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What is an earthquake?
An earthquake is the vibration of the terrestrial crust caused by the sudden liberation of energy that occurs when the earth’s internal tectonic plates move.
Background and nature of the earthquakes
An intense earthquake that occurs in any large city in the United States has the potential to be the most catastrophic natural disaster for the country. Large earthquakes threaten life and damage property, creating a chain of events that upset natural and man made environments. A strong and prolonged shock is a geologic effect that can severely damage buildings or cause them to collapse. The vibratory movements of earthquakes can induce secondary geologic effects such as the liquefaction of the ground, landslides and structural damage to buildings. They can also trigger marine seismic waves (tsunamis/tidal waves) that can cause coastal destructions thousands of kilometers from the epicenter. Earthquakes can also have massive non-geologic effects that can be more catastrophic than the initial effects of the earthquake (such as fires, floods from damaged dykes, and spills of toxic or radioactive materials).
Relative relevance of disasters from earthquakes
During the past 20 years, earthquakes have caused more than a million deaths worldwide (1). More than 80% of the deaths due to earthquakes during this century have occurred in 9 countries, and almost half in only one, China. On July 28th, 1978 at 3:42 a.m., an earthquake of 7.8 magnitude occurred in Tangshan, in the northeast of China. In just a few seconds, an industrial city with a population of one million was reduced to ruins, with more than 240,000 deceased (2). The accelerated urbanization of areas in the world of seismic activity, whose populations reach 20,000 to 60,000 inhabitants per square kilometer, emphasizes the vulnerability of such areas in the face of a catastrophic number of deaths and injuries due to an earthquakes effect. In the last 10 years, the world has experienced 4 catastrophic earthquakes with massive loss of lives: Mexico City, 1985 (10.000 deaths); Armenia, 1988 (25,000 deaths); Iran, 1990 (40,000 deaths) and India, 1993 (10,000 deaths) (table 8-1).
Table 8.1 Earthquakes that caused more than 10,000 deaths during the 20th Century.
Year |
Size (magnitude) |
Deaths |
1985 |
Mexico City, Mexico (M 8,1 y 7,3) |
10.000 |
1993 |
India (M 6,4) |
10.000 |
1960 |
Agadir, Morocco (M 5,9) |
12.000 |
1968 |
Dasht-i-Beyaz, Iran (M 7.3) |
12.000 |
1962 |
Buyin Zara, Iran (M 7,3) |
12.225 |
1917 |
Indonesia (M 7,0+) |
15.000 |
1978 |
Tabas, Iran (M 7,7) |
18.200 |
1905 |
Kangra, India (M 8,6) |
19.000 |
1948 |
Ashkabad, USSR (M 7,3) |
19.800 |
1974 |
China (M 6,8) |
20.000 |
1976 |
Guatemala City (M 7,5) |
23.000 |
1988 |
Armenia, URSS (M 6,9) |
25.000 |
1935 |
Quetta, Pakistan (M 7,5) |
25.000 |
1923 |
Concepción, Chile (M 8,3) |
25.000 |
1939 |
Chillán, Chile (M 8,3) |
28.000 |
1915 |
Avezzano, Italy (M 7,5) |
32.610 |
1939 |
Erzincan, Turkey (M 8,0) |
32.700 |
1990 |
Iran (M 7,7) |
40.000 |
1927 |
Tsinchai, China (M 8,0) |
40.912 |
1908 |
Messina, Italy (M 7,5) |
58.000 |
1970 |
Ankash, Peru (M 8,3) |
66.794 |
1923 |
Kantto, Japan (M 8.3) |
142.807 |
1920 |
Kansu, China (M 8,5) |
200.000 |
1976 |
Tang Shan, China (M 7,8) |
242.000 |
Total |
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Aprox.1´500.000 |
Contributing factors to earthquake disasters
An earthquake can cause a great number of outcomes, depending on its magnitude, its proximity to an urban center and the level of preparation and implemented measures of mitigation. At 5:04 p.m. on Tuesday October 17th, 1989, an earthquake of 7.1 magnitude with the epicenter near the tip of Loma Prieta in the mountains of Santa Cruz to the north of California, caused 62 deaths and 3,000 injured (5). This was the most destructive earthquake in the United States since an earthquake in the San Fernando Valley to the south of California in 1971. The earthquake in Loma Prieta brought forth comparisons to the earthquake in Armenia on December 7th, 1988. The Armenian earthquake released less than half of the energy (magnitude 6.9) as Loma Prieta yet resulted in 25,000 deaths and 18,000 injured. The difference in the impact between the two earthquakes is directly related to the level of preparation and mitigation between the California and the old Soviet Union (6-7). The strict fulfillment of building codes in the last two decades has undoubtedly saved many lives and thousands of earthquake resistant buildings (8-9).
Given the increase in populations in high risk areas, the number of affected will continue to rise in the future.
Factors affecting the occurrence and severity of earthquakes
Natural factors
Earthquakes tend to be concentrated in particular regions of the earth’s surface that coincide with the edges of the tectonic plates in which the terrestrial crust is divided. The tectonic plates do not slide smoothly along their overlapping edges. This creates deformations in the rocks on each side of the borders of the plates. As the rocks become deformed they store massive amounts of energy. When the faults yield, all of the energy stored in the rocks is released in a matter of seconds, in part as heat, and in part as shock waves. The shock waves constitute an earthquake (10). The resulting vibratory energy is then transmitted to the earth’s surface. When it reaches the surface, it can cause damage and collapse of buildings which can kill or injure the occupants. These massive and relentless forces are responsible for the seismic activity belt that extends along the Pacific Ocean from South American to Japan.
Force of Earthquakes
The magnitude and the intensity of earthquakes are two separate measures of their strength which are frequently confused (11). The magnitude is a measurement of the physical energy that is released from its origin, estimated by instrumental observations. Several scales of magnitude are used. The oldest and most widely used is the Richter scale, developed by Charles Richter in 1936. Although the scale is open, the greatest registered force to date is 8,9.
On the other hand, the intensity of an earthquake is a measurement of the resulting consequences more than of its force. It is a measurement of the severity of the impact in a specific place. While the magnitude refers to the force of the earthquake as a whole (an earthquake can have only one magnitude), the intensity refers to the effects in a particular site. The intensity is usually greatest closest to the epicenter. The intensity is determined by classifying the degree of shock on a scale based on the visible consequences left by the earthquake and of the subjective reports from people who experience the shock. There are many scales of intensity utilized in the world. The most widely used in the United States is a modification of the Mercalli scale (MM), which goes from slightly perceivable (MMI) to total destruction (MMXII) (table 8.3).
La intensidad de un terremoto está más relacionada con sus consecuencias en la salud pública que con la magnitud.
Las escalas de intensidad han permitido hacer comparaciones con terremotos ocurridos antes del desarrollo de los instrumentos de monitorización. La destrucción causada por un terremoto está en función de su intensidad y la resistencia de las estructuras a las sacudidas.
The intensity of an earthquake is more closely related than the magnitude, to its consequences for public health. The intensity scales allowed for comparisons between earthquakes before the development of monitoring instruments. The destruction caused by an earthquake is based on its intensity and the resistance of the structures to the shocks.
Table 8.3 Categories of the modified Mercalli scale (MM)
I |
Perceived by very few people under particularly favorable circumstances. |
II |
Perceived only by few people lying down, particularly on high floors of buildings. Suspended objects can swing. |
III |
Noticeably perceived inside. A motor vehicle can be rocked gently. A vibration similar to that of a passing truck. |
IV |
It is perceived inside by many and outside by a few. At night, some people awaken. Stone, glasses, windows and doors rattle. |
V |
It is perceived by almost everyone; damage to contents and structures is rare but possible. |
VI |
It is perceived by everyone; many are scared and run outside; slight damages. |
VII |
Everyone runs outside; damages of little importance to buildings seismically designed and well constructed; slight to moderate damages to ordinary structures; considerable damages to poorly designed or constructed structures. |
VIII |
Slight damages to well designed structures, considerable damages to ordinary constructions and large damages to poorly constructed ones; chimneys, monuments, walls, etc. fall. |
IX |
Considerable damage to large and well designed structures (including partial or complete collapse) in other buildings; constructions move off of their foundations; underground pipes are broken. |
X |
Some well constructed wood structures are destroyed; most of the masonry and ordinary structures are destroyed; rails are bent; landslides are common, water breaches banks of dykes and lakes, etc. |
XI |
Few, if any, masonry structures remain standing; bridges are destroyed, great cracks open in the land; underground pipes are completely dysfunctional; the Earth sinks. |
XII |
The damage is total; waves break over the surface of the earth; it is almost impossible to remain standing; objects are thrown into the air. |
Topographic factors
Topographic factors substantially influence the impact of earthquakes. Violent shocks in areas constructed on alluvium soils or garbage dumps tend to liquefy and to exacerbate the seismic oscillations. This can generate extensive damages and injuries that are far from the epicenter (12). The impact of the earthquake in Mexico City in 1985, where 10,000 people are estimated to have died, and the one of 1989 in Loma Prieta, are good examples of how local soil conditions influence damages caused to buildings.
Meteorological Factors
Meteorological factors play a smaller direct role but they can substantially affect the secondary consequences of earthquakes. The water swells and the high levels of water from storms exacerbate the impact of marine seismic waves. The water saturation of soils increases the probability of landslides and damage to dykes as well as the possibility of the liquification of the ground during the shocks. The damage from an earthquake to dykes, when the waters are near a flood state can be catastrophic. If houses are left with considerable damage, rain and low temperatures would be, at the very least, uncomfortable and could contribute to the increase of morbidity and mortality, as was observed in Armenia in December of 1988.
Volcanic activity
Earthquakes are often associated with active volcanoes, and on occasion by the flow of magma or an increase in the pressure that follows the magma intrusion. Generally, the harmonic call vibrations that are associated with magma flow are not harmful; however, relatively severe earthquakes can precede or accompany volcanic eruptions and contribute to devastating landslides.
Man made factors (artificial causes of earthquakes)
Four human activities, or their consequences, are known to induce earthquakes: 1) the filling of great water reservoirs; 2) deep well injection; 3) underground blasts of nuclear projects, and 4) collapse of mines or underground work. Some observers have speculated that nuclear detonations along a geological fault could release energy in a controlled form and thereby prevent a large earthquake. However, the potential risk of error with such experiments has discouraged even the most intrepid earthquake investigators (13).
Factors that influence morbidity and mortality from earthquakes
Natural factors
Land sliding
Mud and landslides triggered by earthquakes have been the cause of the majority of deaths and serious injuries in several recent earthquakes, including those of Tajikistan (1989), the Philippines (1990) and Colombia (1994) (17). At the beginning of this century, landslides were clearly the most devastating effect from earthquakes in China, where 100,000 people died in 1920,and in Peru in 1970 where 66,000 people died (18). Landslides can bury whole towns and houses in slopes, and sweep vehicles far from their routes, into precipices, particularly in mountainous areas. The detritus flows caused by earthquakes can also dam rivers. Those dams can create flooding and, if dykes suddenly burst, large waves and flooding can result. Both events put human settlements at risk.
Tsunamis (seismic marine waves)
Submarine earthquakes can generate destructive tsunamis (seismic waves) that travel thousands of miles without diminishing before causing destruction to the coastal areas and the environs of bays and ports. A tsunami can be created directly by the movements of earth below the water during earthquakes or by slides, including those that occur under water. They can travel thousands of miles at 483-966 km/h with very little loss of energy. Tall waves in deep ocean waters can be only a few meters and pass under boats with little disturbance. However, in shallow coastal waters they can reach 30.48 meters, with a devastating impact on boats and coastal communities. The successive crests can hit shore at intervals of 10 to 45 minutes and have a continued destruction over several hours.
The Pacific coast of the United States is at a high risk of tsunamis, primarily from earthquakes in South America and the region of Alaska and the Aleutian Islands. For example, in 1964, Alaska’s earthquake generated 6 meter high tsunamis along the coasts of Washington, Oregon and California and caused great damages in Alaska and Hawaii. The tsunamis killed 122 people while only nine people died near the epicenter of the earthquake. Tsunamis are clearly the main threat related to earthquakes for the inhabitants of Hawaii. More recently, tsunamis caused by earthquakes were responsible for the majority of deaths and serious injuries in Nicaragua (1992), northern Japan (1993) and Indonesia (1992 and 1994) (19-20).
Aftershocks
The majority of earthquakes are followed by aftershocks, some of which can be as strong as the initial earthquake. Many deaths and serious injuries occurred from a strong aftershock two days after the earthquake in Mexico City on September 19th, 1985, which killed 10,000 people (15). In some cases, landslides can be triggered by an aftershock.
Some large landslides begin slowly with a small amount of debris falling that is worsened by further shocks. In these cases there can be sufficient warning for a community aware of the risk, to evacuate opportunely.
Time of day
The time of day of the earthquake is a critical and determining factor for the risk of death and injury due to the probability of being trapped by a collapsed building. For example, the earthquake in Armenia in 1988 occurred at 11:41 a.m. and many people were trapped in schools, office buildings and factories. If the earthquake had occurred at another time of day, the patterns of injuries and deaths would have been quite different. The earthquake at Long Beach, California, in 1933, caused extensive damage to schools but there were no deaths as the earthquake occurred outside of school hours (21). In Guatemala, the 1976 earthquake, with 24,000 deaths, occurred at 3:05 a.m. while most people were sleeping. If the earthquake had occurred later, many people would have been outside and might not have been injured or killed (22). On the other hand, an earthquake in Northridge in 1994, in Southern California, killed 60 people when it occurred at 4:31 a.m. on a holiday (3,23). The death toll and number of injuries would have been much worse if the earthquake had occurred at 9 o’clock in the morning on a workday while 700,000 students and 6 million people were traveling to work and school. Thus, the hour of the day in which an earthquake occurs is a crucial factor in the number of victims.
Man made factors
Fires and dyke destruction during an earthquake are good examples of complications caused by man that exaggerate the destructive effects of an earthquake. In industrialized countries, an earthquake can also be the cause of a devastating technological disaster by damage or destruction of nuclear power stations, research centers, areas of hydrocarbon storage and factories that produce chemical and toxic agents. In some cases, these “posterior” disasters can cause many more deaths than were caused by the earthquake directly (24).
Risk of fires
One of the most severe secondary disasters that can follow earthquakes is fire (25). The severe shocks can overturn stoves, heaters, lights and other elements that can ignite flames. In Japan, earthquakes that triggered fires have 10 times more deaths than those that do not have related fires (25). The earthquake in Tokyo in 1923, which killed more than 140,000 people, is a classic example of the ability of fire to produce an enormous number of deaths after earthquakes. Similarly, the huge fire after the San Francisco earthquake in 1906 was responsible for many more deaths. More recently, the earthquake in 1994 in Northridge, California, demonstrated how strong vibrations can disconnect underground fuel and gas lines causing leaks of explosive and volatile materials and resultant fires (4,23). As well, during the first 7 hours after the Loma Prieta earthquake in 1989, in Northern California, San Francisco had 27 structural fires and more than 500 reported fire incidents (8). In addition, the water supply for the city was interrupted, seriously jeopardizing the capacity for fire fighting (26).
Perhaps, the most vulnerable areas are the sectors of slums on the outskirts of many populated cities in developing countries (`illegal establishments' or `squatters). Many of them have the potential to present catastrophic consequences after earthquakes.
Structural factors
The trauma caused by the partial or complete collapse of manmade structures is the most common cause of death and injury for most earthquakes (1). Nearly 75% of deaths attributed to earthquakes in this century were caused by the collapse of buildings that were not suitably designed to be seismo-resistant, were built with inadequate materials, or were poorly constructed (27). The results of field studies after earthquakes have demonstrated that different types of construction deteriorate in different ways when they are subjected to strong vibrations and movements of the earth. There is also evidence that different types of constructions inflict injuries in different ways and with varying degrees of severity when they collapse. (14,28,29).
Non structural factors
It is known that nonstructural features of buildings have failed and caused extensive damages in past earthquakes.
The layer of stucco or cement on building facades, dividing walls, roof coverings, external architectural ornaments, non reinforced chimneys, ceilings, elevator wells, ceiling tanks, hanging lights, and interior contents such as accessories in hospitals, are among the numerous nonstructural items that can fall during an earthquake and can cause injuries or death (30). The frequent collapse of stairs makes it difficult to escape as many buildings are equipped with only one staircase (31). In addition, heavy furniture, appliances, bookshelves, equipment, and other objects located in high areas can fall and cause injury unless they are affixed (16). Although recent studies indicate that nonstructural objects such as ceilings and building contents such as home and office equipment, are rarely fatal. Such objects are responsible for numerous mild and moderate injuries that have implied economic costs for their attention (32).
Referencias
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2. Chen Y, Tsoi KL, Chen F, et al. The Great Tangshan Earthquake of 1976: an anatomy of disaster. Oxford: Pergamon Press; 1988.
3. Tierney K. Social impacts and emergency response. In: Hall JF, editor. Northridge earthquake, January 17, 1994: preliminary reconnaissance report. Oakland: Earthquake Engineering Research Institute; 1994. p.86-93.
4. Goltz JD. Tile Northridge, California, earthquake of January, 17, 1994: general reconnaissance report. Technical Report NCEER 94-0005. Buffalo, NY: National Center for Earthquake Engineering Research; 1994.
5. Centers for Disease Control. Earthquake-associated deaths: California. MMWR 1989;38:767-70.
6. Noji EK. The 1988 earthquake in Soviet Armenia: implications for earthquake preparedness. Disasters: The International Journal of Disaster Studies and Practice 1989;13:255-62.
7. Palafox J, Pointer JE, Martchenke J, et al. The 1989 Loma Prieta earthquake: issues in medical control. Prehospital and Disaster Medicine 1993;8:291-7.
8. Benuska L, editor. Loma Prieta earthquake reconnaissance report. Earthquake Spectra 1990;6(Suppl.):1-448.
9. Noji EK, Armenian HK, Oganessian AP. Mass casualties and major injuries: medical management in the Armenian earthquake. In: Proceedings of the International Symposium on “Medical Aspects of Earthquake Consequences” in Yerevan, Armenia, 9-11 October 1990. Yerevan, Armenia: Armenian Ministry of Health; 1990.
10. Perez E, Thompson P. Natural hazards: causes and effects. Earthquakes. Prehospital and Disaster Medicine 1994;9:260-71.
11. California Department of Commerce, Division of Mines and Geology. How earthquakes are measured. California Geology, February 1979. p.35-7.
12. U.S. Atomic Energy Commission. Report on structural damage in Anchorage, Alaska caused by the earthquake of March 27, 1964. Nevada Operations Office, NVO-9949. (Contract AT (2Sl) 99), Jan. 1966.
13. Stratton JW. Earthquakes. In: Gregg MB, editor. The public health consequences of disasters. Atlanta: Centers for Disease Control; 1989. p.13-24.
14. Noji EK, Kelen GD, Armenian HK, et al. The 1988 earthquake in Soviet Armenia: a case study. Ann Emerg Med 1990;19:891-7.
15. Diaz de la Garza JA. Earthquake in Mexico, Sept. 19 and 20 of 1985. Disaster Chronicles. No. 3. Washington, D.C.: Pan American Health Organization; 1987.
16. Rahimi M, Azevedo G. Building content hazards and behavior of mobility-restricted residents. In: Bolton P, editor. The Loma Prieta, California, earthquake of October 17, 1989-public response. USGS Professional Paper 1553-B. Washington, D.C.: U.S. Government Printing Office; 1993. p.BS1-B62.
17. Garcia LE. The Paez, Colombia earthquake of June 6, 1994. Earthquake Engineering Research Institute Newsletter 1994;8:7.
18. Blake P. Peru earthquake, May 31, 1970. Report of the CDC epidemiologic team. Atlanta: Center for Disease Control; 1970.
19. U.N. Economic Commission for Latin America (ECLAC). The tsunami of September 1992 in Nicaragua. Santiago, Chile: ECLAC; 1992.
20. Synolakis C. The June 3, 1994, East Java earthquake-tsunami takes it toll on local villages. Earthquake Engineering Research Institute Newsletter 1994;28:6-7.
21. Jones NP, Noji EK, Krimgold F, Smith GS, editors. Proceedings of the International Workshop on Earthquake Injury Epidemiology for Mitigation and Response, 10-12 July, 1989. Baltimore, MD: Johns Hopkins University; 1989.
22. U.S. Agency for International Development (USAID). Case report: Guatemala earthquake 1976. Washington, D.C.: USAID; 1978.
56. Synolakis C. The June 3, 1994, East Java earthquake-tsunami takes it toll on local villages. Earthquake Engineering Research Institute Newsletter 1994;28:6-7.
23. Hall JF. The January 17, 1994 Northridge, California earthquake: an EQE summary report. San Francisco: EQE International; 1994.
24. Alexander DE. Death and injury in earthquakes. Disasters 1985;9:57-60.
25. Coburn AW, Murakami HO, Ohta Y. Factors affecting fatalities and injuries in earthquakes. Internal Report. Engineering Seismology and Earthquake Disaster Prevention Planning. Hokkaido, Japan: Hokkaido University; 1987.
26. EQE Engineering. The October 17, 1989 Loma Prieta earthquake: a quick report. San Francisco: EQE Engineering; 1989.
27. Coburn AW, Spence RJS, Pomonis A. Factors determining human casualty levels in earthquakes: mortality prediction in building collapse. In: Proceedings of the First International Forum on Earthquake related Casualties. Madrid, Spain, July 1992. Reston, VA: U.S. Geological Survey; 1992.
28. Armenian HK, Noji EK, Oganessian AP. Case control study of injuries due to the earthquake in Soviet Armenia. Bull World Health Organ 1992;70:251-7.
29. Roces MC, White ME, Dayrit MM, Durkin ME. Risk factors for injuries due to the 1990 earthquake in Luzon, Philippines. Bull World Health Organ 1992;70;509-14.
30. Ohashi T. Importance of indoor and environmental performance against an earthquake for mitigating casualties. In: Proceedings of the Eighth World Conference on Earthquake Engineering. Vol. VII. Englewood Cliffs; NJ: Prentice-Hall; 1984:7. p.655-62.
31. Durkin ME, Thiel CC. Improving measures to reduce earthquake casualties. Earthquake Spectra 1992;8:95-113.
32. Wagner RM, Jones NP, Smith GS, Krimgold F. Study methods and progress report: a case-control study of physical injuries associated with the earthquake in the County of Santa Cruz. In: Tubbesing SK, editor. The Loma Prieta, California, earthquake of October 17, 1989-Loss estimation and procedures. USGS Professional Paper 1553-A. Washington, D.C.: U.S. Government Printing Office; 1993. p.A39-A61
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