Earthquakes and Society

by Courtney Brunious and Amanda Warner
Introduction
         An earthquake is a vibration of the Earth produced by a rapid release of energy (Tarbuck 378). The main features include the focus, the location within the Earth where the earthquake rupture starts, and the epicenter, the point on the Earth's surface directly above the focus. Earthquakes have a greater effect on society than most people think. These effects range from economical to structural to mental. An earthquake only occurs for a few brief moments; the aftershocks can continue for weeks; the damage can continue for years.

Scientific Aspects
         An earthquake is a rupture within the Earth caused by stress (Doyle 66). Earthquakes occur principally by the sudden displacement on faults (Yeats 40), when there is a build-up of stress in the crust caused by plate movement at a subduction zones or other fault lines (Moores 34). The Earth's crust is part of a collection of well-defined crustal plates that grind past each other, under and over each other and recede from each other (Zebrowski 174). A fracture occurs when the stress increases to beyond the strength of the brittle lithospheric rock (Doyle 67). Most of this motion can be explained by the theory of plate tectonics, which explains that an outermost sphere (lithosphere) is divided into a number of relatively rigid plates that collide with, separate from, and translate past one another at their boundaries. The disruption produced at the boundaries between plates results in earthquakes (Yeats 11). The origin of earthquakes is ultimately the jostling between moving plates which produces the strain within the lithosphere that must be relieved by earthquakes (14).

         Pressure builds up along the fault lines and eventually slips, sending shock waves (Tobin 77). Wave motion is the way in which energy is transmitted from the earthquake source to the Earth's surface. Elastic waves are classified based on the motion of individual particles with respect to the direction of propagation of the wave (Yeats 60). There are two basic types of earthquake waves, body waves and surface waves. Body waves propagate through the earth's interior. The two types of body waves are primary ("P") waves and secondary ("S") waves. Primary waves travel the fastest and are the first movement felt in an earthquake (Robinson 54). They are propagated by longitudinal forces that displace particles by extension and compression. S waves (also referred to as shear or transverse) displace particles at right angles to wave direction, moving at one-half the P wave velocity, but having a greater wave amplitude; they cause greater damage than P waves (Tobin 77). They make the ground to move vertically and horizontally, causing more damage than P waves because buildings can only stand minimal horizontal stress (Robinson 54). The two types of surface waves, following body waves, are Love and Rayleigh (55). Love waves propagate a shearing motion in a horizontal plane. Rayleigh waves generate elliptical action in a vertical plane. A combination of these waves cause ground shaking damage (Tobin 78). Earthquakes generate an array of primary, secondary and surface waves due to the complexity in the strain-release pattern at the source and complexity in the earth materials through which the waves pass to reach a seismograph (Yeats 62).


Locating Earthquakes
         Seismographs record the ground shaking that results from the release of energy from earthquakes and help locate the epicenter and focus of an earthquake (Yeats 60). They have a mass freely suspended from a support that is attached to the ground. When the vibration from a distant earthquake reaches the instrument, the movement of Earth in relation to the stationary mass is recorded. The greater the interval between the arrival of the first primary wave and first secondary wave, the greater distance to the earthquake (Tarbuck 384). Modern seismology uses P wave's first motions and the amplitudes of direct P and S waves, surface waves and waves reflected many times from the surface of the earth to understand the earthquake source (Yeats 67).

         An earthquake may be initially located by comparing the differences in arrival times of various phases with standard time-tables and curves (Doyle 42). From there, earthquake depths are estimated by the arrival time of reflected waves from the surface above the focus. Earthquake depths vary and are generally categorized as shallow, with a focus within 70 km of the surface, intermediate, with a focus between 70 and 300 km of the surface, and deep, with a focus greater than 300 km below the surface. Ninety percent of all earthquakes occur at depths less than 100 km and almost all of the very damaging earthquakes appear to originate at shallow depths (Tarbuck 387).  The majority of earthquakes are "shallow," within the upper cooler crust and in the most brittle part of the lithosphere (Doyle 44).


Measuring Earthquakes
         The magnitude of an earthquake is an instrumental measure of its size on a logarithmic scale introduced by Charles Richter in 1935. The magnitude is estimated by measuring the ground amplitudes recorded at stations and allowing for distance, depth and wave period. Earthquakes have a huge energy range with 15 orders of magnitude (Doyle 45). The Richter scale is a logarithmic scale; an increase of one in magnitude corresponds to a ten-fold increase in the amplitude of shaking (Robinson 56). Earthquake intensity, on the other hand, is an estimate of the violence of an earthquake shaking at a given site (Yeats 71). This reflects the size of the seismic wave, distance from epicenter, geological structures and social factors (Tobin 82).  This generally decreases with distance from the epicenter. Earthquake intensity is measured with the Modified Mercalli Intensity Scale, looking at the amount of shaking and damage during an earthquake (Moores 37). Earthquakes can have only one magnitude but many intensities (Robinson 55) because magnitude is a set calculation, whereas intensity varies with location, calculating the amount of damage done to structures, the degree to which the earthquake was felt by individuals and the presence of secondary effects (Yeats 71).
 
 
Richter Scale
Richter Magnitudes
Effects Near Epicenter
Estimated Number per Year
<2.0 Generally not felt, but recorded 600,000
2.0-2.9 Potentially Perceptible 300,000
3.0-3.9 Felt by some 49,000
4.0-4.9 Felt by most 6200
5.0-5.9 Damaging shocks 800
6.0-6.9 Destructive in populous regions 266
7.0-7.9 Major earthquakes; inflict serious damage 18
>8.0 Great earthquakes; cause extensive distruction near epicenter 1.4
 
 
 
Modified Mercalli Intensity Scale (MMI)
I.  Not felt except by a very few under especially favorable circumstances. 
II.    Felt only by a few persons at rest, especially on upper floors of buildings. 
III.  Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. 
IV.  During the day felt indoors by many, outdoors by few. Sensation like heavy truck strinking building. 
V.   Felt by nearly everyone, many awakened. Disturbances of trees, poles and other tall objects sometimes noticed. 
VI.  Felt by all; many frightened and run outdoors. Some heavy furniture moved; few instances of fallen plaster or damaged chimneys. Damage slight. 
VII.  Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures. 
VIII.  Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures. (Fall of chimneys, factory stacks, columns, monuments, walls.) 
IX.   Damage considerable in specially designed structures. Buildings shifted off foundations. Ground cracked conspicuously. 
X.   Some well-built wooden structures destroyed. Most masonry and frame structures destroyed. Ground badly cracked. 
XI.  Few, if any (masonry) structures remain standing. Bridges destroyed. Broad fissues in ground. 
XII.  Damages total. Waves seen on ground surfaces. Objects thrown upward into air.
 
( both tables courtesy of Tarbuck)

Aftershocks
         Aftershocks are smaller earthquakes generated by adjustments following a major earthquake (Tarbuck 381). They are caused by the strain of an earthquake not being fully released in the main event and by a rearrangement of stresses in the region. Aftershock patterns indicate the position of the fault or moved block (Doyle 70). Along with earthquakes, aftershocks can cause secondary effects, the nontectonic surface processes that are directly related to earthquake shaking, including tsunamis, landslides, liquefaction and ground cracking (Yeats 427). Tsunamis are long-period ocean waves usually caused by sea-floor movements during an earthquake. Most are a result of earthquakes within subduction zones, where a sudden uplift of the sea floor by faulting causes a vertical displacement of the water above the sea floor (Doyle 86). The wave gets higher as it reaches shallower water because the velocity decreases as the height increases (Tarbuck 331). Landslides  include rock falls, rock slides, rock avalanches, soil block slides, slow earth flows and soil lateral spreads (Yeats 433). Liquefaction is a result of increased pore pressure which decreases effective stress (436).

Earthquake Theories
         There are two main theories about earthquakes, the Elastic Rebound Model and the Seismic Gap Theory. The Elastic Rebound Model states that at a geological fault between two moving plates, stress occurs and deforms the rocks (Yeats 61). This occurs in four main steps of rock deformation, the original position, buildup of strain, slippage, and strain release (Tarbuck 380). If the fault creeps, it will produce frequent micro-earthquakes; if it binds together and then slips, it will produce large earthquakes. Stress will then quickly be released; sides of the fault will become offset; rocks will rebound to their initial state of stress (Yeats 61). The problem with this theory is that earthquakes do not produce the large drop in stress required for this model (65). The Seismic Gap Theory states that strong earthquakes are unlikely in regions where weak earthquakes are common and the longer the quiescent period between earthquakes, the stronger the earthquake will be when it finally occur (Yeats 176). The complication is that the boundaries between crustal plates are often fractured into a vast network of minor faults that intersect the major fault lines. When an earthquake relieves the stress in any of these faults, it may pile additional stress on another fault in the network. This contradicts the Seismic Gap Theory because a series of small earthquakes in an area can then increase the probability that a large quake will follow (178).
 

Earthquake Subduction Zones
         The subduction zone of an earthquake is the area of intense activity caused by the movement of two plates where one plate subducts beneath the other. There are three types of plate boundaries where subduction occurs: ocean to ocean, ocean to continent and continent to continent (Moores 12). In an ocean to ocean subduction, the plates are pulled apart. In an ocean to continent subduction, one plate is thrust under the other. In a continent to continent subduction, the plates scrape past one another (USGS). An earthquake begins at a contact between two rock surfaces; one side of the fault overhangs the other. If the overhang side moves downward, the fault is normal. If the overhang side moves upward, the fault is reverse. If the movement is along the vertical axis, the fault is a dip-slip. If the movement is along the horizontal axis, the fault is strike-slip. The friction between the two planes of the fault controls its movement or lack thereof. The lower the friction, the weaker the fault and the easier it slips. Fault creep is when the friction is low enough so that the fault may slip constantly and aseismically. Medium friction produces small earthquakes and high friction will produce a fault that will slip occasionally, but with a few large earthquakes (Robinson 60).
 

         Most earthquakes are caused by the release of elastic strain accompanying sudden displacements on faults (Yeats 60). Crustal earthquakes are caused by sudden displacement on faults (42). The different types of fault lines include strike-slip faults, ridge-ridge transformations, trench-trench boundary transformations, ridge-trench boundary transformations, horizontal extrusions and trench-parallel strike-slip faults (180). In the continental crust, strike-skip faults commonly occur as connecting elements between active reverse or normal faults (184). Normal faults develop in crust undergoing extension in which the maximum principal compressive stress is vertical. Normal-faulted regions are characterized by high heat flow, relatively low-velocity upper mantle and volcanism. The most common setting is at sea-floor spreading centers, where new oceanic crust is created. Most displacement on normal faults occur soon after formation by sea-floor spreading. Continental breakup is preceded by the development of rift zones in continental crust characterized by grabens bounded by normal faults. Rift valleys, such as the Rio Grande rift in New Mexico or the Baikai rift in Siberia, comprise zones of high seismicity and are incipient spreading centers (258).

         Reverse faults form in an environment where the maximum principal compression stress is horizontal and the minimum is vertical (301). There are four categories of active reverse faults: active reverse faults that constitute the boundaries between plates of continental crust; reverse faults that occur continentward of the volcanic arc in association with a subduction zone; reverse faults that are associated with strike-slip faults and reverse faults in continental shield areas (302).
 Ninety-five percent of all earthquake energy is released at plate boundaries; only five percent are intraplate. Eighty-six percent occur at subducting plate margins where oceanic plates have cooled, become dense and sunk into the mantle. Only nine percent of the Earth's seismicity occurs at the mid-ocean ridges where new plate is formed by rising magma. (Yeats 57). Knowing the types of seismic zones and their occurrence patterns, scientists know where ninety percent of the major earthquakes are likely to occur (USGS).


Societal Aspects
         The effects of earthquakes on society reach farther than the destruction that they cause and the fear that they place on the population. As a result of the fear that they cause, society has come up with many ways to protect against the damage that is possible during earthquakes. Warning systems, research, and advances in engineering have done an amazing job of giving people the resources necessary to survive an earthquake. Warning systems and research are often found in the areas that are most prone to this sort of disaster. In the following paragraphs we will discuss how society has found ways to maintain the safety of people during earthquakes and how this affects other parts of society. We will also discuss how people can protect themselves against earthquakes and what the consequences are of the government warning of possible seismic activity in a community.

         How people perceive earthquakes has a huge impact on the precautionary measures that can be taken to reduce the possibilities of injury, property damage, and loss of life. Protecting yourself with earthquake insurance, building structures with seismic activity in mind, having earthquake drills, and being prepared with emergency supplies can have a great affect on how people survive an earthquake. Ignorance of what can be done to protect yourself from an earthquake can result in unnecessary damage and/or pain and suffering. In the chart we can see that people will often not take precautionary measures unless they have been greatly affected in the past by an earthquake. Because many people are in denial about the importance of precautions in the case of earthquakes, damage is done that could have been prevented. The survey that was done shows that when residents that lived along a fault line were asked of disadvantages of living in their place of residence, 44.2 percent could not think of any problems and out of the remaining people, nobody mentioned earthquakes (Berlin 68). Unfortunately it seems as if people are unaware of the great danger that earthquakes present. According to this survey, even when people do realize the dangers all they seem to do for precaution is pray. This is not the most effective way of protecting against earthquake damage. This lackadaisical attitude toward the effects of earthquakes can be seen in a summary of earthquakes hazard survey in San Mateo California.

 
Relationship Between Adoption of Precautionary Measures and Intensity of Past Experience
                            Losses over $1000    Losses under $1000   No Past experience     Total 
                                No.            %            No.           %             No.          %         No.    %
No action taken        36             48.0          58           69.9            115          79.9     209  69.2
Action taken             39             52.0          25           30.1              29          20.1       93  30.8
Total                         75            100.0         83         100.0            144         100.0   302 100.0
         This  upcoming survey gives an idea of public feelings about what to do in an earthquake. Unfortunately many people do not understand what is needed to protect themselves from earthquake damage. Most of the people that responded in this survey indicated they would probably just leave the area in the event of an earthquake prediction was made by government scientists (Berlin 73). Also it turns out that the young people would leave the area while the older people would not take action and stay. The moving away of people because of an earthquake prediction brings out problems with an earthquake warning system.
 
Summary of Earthquake Hazards Survey, 1970 and 1976, San Mateo County, California
Respondents                                                                                    1970                1976
1. Residents                                                                                     Percentageof Respondents 
  Own residence                                                                                 93                      82 
   Rent or lease                                                                                     7                      18 
2. Were informed of potential earthquake hazards by previous owner    8                      16 
3. Would inform future residents of potential earthquake hazards          22                     51 
4. Would react to an earthquaker by: 
       Standing under a doorway                                                            43                     43 
       Getting under a desk, table or bed                                                  8                     15 
       Staying away from windows, mirrors, and chimneys                        5                       3 
       Remaining stationary and calm                                                       20                    16 
       Running outside                                                                             12                    14 
       Unsure                                                                                          10                      5 
       Other responses                                                                              2                      4 
5. Reaction to prediction of a major earthquake within one week 
        Do Nothing                                                                                  52                      38 
        Move or leave the area temporarily                                               36                      40 
        Unsure                                                                                           9                        8 
        Reinforce the residence                                                                   1                        3 
        Purchase insurance                                                                          1                        1 
        Other responses                                                                              1                      10 
6. Had received earthquake hazard information directly from their           22                      28 
     local government. 
 
         Federal warnings of possible earthquakes seem like a reasonable way to protect against the effects of a disaster. However with this warning system comes possible social disruptions and a decline in the local economy from the time of the warning to the expected date of the event.  People would not be able to buy earthquake insurance during this period of uncertainty. Another economic problem that this presents is that there would be adverse changes in mortgage availability, property values, investment patterns, and employment opportunities. The town would feel the effects of this with a reduction in tax revenues for the local government, which would lead to a reduction in public services (Berlin 76). All of this happens because an earthquake was predicted. Although this seems like a terrible burden to a community, it would lower the risk of casualties and damage. An attempt to lessen the economic effects of an earthquake prediction problem is the only way to get around this problem.

         Economics are not the only things affected by earthquakes. People often experience emotional side effects of this natural hazard. The emotional problems that come with earthquakes differ from other natural hazards because they usually strike without warning and because of the force and destructive effects that come with it. Many people who have done research on the human reaction to earthquakes have found many similar observations of a so called "earthquake syndrome."Studies were done on this by the San Fernando Valley Child Guidance Clinic and the ENKI Research Institute following the San Fernando, California earthquake on February 9, 1971. Through offering its services to help parents and children with their fears and anxieties, the clinic received over 1200 calls by the end of the second week (Berlin 94). After observing people that came in for counseling, the staff of the clinic observed that more girls were affected by the earthquake than boys. They realized this could be because girls can express their fears more easily (95).

         Most of the studies done by this clinic revolved around children's reactions to the earthquakes. It was noticed that many children, whom parents thought were competent and unafraid, were affected by the earthquake. After all of the research, the clinic found that increased knowledge of earthquakes lessened the fears in many of the children (96). They also found that mental health services were needed following a disastrous earthquake because many people cannot cope with the disaster (97).

         Safety and survival in an earthquake can be broken down into three sections: before an earthquake, during and earthquake, and after an earthquake. First of all before an earthquake ever happens, buildings have to be engineered specifically with earthquakes in mind. Major problems during earthquakes that can be avoided with a little thought are failure of structural systems, failure of nonstructural systems, damage from unanchored equipment and furniture, and loss of electricity. A well-engineered building will not have these results. The reason for the majority of most structural failures due to earthquakes occur because of human failure in engineering (Nowak 168). Minimizing hazards in an earthquake is imperative for reducing the risk of loss of life and limb to building occupants. The knowledge of where to seek shelter and anchoring essential equipment and bracing shelves and cabinets can make the difference between life and death. Knowing how to give first aid, shut off gas, electricity, and water, and being prepared with transistor radios and flashlights can be done to become prepared if an earthquake ever happens (169).

         During an earthquake, people should watch for falling objects and stay away from windows and other glass objects (170). Also people should attempt to hide under a stable structure that will protect them from objects that are falling. After an earthquake, people should check for injuries or any other fire hazards (176). Also people should be prepared for aftershocks, which are common following earthquakes.

         Earthquakes are a natural disaster that unfortunately the majority of society takes  much too lightly. And because of peoples perceptions of earthquakes, much more damage than is necessary can happen. The most important thing a person can do in preparation for an earthquake is to learn about the dangers that exist during and after and earthquake. Society is greatly affected by what an earthquake does. Buildings have to be made with them in mind and economics and mental health are also changed according to how society responds to this natural hazard. The problem with most of society is that no matter how much they are told about the dangers of something, it takes experiencing this event to change their minds. This holds true when it comes to earthquakes. Because many people have not experienced the despair and damage that earthquakes bring, they cannot relate to the great danger that earthquakes can put them in. Midwesterners do not necessarily have to worry about earthquakes and therefore tend to take them lightly. But what if a person from the Midwest gets caught in an earthquake? He would not know what to do. This is why education about earthquakes is so important to protecting against the dangers. Of course earthquakes are not the "glamour" natural disasters like volcanoes or tornadoes. And even though they don't receive names like hurricanes, they can do the same or more damage. Society overall needs to wake up and inform itself of the dangers of earthquakes and what precautions need to be taken. The worst thing that people can do is be ignorant during this situation and do nothing at all. Therefore people need to learn about what makes an earthquake and what can be done to minimize their impact. 


Discussion: Integration of Aspects
         The Prince William Sound Earthquake of 1964, also known as the Great Alaskan Earthquake, clearly illustrates the integration of society and science in understanding earthquakes. The earthquake occurred on March 27, 1964 at 5:36 p.m., local time, and lasted for four minutes. The epicenter was about 10 km east of the mouth of the College Fiord, approximately 90 km west of Valdez and 120 km east of Anchorage (polarnet.com). The earthquake measured 8.3 on the Richter Scale and, its intensity ranged from IX-X on the Modified Mercalli Intensity Scale (Coffman 108).
 
         The vertical displacement and horizontal shifts can be classified at subsidiary reverse faulting with dip-slip offsets. The zone of subduction covered 110,000 square miles. The primary fault along which displacement occurred is probably a complex thrust fault approximately 575 miles long and 145 miles wide, near the Aleutian Trench beneath the Aleutian Arc (108). The main shock and its aftershocks occurred on a fault which is part of the boundary between the Pacific and North American plates. The northwestward motion of the Pacific plate at about 5 cm per year causes the crust of southern Alaska to be compressed and warped. After periods of hundreds of years, this compression is relieved by the sudden southeastward motion of portions of costal Alaska as they move back over the subducting Pacific plate (polarnet.com).

         Violent ground motion triggered rockslides, snow avalanches and landslides throughout central Alaska. Fractures and cracks developed, creating mud spouts, slumping and sand boils. Cracks also were observed in thick lake and river ice. These damages severely destroyed about 30 blocks of dwellings and commercial buildings in downtown Anchorage (Coffman 108). Landslides caused the greatest devastation in Anchorage, the largest being the Turagain Heights slide which extended 8,600 feet west to east (109).

         The aftershock zone was about 250 km wide and extended 800 km long. Thousands of aftershocks were recorded in the months following the main shock. Through April 30, 1964, 19 aftershocks were recorded with a magnitude of 6 or above. Smaller aftershocks continued for more than a year (polarnet.com). The earthquake generated a tsunami (Coffman 109), which caused most of the damage. The 1964 Alaskan tsunami was the second largest ever recorded, composed of two types of waves: open-ocean sea waves, generated by large-scale motion of the sea floor, and local waves, generated by underwater landslides in bays of fiords.  The death toll was extremely small for a quake of this size, due to low population density, time of day (Good Friday) and type of material used to construct many buildings (wood) (polarnet.com).

         Although the death toll was small for this earthquake, the mental problems contributed to this disaster effected a large number of the population. Research done on public reaction to the March 27, 1964 Earthquake in Alaska showed that there was no panic by people during the impact and immediate post disaster periods (Berlin 92). Researchers Langdon and Parker found that it was generally four days after the disaster that people started showing the effects of what had happened. They reasoned that this was because people were so involved in activity after the earthquake that they had no time for a state of depression (93). They also found that the most common reaction to the earthquake was a "separation phenomena" which separated the person from the overwhelming part of the disaster. Humor also occurred as a defensive mechanism to the earthquake. As in many other very emotional situations, people often try to make light of the situation to ease the pain or anxiety that they feel. Langdon and Parker also chronicled how people continued to react during a 2-month period. During this time, people felt an acute feeling of fear, anxiety, concern for others, depersonalization, thirst for information, avoidance of loneliness, heightened appreciation of reality, gilt for being lucky, and depression. People also experienced excess fatigue and changes in eating and drinking patterns (94).

         Additional studies were done on how mental patients at the Alaska Psychiatric Institute in Anchorage responded to the earthquake. These studies showed that although several of the patients were quite frightened, they had no panic reaction. The patients were also not concerned about their personal safety, but were concerned about the safety of their families. These people were so self-involved and withdrawn that they did not react fully and immediately to the external threat (94). As a result of the Earthquake and Tsunami in Alaska, many changes were made in the Alaskan Regional Tsunami Warning System. Deep ocean sensors for evaluating the destructive potential of tsunami were employed along with additional seismograph and tide stations being established to improve the detection capability system (90).The addition of a data collector for seismic and tide stations and communications relay for transmitting data helped to improve the warning time by as much as sixty percent. A final major change is the confusion that some of the warning systems terminology caused has now been replaced. Much like a tornado watch in the Midwest, a tsunami watch is used for the first alerting message. Following this would be a tsunami warning when a tsunami has been generated. This warning is issued to system participants that a potentially dangerous tsunami can be expected at their location (91). These warning systems have helped in preparing people better for the possibility of a tsunami occurring.

         Although the Great Alaskan Earthquake ranks up there as one of the most powerful earthquakes in the history of the world, its destruction has given society more information on how to prepare people for earthquakes and tsunamis. Disasters such as these keep pushing society toward new ways to protect and inform people in cases of emergency.


Conclusion
         In conclusion, education is society's best tool in minimizing the effects of earthquakes. Seismologists have instruments, including the seismograph, that record earthquake movement to further study the emerging patterns. Earthquake prediction, based on the time interval between body and surface waves, is still primitive, yet is promising. Engineers have also conducted research about building structure in relation to earth movement. Along with the benefits of possible earthquake prediction, the potential for social and economic disaster remains.

Definitions

Bibliography
 
 
If you have comments or suggestions, email us at
warnera@umich.edu or  cbruniou@umich.edu