engineering and is dealt with in detail by other papers in this volume.

The situation is totally different in the case of old, and even many recent, buildings whose dynamical characteristics are unknown and cannot easily be discovered. In many parts of the world, including Europe, such buildings represent a large proportion of those in which people live and work. Any serious attempt to assess seismic risk must therefore take them into account.

In the ongoing UNDP/UNESCO project for earthquake risk reduction in the Balkan region, this problem is being attacked in the following manner:

(a) Data on the damage suffered by individual buildings in the region during recent earthquakes (e.g. Skopje, 1963; Adapazari (Turkey), 1967 Gediz (Turkey), 1969; Vrancea (Romania and Bulgaria), 1977;Montenegro (Yugoslavia and Albania), 1979; Corinth (Greece), 1981) are being compiled and reduced to a standard format (b) The degree of damage (specific loss) to each building is expressed on a standard scale. (c) Buildings are classified according to type of structure, materials used, etc., into 12 categories. (d) Vulnerability functions, relating the degree of damage to the intensity of ground motion, are derived for each category of building by statistical analysis of the data.

The major difficulty in carrying through this work is the definition of intensity. Only in the case of the most recent earthquakes (Montenegro, 1979; Corinth, 1981), have instrumentatl records of ground motion been obtained. In these two cases, the degree of damage can be compared directly with objective data on ground motion intensity.

In the remaining cases, one is obliged to fall back on macroseismic scales to express intensity;but since intensity assessments on such scales are based precisely on observed degrees of damage to typical structures, one is caught within a vicious circle. One is trying to discover a relation between vulnerability (a property of the building) and intensity (a parameter expressing ground motion) when intensity is measured on a scale which subsumes certain vulnerabilities for certain types of structure.

Escape from this logical trap will be possible only when instrumental records of ground motion are available for a much larger number of damaging earthquakes. In the mean- time, we shall have to be content with vulnerability functions which simply indicate departures from the vulnerabilities subsumed in the commonly-used macroseismic intensity scales.

The number of strong-motion recorders in operation is increasing rapidly. From less than 10 in 1970, the number in the Balkan region has by now risen to over 300. We may therefore expect a more rapid accumulation of objective data in the future, which will make it possible to derive

reliable vulnerability functions for various types of buildings, relating specific loss to one or more physical parameters of ground motion.

We have spoken so far of the vulnerability of buildings and structures, but no analysis of seismic risk to an urban community would be complete without consideration of the vulnerability of urban lifelines (electricity and water supply lines, heating and sewerage systems, telecommunica- tions, etc). This introduces problems of great difficulty, not only because of the physical complexity of the lifelines themselves but because of the numerous secondary effects of interruptions in their functioning. Further research into this problem is urgently needed.

Risk assessment

As we have already seen, the specific risk to a building or structure is the product of a convolution of its vulnerability with the seismic hazard. Provided that adequate data are available on these two factors, and that intensity of ground motion is expressed by the same parameters in the two sets of data, this convolution presents no particular difficulty.

However, as we have seen above, the assessment of the vulnerability of older buildings presents problems of great complexity, and in most parts of the world there are simply no data available on vulnerability. On the other hand, urban construction goes ahead and architects and planners urgently need information on seismic risk.

The situation is perhaps not quite so desperate as may appear at first sight. Records of earthquake damage have been kept for centuries in many parts of the world, and such records can be used to compile catalogues and to prepare maps on which the observed 'intensity' is expressed on a macroseismic scale. In many cases, the amount of data available is sufficient to permit the derivation of an intensity-frequency curve. But, as we have seen, 'intensity' maps of this kind are, in fact, maps of specific risk to buildings of certain types rather than maps of seismic hazard, and specific risk is precisely what interests planners.

Planners and economists need not therefore despair if precise data on seismic hazard and vulnerability are lacking. In order to obtain an approximate evaluation of the risk to existing buildings, it is not absolutely necessary to know whether earthquakes are caused by convection in the upper mantle of the earth or by ancient heroes turning in their graves. Data on the 'intensity' of past earthquakes are almost certain to be available wherever a significant seismic hazard exists, and such data may be used directly to derive an approximate assessment of risk.

A word of caution must nevertheless be added. The analysis of macroseismic data will not make possible the evaluation of the specific risk to any individual building but only that of the average risk to buildings in the broad categories specified in whichever scale has been used to express intensity.

We have spoken so far of the risk to individual buildings and structures. In order to assess the risk to an aggregate of buildings, such as a town or village, it is first necessary to assign a value to each element at risk, since specific risk is not an additive quantity, and then only can one add together the risks to a number of elements. At this stage it must be borne in mind that the risk to an urban settlement is far greater than the sum of the risks to the individual elements, and that one must also take into account:

possible losses due to secondary causes such as fire following earthquake;


An approach to earthquake risk management: E. M. Fournier d'Albe

possible losses due to the interruption of essential supplies or communications; possible losses due to interruption of industrial production and eventual loss of markets; etc.

It will no doubt have been noted that there has been no reference so far in this paper to the risks to human life. This is because we have taken as our point of departure a definition of risk couched in economic terms. It has been made abundantly clear by others how difficult it is to attach an economic value to human life (indeed, it is easy to show that, in some circumstances, such a value may be negative). This does not mean that the method of analysis used in this paper cannot be adapted to evaluate risk to life.

Human beings are vulnerable to the extent that they live and work inside vulnerable buildings, and there are sufficient data on casualties in past earthquakes for one to estimate the specific risk to human life (i.e. probable percentage of population killed or injured) in a given period of time), if adequate data are available on seismic hazard and the vulnerability of buildings and structures. However, there is one important factor determining human vulnerability which does not affect that of buildings or structures: the time of day at which an earthquake occurs. Experience has shown that this may be the most important single factor determining the number of casualties. Since it is quite impossible at the present time to foresee at what time of day an earthquake may occur, there must necessarily be a great degree of uncertainty in estimating the risk to human life.

Responses to risk The notion of acceptable risk

The seminar on 'lessons learnt from recent earthquakes' would probably never have been held if, in the general opinion, earthquakes presented no more risk than what can be accepted as part of everyday life. Response to earthquake risk is motivated by a feeling that the existing risk is unacceptable and that something should be done to reduce it. But to what level should it be reduced?

The author knows of no country or community in which there exists an established procedure for deciding consci- ously and deliberately on levels of risk which it and its members, collectively and severally, are prepared to accept. One cannot decide what is an acceptable level of risk, one can only observe and take note of what is or is not accepted. This is particularly true of risks to human life, for which the level of acceptance varies greatly from one type of hazard to another (compare, for instance, the accepted risks of death or injury in car accidents with that of an accident to a nuclear power plant).

The method of risk analysis summarized in the present paper offers, on the other hand, a means by which decisions may be reached objectively with regard to the risk of material loss through earthquakes. Such risks can be reduced through control of the spatial distribution of elements or through control of their vulnerability. The additional costs of such action to reduce risk can be balanced against reduction in losses, and an acceptable risk level determined by normal cost-benefit analysis.

Degrees or freedom Of the three factors determining risk (see equation (8)),

we have to accept that first, seismic hazard, lies outside human control. There remain two possibilities for action to reduce seismic risk:

Control of the value and of the spatial distribution of new investments (settlements, factories, etc); Control of the vulnerability of the elements at risk.

In addition, the financial risk borne by those who live or invest in seismically hazardous areas may be reduced by arranging, through the mechanism of insurance, for it to be shared by the population as a whole.

The distribution of elements at risk Since seismic hazard is a function of position, it is

always possible to exert some control over seismic risk by an appropriate choice of site for new constructions.

Seismic risk is, however, only one of many factors to be taken into account in the planning of new settlements or production facilities, and siting in a low-hazard locality may entail additional costs which outweigh or annul the advantage to be gained by reducing risk. This is a matter for analysis by economists and physical planners and has been discussed in some detail by Ciborowski in his companion paper.

Here we need only draw attention to the fact that the seismic hazard at any site depends on two factors: (a) the location of the site with respect to the seismic origin zones; and (b) the local subsoil characteristics.

The first of these two factors, which will vary significantly only over distances of the order of 10 km or more, will be of concern mainly at the level of regional planning. The second, which may vary significantly over distances of 100 m or less, will be of great importance in detailed urban planning. Very little experience has yet been acquired in the use of seismic hazard data in regional or urban planning, and it remains to be discovered exactly what information is required, and in what form, for use in the planning process. An attempt to clarify these issues is now being made in connection with the elaboration of a physical development plan and of urban master plans for the Republic of Monte- negro (Yugoslavia). The main questions appear to be: (a) map scales; (b) the period of time for which seismic hazard should be calculated and (c), whether absolute values of hazard are required, or only relative variations from place to place.

The control and reduction of vulnerability Measures to control and reduce the vulnerability of

structures, of their contents and of the people living and working in them, may usefully be divided into two categories: (a) long-term or permanent, and (b) short-term or temporary.

Long-term measures. Seismic building codes and regulations are the principal means by which vulnerability can be reduced in the long term, provided that they are technically sound and effectively enforced. Such codes already exist in many countries.

The rules of design and construction embodied in such codes and regulations are derived from the technology of earthquake engineering and will not be discussed here. What is of greater interest, within the context of this paper, are their modes of apolication.

Most national building codes are accompanied by a map or table indicating the seismic intensity to be used as a basis for design in various parts of the country or in various administrative areas. In many parts of the world this is still the best that can be done to specify the seismic hazard. It is obvious, however, that the use of a single parameter such as a degree on a macroseismic scale is a gross over-simplification.

Eng. Struct., 1982, Vol. 4, July 151

An approach to earthquake risk management." E. M. Fournier d AIbe

For instance, MSK intensity VIII in Romania represents a very different type of ground motion to that represented by the same intensity in California or Turkey, whereas that in Algeria is likely to be different again. This is due to the differences in earthquake focal mechanism in different parts of the world, and even in different parts of the same country.

Building codes and regulations have to be used by engineers and architects, most of whom cannot be expected to have any detailed knowledge of seismology or earthquake engineering. They must therefore be relatively straightforward and easy to apply in practice. On the other hand, they must be effective, and to be effective they must take into account the extreme complexity of seismic ground motion and of the dynamic response of structures to this motion. Their formulation thus presents problems whose difficulty seems likely to increase rather than decrease with the acquisition of new knowledge. The introduction of the concepts of risk management may complicate the matter still further.

The vulnerability of new buildings may be reduced by the publication of appropriate building codes, but what can be done about the older buildings which will remain in use for a long time to come? Their vulnerability is difficult to assess and their strengthening often presents very difficult technical problems. Furthermore, the cost of strengthening may some times exceed the cost of demolition and replacement.

In any case, a choice has to be made between three alternatives: (a) to strengthen the building; (b) to demolish and replace it; or (c) to leave it as it is. In the case of older buildings of historical or architectural interest, the decision will depend not only on risk analysis in economic terms but on the cultural value attached to the building in its present form. This will involve subjective judgement, individual or collective, which is likely to vary widely from case to case.

Short-term and temporary measures. Earthquake prediction may still be far from becoming an exact science, but the fact remains that predictions are being made and com- municated to the public. Whatever their scientific value, they result in great pressure being put on the authorities responsible for public safety, to take some action to mitigate the effects of the expected catastrophe. Certain measures can in fact be taken to reduce vulnerability during periods of exceptional hazard, real or imaginary, for example:

lowering of water level in reservoirs, protection of valuable property within buildings; safe storage of toxic or inflammable substances; reinforcement or mobilization of fire-fighting and medical services; stockpiling of food and emergency shelters; evactuation of people from vulnerable buildings.

All such measures entail some degree of disturbance to normal life and, if not justified by the occurrence of the expected earthquake, will have to be in terms of acceptable risk. The responsibility for decision-making in such circumstances is extremely heavy and may have important social and economic consequences. Situations of this kind must clearly' be envisaged and prepared for in advance.

The basic rule of decision-making may be:

'Take a particular action when the risk to a given area exceeds a specified value'

The problem before a government or a community is there-

fore to decide on a series of threshold values of risk whose attainment will automatically trigger appropriate precau- tionary measures planned in advance. A procedure of this kind seems indispensable if one is to avoid irrational or 'Panic' reactions to emergency situations.

It remains to be seen how such a procedure can be tbrmulated and put into practice. In most countries, the civil defence organization is probably the most appropriate body to take the leading role, on condition that it works in close cooperation with seismological and engineering institutions.

Risk-sharing

Insurance and re-insurance offer a means of relieving those whose property is damaged or destroyed by an earthquake from bearing the full brunt of the disaster, and of spreading the burden of risk among the population as a whole. However, except in certain countries (e.g. New Zeland) where national earthquake insurance schemes exist, the availability and conditions of insurance against earthquake risk are extremely variable. There is indeed a growing reluctance among re-insurance companies to underwrite earthquake insurance because of the magnitude of the risks involved and the lack of data on vulnerability and seismic hazard. The Unesco working group which examined this subject identified the main problem as:

lack of sufficient exchange of knowledge between scientists and the insurance industry; insufficient coordination within the insurance industry; insufficient coordination of ways and means between the insurance industry and governments for the reduction of risk; lack of readily available information on the location and vulnerability of buildings at risk; insufficient use of the data available, impact of inflation on insurance values and catastrophe reserves.

Furthermore, the existence of predictions may profoundly modify the conditions under which insurance operates.

Several large re-insurance companies have now set up their own scientific groups to study and advise on these problems. At all events, it is clear that insurance is one of the most important elements of earthquake risk management.

Conc lud ing r e m a r k s

In this paper we have attempted to formulate a conceptual basis for regarding earthquakes not as a random series of catastrophes but as a problem of social and economic risk which can be dealt with as part of everyday life. The main idea is that one may act to mitigate their impact rather than simply react to the losses that they inflict.

Given the complexity of the physical phenomena themselves, and of their social and economic effects, we have had to gloss over or omit discussion of many important technical problems. The general principles that we have adopted will, we hope, withstand closer scrutiny than it has been possible to give them here.

Refe rences

1 UNDRO. 'Natural disasters and vulnerability analysis: Report of expert group meeting', Geneva, 1979

2 UNESCO. 'Proceedings of the seminar on seismic zoning maps', Skople, 1976


an approach to earthquake risk management

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