Injury (1990)21, 17-20
Printed in Great Btitain
17
Earthquake occurrence
and effects
R. D. Adams Seismologist
International
Seismological
Centre, Newbury,
Berks and Department
Although earthquakesare mainly concentrated in zones close to bouna%es of tectonic plates of the Earth’s lithosphere, infrequent events awayfrom the main seimic regions can cawe wmjor disasters. The major cause of damage and injury following earthquakes is elastic vibration, rather than fault displacement. This vibration at a particular site will depend not only on the size and distance of the earthquake but also on the local soil conditions. Earfhquake prediction is not yet generally fruitful in avoiding earfhquake disasters, but much useful planning to reduce earfhqtu.&e effects can be done by studying fhe general earthquake hazard in an area, and faking SOWE simple precautions.
Introduction Earthquakes are among the major natural disasters that produce casualties, and involve the medical profession in relief work. They always appear to be unexpected, and are naturally terrifying happenings. There are still many misconceptions about their occurrence and effects, and this paper attempts to give some basic information that could be useful to those who may be called to an earthquake-stricken area, or who plan for the possibility. Seismologists know almost immediately when a major earthquake occurs, and a preliminary position and size can often be calculated within I h. It is not always possible to tell from such a preliminary location, however, whether a civil disaster has occurred - this will depend on the earthquake’s position with respect to centres of human activity, and on the vulnerability of these centres to earthquake shaking. Disasters caused by small events very close to remote centres of population may be hard to locate by seismological means alone. For major events the first reliable information about the location and severity of the disaster is often available through the news channels, and at these times the media and seismological agencies work closely together to pool available information.
Global earthquake distribution The distribution of world earthquakes is now well known, with the main boundaries between the ‘plates’ of the Earths lithosphere marked by clear zones of activity along midoceanic ridges, and often at continental boundaries where one plate is being pushed beneath another. The distribution of earthquakes along these zones, however, is very variable, 0 1990 Butterworth & Co (Publishers) Ltd 0020-1383/90/010017~4
of Geoscience
University
of Reading, UK
with most of the global energy release around the edge of the Pacific Ocean, and about 80 per cent along the western rim. A second major zone of activity extends from Indonesia, through the Himalayas, Middle East, Turkey and Southern Europe. Detailed maps of seismicity show a large number of events. Each year the International Seismological Centre, in Berkshire, documents about 25 000 to 30 000 earthquakes. Naturally, most of these are far too small to be felt, let alone produce any adverse effects. In a typical year about one earthquake will reach magnitude 8, about one a month magnitude 7, two a week magnitude 6 and so on. Richter magnitude is a logarithmic estimate of the total energy released, with each change of one unit corresponding to a tenfold change in ground motion, or a thirtyfold change in radiated energy. Magnitude, which is determined from seismographs, must not be confused with felt intensity, which is a description of earthquake effects at a particular place. Intensity is generally given on the Modified-Mercalli (MM) scale, or in Europe on the Medvedev-SponheuerKamik (MSK) scale. The largest earthquakes have magnitudes about 9, while most intensity scales rise to 12 for total destruction. Seismologists never use the word ‘force’. Figures I and 2 show parts of the global plot of earthquakes computed by ISC. Although most earthquakes occur near plate boundaries, a significant number occur in the more stable ‘intraplate’ regions, and these often have unexpectedly severe effects, as will be explained later.
Earthquake effects Much attention is paid to the ground breakage or surface faulting that sometimes accompanies large earthquakes. These breaks are spectacular, but unless a structure is actually straddling such a break, the effects near a fault need not be too severe. It is the vibrational energy accompanying the earthquake rupture that causes the damage, and it is almost invariably the failure of structures from shaking that causes death and injury - exceptions are earthquake-induced landslides, avalanches and tsunamis. The shaking experienced during an earthquake will depend on three factors. First the size, or magnitude of the event itself, second, the distance from the point of release of energy, and last, but probably most important, the type of
Injury: the British Journal of Accident Surgery (1990) Vol. 21/No.
18
Figure 1. Earthquakes located by International in the Earth’s outer layers.
Seismological
Centre for period 1964-1983,
soil at the site. Good solid rock transmits earthquake energy with a minimum of vibration, but poor, unconsolidated soils, such as are often found in river valleys or in areas of poorly reclaimed land can amplify the earthquake vibrations to Surface layers can also produce alarming proportions. resonances that match the natural period of buildings of particular heights. An extreme example of this is the Mexico City event of 1985, where although the earthquake was several hundred kilometres away, the properties of the old lake bed on which the city is founded were such that bu~~-~~~s 6-10 storeys high were particularly badly The most common cause of earthquake injury is vibration-induced failure of structures, which results in entrapment of victims. These structures may range from
delineating
main boundaries
between
I
‘plates’
simple dwellings, with heavy roofing material, to large multistorey buildings which collapse totally. Earthquakes will also cause loss of community services, such as power, gas, water, sewerage, and disrupt communications. These will not only affect the surviving population, but will hinder rescue work, and often encourage the spread of disease.
Earthquake countermeasures We must face the fact that we cannot stop earthquakes from occurring. Much interest has always been shown in earthquake prediction, but this has had minimal success and is of doubtful practical value. Even if seismologists could predict precisely the place, time and size of an earthquake, it is not
Adams: Earthquake occurrence and effects
19
Figure 2. Earthquakes located by International Seismological Centre in the Pacific area for period 19’7-1983. Pacific contains about 80 per cent of the world’s earthquakes.
clear what steps to take, and western societies would find it hard to accept the social disruption associated with prediction, particularly with false alarms. The only successful prediction of a major earthquake in the social sense remains that in northeast China in 1975. Following a long-term prediction from seismicity patterns and mid-term study of ground deformation, a successful identification of foreshocks resulted in the 100000 inhabitants of Haicheng moving outside during the afternoon of 4 February, and almost all were unharmed when the magnitude 7.3 earthquake occurred in the early evening. No such warning preceded the major disaster at Tangshan 18 months later when at least 250 000 people were killed. Prediction in its broadest sense remains a scientific aim, and many smaller events have been successfully predicted in the scientific sense, often within limits of time and place that are acceptable mathematically, if not for administrative purposes. Even if not of social value, these studies provide a growing insight into the fundamental mechanism and cause of earthquakes. At present the US Geological Survey has made a long-term prediction for a section of the San Andreas fault near the small town of Parkfield. Indications are that an earthquake of magnitude 6 will happen before 1993, and monitoring systems of all types have been set up in an effort
The western
side of the
to obtain a more precise warning. The lack of success at earthquake prediction does not mean, however, that there is nothing we can do to counteract earthquake threat. First we must define the threat. This can be expressed in a variety of ways. Seismologists use the term ‘hazard to express the likelihood of earthquake shaking. One useful way is to determine the probability that a certain place will experience shaking of a certain severity in a given time interval. Other maps show the maximum shaking expected. Hazard alone, however, does not define the likelihood of disasters. This is determined by a combination of hazard, the distribution of human population and their structures and the construction standards. Thus, in areas of frequent earthquake occurrence like California and Japan, there is great public awareness of eathquake danger, and efforts are made to take appropriate countermeasures. On the other hand, in areas away from the main plate boundaries, infrequent earthquakes may affect structures that have not previously been subjected to such a degree of shaking and are poorly constructed, resulting in a major disaster. Also, human memory is not longer than a decade or two, and communities tend to forget disasters that have affected earlier generations. It is not always the biggest earthquakes that attract the
20
Injury: the British Journal of Accident Surgery (1990)
most attention. The Armenian earthquake of December 1988, which killed at least 25 000 people, was of magnitude 6.8, a size that occurs somewhere in the world every few weeks. In May 1989, a magnitude 8+ shock south of New Zealand, which released more than 150 times the energy of the Armenian event, passed almost unnoticed, because of its position. At the other end of the scale, the Agadir earthquake in 1960, which killed 12 000 Moroccans, was of magnitude only 5.6, but was immediately beneath a city of inadequate construction standards. In compiling hazard maps, it is important to combine information from as many sources as possible, and recent seismological information must be combined with reevaluations of early instrumental and historical earthquake studies, in addition to geological and geodetic studies, the determinations of soil conditions and slope stability. Hazard information has to be made available to planners, architects and engineers who have to incorporate these ideas into their activities. Many precautions are simple common sense. The most unstable ground should be reserved for parks or other open areas. Structures should be designed so that if they fail they will not harm people. Buildings should be symmetrical, and should not have open lower storeys. The use of timber-framed houses and light roofing materials should be encouraged. Planning in earthquake-prone areas is essential. In California and in Japan there are Departments of Preparedness education and community concerned with planning, response. In New Zealand, Civil Defence does not mean only measures against nuclear attack, but is primarily aimed towards help in earthquakes, fires and floods. Communities hold Civil Defence exercises and every school has regular earthquake drill. In private houses in earthquake areas, simple precautions can help reduce earthquake effects. Cupboards should be he!d by bolts, not latches, and dangerous items such as
Vol. t l/No.
1
paraffin heaters should be securely fastened to the wall. Each dwelling should also have an emergency supply of water, food and medical supplies.
Conclusions Careful precautions such as those mentioned above will reduce earthquake effects, but disasters will still occur. In these instances, relief teams including medical staff will be rushed to the area, and the following points may be relevant to note. The seventy of shaking in a small earthquake can be just as high as in a large one, but over a small area. The larger the earthquake, the larger the area of high intensity. The area affected is roughly (M - 4) km2, so a magnitude 6 earthquake will have a linear dimension of 10 km, a magnitude 7, 30 km, magnitude 8,100 km and so on. Do not neglect the effect of aftershocks, both in compounding damage and their effects on people, both the local population and relief workers. Aftershocks can have a very disturbing psychological effect on people who have been through the main event. Aftershocks may keep one awake at night, and lack of sleep affects efficiency and judgement. Remember that scientists and engineers will be leaving to other experts the humanitarian activities and trying to concentrate on gathering technical information that may help to alleviate future disasters. This does not mean that they are heartless, but they have to gather information while it is available. These are a few points on earthquakes as seen by a seismologist, which may help medical staff to better plan and carry out their relief work in earthquake-stricken areas.
Requests for reprinfs shauti be addressed to: R. D. Adams, International Seismological 4NS, LJK.
Centre,
Pipers Lane, Thatcham,
Newbury
RG13
Printed in Great Btitain
17
Earthquake occurrence
and effects
R. D. Adams Seismologist
International
Seismological
Centre, Newbury,
Berks and Department
Although earthquakesare mainly concentrated in zones close to bouna%es of tectonic plates of the Earth’s lithosphere, infrequent events awayfrom the main seimic regions can cawe wmjor disasters. The major cause of damage and injury following earthquakes is elastic vibration, rather than fault displacement. This vibration at a particular site will depend not only on the size and distance of the earthquake but also on the local soil conditions. Earfhquake prediction is not yet generally fruitful in avoiding earfhquake disasters, but much useful planning to reduce earfhqtu.&e effects can be done by studying fhe general earthquake hazard in an area, and faking SOWE simple precautions.
Introduction Earthquakes are among the major natural disasters that produce casualties, and involve the medical profession in relief work. They always appear to be unexpected, and are naturally terrifying happenings. There are still many misconceptions about their occurrence and effects, and this paper attempts to give some basic information that could be useful to those who may be called to an earthquake-stricken area, or who plan for the possibility. Seismologists know almost immediately when a major earthquake occurs, and a preliminary position and size can often be calculated within I h. It is not always possible to tell from such a preliminary location, however, whether a civil disaster has occurred - this will depend on the earthquake’s position with respect to centres of human activity, and on the vulnerability of these centres to earthquake shaking. Disasters caused by small events very close to remote centres of population may be hard to locate by seismological means alone. For major events the first reliable information about the location and severity of the disaster is often available through the news channels, and at these times the media and seismological agencies work closely together to pool available information.
Global earthquake distribution The distribution of world earthquakes is now well known, with the main boundaries between the ‘plates’ of the Earths lithosphere marked by clear zones of activity along midoceanic ridges, and often at continental boundaries where one plate is being pushed beneath another. The distribution of earthquakes along these zones, however, is very variable, 0 1990 Butterworth & Co (Publishers) Ltd 0020-1383/90/010017~4
of Geoscience
University
of Reading, UK
with most of the global energy release around the edge of the Pacific Ocean, and about 80 per cent along the western rim. A second major zone of activity extends from Indonesia, through the Himalayas, Middle East, Turkey and Southern Europe. Detailed maps of seismicity show a large number of events. Each year the International Seismological Centre, in Berkshire, documents about 25 000 to 30 000 earthquakes. Naturally, most of these are far too small to be felt, let alone produce any adverse effects. In a typical year about one earthquake will reach magnitude 8, about one a month magnitude 7, two a week magnitude 6 and so on. Richter magnitude is a logarithmic estimate of the total energy released, with each change of one unit corresponding to a tenfold change in ground motion, or a thirtyfold change in radiated energy. Magnitude, which is determined from seismographs, must not be confused with felt intensity, which is a description of earthquake effects at a particular place. Intensity is generally given on the Modified-Mercalli (MM) scale, or in Europe on the Medvedev-SponheuerKamik (MSK) scale. The largest earthquakes have magnitudes about 9, while most intensity scales rise to 12 for total destruction. Seismologists never use the word ‘force’. Figures I and 2 show parts of the global plot of earthquakes computed by ISC. Although most earthquakes occur near plate boundaries, a significant number occur in the more stable ‘intraplate’ regions, and these often have unexpectedly severe effects, as will be explained later.
Earthquake effects Much attention is paid to the ground breakage or surface faulting that sometimes accompanies large earthquakes. These breaks are spectacular, but unless a structure is actually straddling such a break, the effects near a fault need not be too severe. It is the vibrational energy accompanying the earthquake rupture that causes the damage, and it is almost invariably the failure of structures from shaking that causes death and injury - exceptions are earthquake-induced landslides, avalanches and tsunamis. The shaking experienced during an earthquake will depend on three factors. First the size, or magnitude of the event itself, second, the distance from the point of release of energy, and last, but probably most important, the type of
Injury: the British Journal of Accident Surgery (1990) Vol. 21/No.
18
Figure 1. Earthquakes located by International in the Earth’s outer layers.
Seismological
Centre for period 1964-1983,
soil at the site. Good solid rock transmits earthquake energy with a minimum of vibration, but poor, unconsolidated soils, such as are often found in river valleys or in areas of poorly reclaimed land can amplify the earthquake vibrations to Surface layers can also produce alarming proportions. resonances that match the natural period of buildings of particular heights. An extreme example of this is the Mexico City event of 1985, where although the earthquake was several hundred kilometres away, the properties of the old lake bed on which the city is founded were such that bu~~-~~~s 6-10 storeys high were particularly badly The most common cause of earthquake injury is vibration-induced failure of structures, which results in entrapment of victims. These structures may range from
delineating
main boundaries
between
I
‘plates’
simple dwellings, with heavy roofing material, to large multistorey buildings which collapse totally. Earthquakes will also cause loss of community services, such as power, gas, water, sewerage, and disrupt communications. These will not only affect the surviving population, but will hinder rescue work, and often encourage the spread of disease.
Earthquake countermeasures We must face the fact that we cannot stop earthquakes from occurring. Much interest has always been shown in earthquake prediction, but this has had minimal success and is of doubtful practical value. Even if seismologists could predict precisely the place, time and size of an earthquake, it is not
Adams: Earthquake occurrence and effects
19
Figure 2. Earthquakes located by International Seismological Centre in the Pacific area for period 19’7-1983. Pacific contains about 80 per cent of the world’s earthquakes.
clear what steps to take, and western societies would find it hard to accept the social disruption associated with prediction, particularly with false alarms. The only successful prediction of a major earthquake in the social sense remains that in northeast China in 1975. Following a long-term prediction from seismicity patterns and mid-term study of ground deformation, a successful identification of foreshocks resulted in the 100000 inhabitants of Haicheng moving outside during the afternoon of 4 February, and almost all were unharmed when the magnitude 7.3 earthquake occurred in the early evening. No such warning preceded the major disaster at Tangshan 18 months later when at least 250 000 people were killed. Prediction in its broadest sense remains a scientific aim, and many smaller events have been successfully predicted in the scientific sense, often within limits of time and place that are acceptable mathematically, if not for administrative purposes. Even if not of social value, these studies provide a growing insight into the fundamental mechanism and cause of earthquakes. At present the US Geological Survey has made a long-term prediction for a section of the San Andreas fault near the small town of Parkfield. Indications are that an earthquake of magnitude 6 will happen before 1993, and monitoring systems of all types have been set up in an effort
The western
side of the
to obtain a more precise warning. The lack of success at earthquake prediction does not mean, however, that there is nothing we can do to counteract earthquake threat. First we must define the threat. This can be expressed in a variety of ways. Seismologists use the term ‘hazard to express the likelihood of earthquake shaking. One useful way is to determine the probability that a certain place will experience shaking of a certain severity in a given time interval. Other maps show the maximum shaking expected. Hazard alone, however, does not define the likelihood of disasters. This is determined by a combination of hazard, the distribution of human population and their structures and the construction standards. Thus, in areas of frequent earthquake occurrence like California and Japan, there is great public awareness of eathquake danger, and efforts are made to take appropriate countermeasures. On the other hand, in areas away from the main plate boundaries, infrequent earthquakes may affect structures that have not previously been subjected to such a degree of shaking and are poorly constructed, resulting in a major disaster. Also, human memory is not longer than a decade or two, and communities tend to forget disasters that have affected earlier generations. It is not always the biggest earthquakes that attract the
20
Injury: the British Journal of Accident Surgery (1990)
most attention. The Armenian earthquake of December 1988, which killed at least 25 000 people, was of magnitude 6.8, a size that occurs somewhere in the world every few weeks. In May 1989, a magnitude 8+ shock south of New Zealand, which released more than 150 times the energy of the Armenian event, passed almost unnoticed, because of its position. At the other end of the scale, the Agadir earthquake in 1960, which killed 12 000 Moroccans, was of magnitude only 5.6, but was immediately beneath a city of inadequate construction standards. In compiling hazard maps, it is important to combine information from as many sources as possible, and recent seismological information must be combined with reevaluations of early instrumental and historical earthquake studies, in addition to geological and geodetic studies, the determinations of soil conditions and slope stability. Hazard information has to be made available to planners, architects and engineers who have to incorporate these ideas into their activities. Many precautions are simple common sense. The most unstable ground should be reserved for parks or other open areas. Structures should be designed so that if they fail they will not harm people. Buildings should be symmetrical, and should not have open lower storeys. The use of timber-framed houses and light roofing materials should be encouraged. Planning in earthquake-prone areas is essential. In California and in Japan there are Departments of Preparedness education and community concerned with planning, response. In New Zealand, Civil Defence does not mean only measures against nuclear attack, but is primarily aimed towards help in earthquakes, fires and floods. Communities hold Civil Defence exercises and every school has regular earthquake drill. In private houses in earthquake areas, simple precautions can help reduce earthquake effects. Cupboards should be he!d by bolts, not latches, and dangerous items such as
Vol. t l/No.
1
paraffin heaters should be securely fastened to the wall. Each dwelling should also have an emergency supply of water, food and medical supplies.
Conclusions Careful precautions such as those mentioned above will reduce earthquake effects, but disasters will still occur. In these instances, relief teams including medical staff will be rushed to the area, and the following points may be relevant to note. The seventy of shaking in a small earthquake can be just as high as in a large one, but over a small area. The larger the earthquake, the larger the area of high intensity. The area affected is roughly (M - 4) km2, so a magnitude 6 earthquake will have a linear dimension of 10 km, a magnitude 7, 30 km, magnitude 8,100 km and so on. Do not neglect the effect of aftershocks, both in compounding damage and their effects on people, both the local population and relief workers. Aftershocks can have a very disturbing psychological effect on people who have been through the main event. Aftershocks may keep one awake at night, and lack of sleep affects efficiency and judgement. Remember that scientists and engineers will be leaving to other experts the humanitarian activities and trying to concentrate on gathering technical information that may help to alleviate future disasters. This does not mean that they are heartless, but they have to gather information while it is available. These are a few points on earthquakes as seen by a seismologist, which may help medical staff to better plan and carry out their relief work in earthquake-stricken areas.
Requests for reprinfs shauti be addressed to: R. D. Adams, International Seismological 4NS, LJK.
Centre,
Pipers Lane, Thatcham,
Newbury
RG13
