All of us are aware of the human disasters which are sometimes caused by earthquakes. In the course of history, some of these have been of immense proportions, but the hazard is a continuous one in many parts of the world, and the few examples which follow will suffice to illustrate the magnitude of the problem:
The rational response to any danger is to study the natural phenomenon which gives rise to it. In the case of earthquakes, the study has proceeded outwards from the centres of the most disastrous events. Thus the earliest scientific studies were confined to the observation of gross (macroseismic) effects in the region surrounding the so-called "epicentre", which is defined as the point on the surface of the ground at which the effects of the earthquake were most intense. At a later stage of development, it was found that earthquake waves could be detected instrumentally at great distances from the epicentre, and it was realised that the true source, or "focus" was often at a very considerable depth within the earth.
The possibility of remote observation brought a new element of scientific detachment into the study of earthquake phenomena, by separating the contributing factors in the interaction of the natural event on a human environment. Sometimes, the instruments detect great earthquakes in sparsely inhabited parts of the earth when little or no disturbance to human life is reported. At other times, great destruction and loss of life arise from the occurrence of a rather small earthquake in the immediate vicinity of a poorly constructed city. Nowadays, the instruments of the world network of seismograph stations have become so sensitive that the waves from minor earthquakes can be detected thousands of miles from the point of origin. The pattern of earthquake occurrence shows a great preponderance of energy release round the borders of the Pacific Ocean. This is seen to be markedly different from the pattern of earthquake effects on humanity, in which high population density combined with a high proportion of unsuitable buildings tends to concentrate the human toll into the Mediterranean basin, the Middle East and some parts of the Pacific coast (Fig. 1).
Recognising the earthquake as a natural event which sometimes produces disastrous effects, the humanitarian may well place the alleviation of suffering at the head of his list of priorities. To achieve his main objective, he must combine an understanding of the natural phenomenon with the technological means of protecting people from harm. His operations must necessarily be influenced by local conditions, both in relation to the natural hazard and in respect of the resources which each community can deploy in its own defence.
One soon finds, however, that local problems cannot be considered in isolation. Earthquake hazards are related to soil conditions, geological structure and tectonic activity, which must be studied on a regional basis. A regional authority starts with a sphere of interest which may well extend across several national frontiers, but soon finds itself in need of data from other parts of the world. Eventually, one comes to realise that the structure is indivisible, and that organisations at all levels must co-operate within a world-wide pattern. Logical exposition now requires the inversion of the human scale of priorities. The objectives and potentialities of the world network must be discussed first, so that the contribution which it can make to the solution of the regional problems can be understood. When the regional pattern has been revealed, we are in a position to consider the problems of the individual community.
In between the seismologist and the people who need protection from the effects of earthquakes, we find the earthquake engineer, who has the responsibility of ensuring that new structures have the proper degree of resistance to the earthquake hazards of their environment. His interests unite with those of the seismologist in the estimation of the size and frequency of earthquakes in various parts of the world, and with architects, planners and insurance companies at the other end of the scale.
In view of the essential continuity of interest, it is a remarkable fact that interaction between the major groups remained at a comparatively low level until quite recently. Happily, the situation is now improving, and numerous meetings are now taking place with the object of improving the understanding of the overall problem. The Report of the Intergovernmental Meeting on the Assessment and Mitigation of Earthquake Risk (UNESCO, 1976) makes an important contribution in this respect.
The permanent recording stations of the world network are collectively capable of responding to earthquake waves over a range of frequency extending from ten cycles per second down to those of the free oscillations of the whole earth, which has natural periods in excess of an hour. A few stations can detect the permanent distortion of the whole earth that results from the dislocation that occurs at the focus of a distant earthquake.
The waves which reach the station may have travelled through the interior of the earth, or may have been propagated as 'surface waves' around its circumference, but in all cases the information that can be derived from the record will be the time of occurrence, amplitude and frequency of a visible disturbance. It is necessary to combine readings from a number of stations to determine the time, position and magnitude of the source. When these parameters have been determined, the record of any one station can be interpreted in detail, and the information that it yields will relate to the passage of seismic energy along a variety of possible paths from the known source to the known point of detection.
The instruments which remain in active service have a wide range of sensitivity, and the environmental conditions range from those at the most isolated locations in the centres of continents to those which are sited on oceanic islands and in the centres of cities. In consequence, the detection threshold varies over a range of more than 1000:1 between one station and another, and it may be more difficult to detect the passage of a seismic wave across a noisy site within 100 km of the source, than it is to observe the same wave at a quiet station 10 000 km away. A few of the world's most sensitive stations are thereby producing a large proportion of all the available observations of small events, particularly those which occur outside the areas served by sensitive local networks. On the other hand these highsensitivity stations are easily deflected off scale by strong shocks, which are of particular importance for theoretical and practical investigation. It is therefore very desirable for such stations to include instruments of low and medium sensitivity in addition to their high-sensitivity equipment.
The main body of stations in the world network are producing readings of the waves from events which are substantially above the detection threshold of the most sensitive stations. The essential contribution of these stations is to provide simultaneous observations of medium-size events along numerous propagation paths through the earth. These interlocking observations provide a great deal of information about the mechanism of the event, and about the internal structure of the earth. Uniform geographical coverage, wide range in frequency response, extended dynamic range and the highest practicable degree of standardisation between stations are the important requirements for this network. Many recommendations, made at international meetings, have been aimed at producing improvements in this respect.
It is particularly necessary to emphasise the contribution which aseismic countries can make, as a quiet, stable platform in a region of undisturbed geology is the chief requirement for long-range seismological observations. The advantage of placing a seismological station in a quiet position is comparable to that of siting an astronomical observatory far away from the lights of cities.
Short-range networks in seismic areas can also make a contribution to world seismology, by introducing additional precision into the timing and location of the events which occur within their radius of detection. The importance of this contribution is increasing as the world network becomes more sensitive, because events which a few years ago were regarded as of purely local significance are now being detected at great distances.
We can therefore summarise the objectives of the world network as the detection of seismic waves and the location of their sources in space and time. This basic information serves research in the following ways:
The special requirements for regional organisation were first formulated explicitly by Bath in 1960, and have since been discussed at a number of international meetings. The basic postulate is that the earth can be divided into regions, wherein social and technical factors favour a more intimate degree of co-operation between seismological institutions than that which it is practicable to achieve directly on a world-wide scale. Regional centres are therefore suggested as agencies for the collection, storage and interpretation of seismological data relating to the region, for fostering co-operation between the stations of the region and the international centres, and for stimulating all types of seismological work on the regional scale.
The optimum geographical extent of the region depends partly on the extent of seismotectonic features which have to be studied as a whole, partly on the extent to which common social and economic conditions provide a basis for co-operation between neighbouring peoples, and partly on the fact that records made within about 1000 km of the epicentre can be interpreted much more effectively in collections than as separate pieces of data. In some of the largest countries, national centres fulfil the regional functions. The essential power of the new approach is in fostering co-operation across national boundaries between groups of smaller countries.
In the study of seismicity, the regional picture is part of the world pattern, and if the region contributes short-range data, local interest and local knowledge, the difference between the regional and world-wide approach is of priority and detail rather than of kind. Other problems, such as the investigation of crustal structure, or the association of earthquake foci with geological features, inherently require observations to be made inside the region. In problems such as the assessment of an earthquake hazard on a given site the final study may be extremely localised, and it is the regional picture which provides the relatively extensive background.
We have seen that world-wide and regional studies can indicate the distribution of earthquake foci within the earth, and can lead to an understanding of other factors, such as the influence of foundation conditions and type of construction, which define the earthquake hazard in a particular place. The methods of reducing the risk are to make buildings more resistant to earthquake damage, and to ensure that communities in dangerous locations are provided with appropriate emergency services.
It is often assumed that the organisation of protective measures must necessarily be expensive to the community, but there are several factors that can reduce or even eliminate the total cost. If the hazards are not understood, some structures will be designed for excessive resistance, whereas others may be so obviously unsafe as to be uninsurable at normal rates. Considerable improvements in design and construction can be introduced at little cost (ranging from 4% to 20% of the total), and zoning laws can sometimes be framed so as to keep buildings with high human occupancy off unsafe ground. It would be wrong to exaggerate these savings, because a reaction to any danger must of course limit the ways in which a society can use its resources. The crucial argument is that if a hazard is understood the risk of ill effects can be minimised.
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