Earthquake Assignment

0011017 GG3037: Environmental Hazards and Remote Sensing

Select a specific environmental hazard and investigate/examine physical processes and consequences, analyse/assess hazard impacts, and evaluate various mitigation strategies, emergency management and policy issues.

This research project will be divided into four sections. Firstly I will introduce the theory of plate tectonics and define what an earthquake is. I will then investigate and examine the physical processes associated with this hazard and look at the consequences it produces. The impacts will be analysed before looking at possible mitigation strategies, emergency management and policy issues. Case studies from first and third world countries will be used to highlight the theory and comparisons will be made between the two.

An earthquake can be defined as; “Ground shaking caused by the sudden release of accumulated strain by an abrupt shift of rock along a fracture in the Earth or by volcanic or magmatic activity, or other sudden stress changes in the Earth..” (USGS, 2010).

Earthquakes generally occur at plate boundaries where two plates meet (Figure 1). The nature of these plate boundaries is characterised by the movement of each plate in relation to the other. The majority of earthquakes occur where oceanic crust meets continental crust, predominantly in the Pacific Ring of Fire where 90% of the world’s earthquakes occur (USGS, 2009) with the exception of mid-oceanic ridges, where many earthquakes can occur in a small area in a short space of time, ‘swarms’ (Bolt, 1999: 31).

There are four distinct type of plate margin: constructive margins are found where two plates diverge and upwelling magma rises to the surface and cools to form igneous rock. The Mid-Atlantic Ridge serves as a prime example for this boundary. Conservative boundaries are found where plates slide past one another and crust is neither created nor destroyed. This margin is primarily associated with earthquakes as friction builds up over time as the plates ‘stick’ together. The release of pressure can result in a severe earthquake, e.g. along the San Andreas Fault. Destructive margins are found where one plate is subducted under another, for example, Juan de Fuca under the North American Plate. The final plate boundary is a collision boundary, which is where two plates collide head on and fold up at the edges, as seen in the Himalayas.

When the strain becomes too great, primarily along conservative margins where plates ‘stick’ together, and the elastic limit is reached, the rock will rupture back to an unstrained position, in the form of an earthquake, releasing its energy in the form of seismic waves. Eiby (1967: 97) describes the build up of energy like winding up a spring and the earthquake being like a coiled spring being let go.

The point in the Earth’s crust where the pressure is released is referred to as the focus, whilst the point directly above this is the epicentre. Shallow focus earthquakes (<70km deep) are the most destructive, with a few exceptions, because the energy they release has little time to dissipate before reaching the surface (Monroe & Wicander, 2008: 193). Unfortunately, shallow focus earthquakes are more common and are associated with conservative plate margins, whilst deep focus earthquakes are characteristic of subduction margins.

1


Figure 1: The Earth’s major tectonic plates and their direction of movement in relation to one another.

Source: The Geography Site (2006)

There are two types of seismic waves that can be further subdivided. Body waves (P + S) are the less dramatic of the two but can be very useful for analyzing the earth’s interior structure, whereas surface waves (R+L) cause the most significant levels of damage, despite travelling slower (Garrison, 2007: 61). P-waves (pressure), also known as compressional waves, are initiated when rock is pushed/pulled forwards and backwards and are capable of travelling through solids, liquids and gases. S-waves (secondary) are the second fastest after pressure waves but are only capable of travelling through solids. They are shear waves that move material perpendicular to the direction the wave is moving. By contrast, a shear wave is an example of a transverse wave where as P waves are longitudinal.

Figure 2: Body waves (P and S)

Source: Burke Museum of Natural History and Culture (2002)

When P and S waves reach the surface, a number of them are converted into surface waves; Rayleigh and Love waves (R+L), which appear on seismographs after body waves but result

2


in more severe ground movements and therefore cause the most damage. They usually travel just below or on the ground’s surface whilst ‘rolling’ and moving side-to-side (Figure 3) and are capable of travelling round the globe due to low decay rates (Stein, 2003: 87).

Figure 3: Surface waves (L and R)

Source: Burke Museum of Natural History and Culture (2002)

Mandl (2000: 101) describes faults as ‘elongated zones of concentrated shearing, parallel to which the adjacent rocks have been offset’. Strike-slip faulting occurs when two blocks move horizontally in relation to one another along a roughly vertical fracture. Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically; if the rock mass above an inclined fault moves down, the fault is termed normal, where as if the rock above the fault moves up, the fault is termed thrust.

Intraplate earthquakes are those that occur away from plate boundaries and are caused by stresses placed upon the plate from neighbouring plates or by sedimentary loading/unloading (deglaciation). These stresses may be sufficient enough to cause failure along existing fault planes, giving rise to intraplate earthquakes. Building dams and henceforth creating resevoirs can lead to severe intraplate earthquakes such as the largest RTS related 6.3M earthquake that hit Koyna, India, in 1967. Gupta (2002) states how there are 90 identified sites of earthquakes that have been triggered by the infilling of large reservoirs. Jauhari (1999: 12) argues, “ When the pressure of the water in the rocks increases, it acts to lubricate faults which are already under tectonic strain, but are prevented from slipping by the friction of the rock surfaces.” Gupta (2002) believes the depth of reservoirs is the most important factor in RTS, with the US commission on Large Dams stating more specifically that RTS needs to be considered for dams deeper than 80m-100m. Many engineers are reluctant to consider RTS implications when designing dams as they oppose the fact that RTS even exists, and believe accounting for RTS implications will add time and costs to their projects.

3


The effects of an earthquake are influenced by the location of the epicentre and the magnitude on the Richter scale, with the most devastating earthquakes being linked to earthquakes of a high magnitude that have epicentres close to urban hubs.

Shaking and ground rupture are the main consequences of an earthquake, resulting in more or less damage to buildings and other rigid structures. The amplitude and duration of the earthquake will increase in poorly unconsolidated sediments and ultimately lead to higher levels of damage (Figure 4). This can lead to landslides and avalanches that can bury entire villages, as earth material is rapidly transported downslope. The ground shaking associated with earthquakes can loosen sediments, seen during the Mt Huascaran landslide in Peru on the 31 May, 1970, which initiated a gigantic landslide that killed 67,000 people.

Fire is the second largest killer after building collapses (Stein, 2003: 15) as seen during the San Francisco earthquake (1906), when 900 lives were lost (NFPA, 1991) as a result of the fires which burned most of the city. More recently in Los Angeles, California, the 1994 Northridge earthquake (M6.7), 110 earthquake related-fires were reported in the time between 4:31 am (time of main shock) and midnight (Scawthorn et al. 1998). Fires break out after electrical power cables and gas lines are damaged and can become amplified if water manes are also damaged during the earthquake and consequently water pressure is lost.

Figure 4: Illustration showing how seismic intensity increases in poorly consolidated material

Source: Meng (2010)

Soil liquefaction occurs when, due to shaking, water saturated granular material (such as silts and sands) temporarily loses its strength and transforms from a solid to a liquid. It is possible soil liquefaction may cause solid structures like buildings and bridges, to tilt or sink into liquefied deposits. Major property damage occurred due to soil liquefaction during the 1989 Loma Prieta earthquake that struck the San Francisco Bay area of California. The main area affected by soil liquefaction was the Marina District, 60 miles from the epicentre of the earthquake. This was because the soil had been used to create land on the waterfront, and was highly susceptible to liquefaction.

4


Impacts can be broken down into: short-term (immediate) and long term. They can be social, economic and environmental and will ultimately depend to a large extent on the wealth and level of development of the area affected. Social impacts on a short-term time-scale consist of immediate loss of human life and injuries. Homes may be destroyed making people homeless and leading to large amounts of environmental refugees, which may be re-housed if the funding is available. Transport and communication links may be disrupted which can have detrimental consequences on rescue efforts. Burst water pipes can contaminate drinking water and over a longer time-period can lead to the spread of disease. Economic impacts that occur imminently include the destruction of shops and businesses, which may result in looting taking, place, as seen in Chile, 2010. Trading becomes difficult due to damaged transport links. On a longer time-scale, the cost of rebuilding urban settlements can be very expensive which often results in investments being focused on repairing original buildings. Income to the region may decrease. Environmental impacts on a short-term time-scale comprise of fires, landslides and tsunamis. Each of the aforementioned impacts can destroy human and environmental landmarks.

Unfortunately, earthquakes are not easily predicted and therefore forecasting when the next quake might happen is very difficult. Most seismically active areas use probabilities to describe the likelihood of a region experiencing a large earthquake over a given time frame. For example seismic risk maps draw on data of previous earthquakes in an area to determine how likely it is that area is to experience an earthquake in the foreseeable future.

Earthquake prediction programs have been set up by governments in the U.S, Russia, Japan and China, which monitor major fault zones and study the behavior of rocks before, during and after an earthquake. However, it is not 100% accurate; the Chinese failed to predict the 1976 Tangshan earthquake and the 2008 Wenchuan earthquake, which killed 242,000 and 80,000 people respectively.

However, prediction as a technique for mitigation has its limitations; it is very expensive and often not accurate to the year or month. The best mitigation strategies are ones that try to limit the physical damage and human loss through a series of education programs to inform people how to respond when an earthquake hits, and strict building policies that highlight the need for structurally sound homes and office blocks, particularly in built up high rise cities. However, these too come at a price and very often only wealthier Western states can afford to protect their citizens in this way. The remainder of the essay will focus on two specific earthquake case studies from an MEDC and an LEDC; comparing effects, impacts and responses.

Kobe, is the sixth largest city of Japan, which was hit by largest Japanese earthquake since 1923 hit the city on 17 January 1995 at 5:46am, whilst most people were sleeping or making breakfast. “The earthquake was officially named the 1995 Hyogo-ken Nanbu earthquake, but it is better known as the 1995 Kobe earthquake.” (Katayama, 2006:12) It was triggered by the subduction of the Philippine Sea plate under the Eurasian plate (Figure 6) and occurred along a fault known as the Median Tectonic Line, which lies immediately south of Osaka Bay. The epicentre was only 20km south-west of Kobe.

5


Figure 5: The plate interactions involved in the Kobe earthquake with Kobe marked on the map in relation to Tokyo

Source: GeoResources (2002)

The worst hit area was a section of the central part of the city measuring 5km x 20km, alongside the main docks and port because of the soft rocks underlying the surface which underwent liquefaction. The port itself was built on reclaimed land and therefore saw the most intense liquefaction, which consequently lead to large cranes falling into the sea. More than 300 fires were ignited immediately after the earthquake hit as a result of gas explosions initiated by the vigorous ground shaking. The fires, fuelled by dry debris and whipped up by icy conditions, proved to be quite a challenge to the fire services as 75% of water supplies had been severely damaged and emergency access routes all over the city had been cut off (Figure 6).

The 7.2 magnitude earthquake resulted in 6,434 deaths and $102.5 billion of damage, making it the most expensive modern earthquake. It causes significant economic and physical damage as infrastructure was severely destroyed, including major highways in and out of the city, including the Shinkansen overpass. The houses that collapsed were typically old with heavy mud tiled roofs and timber frames that were used to insulated the properties in summer and protect the inhabitants from typhoons in autumn. However, the violent seismic shaking of the earthquake caused these weak materials to collapse (Otani, 1999: 4). In the aftermath, 316000 people were left homeless and had to move into temporary accommodation.

After several detrimental hazard events, the government of Japan invested heavily in risk and vulnerability assessments within the public and private sectors with the help of scientific organizations, research institutes and universities to understand disaster impacts. The Earthquake Disaster Mitigation Research Center (EDM) was set up in 1998. According to the EDM website (2006) it’s main goals are; firstly, to enhance the safety and resiliency of the medical system, secondly; to develop IT-based tools for disaster mitigation, and finally to build an international disaster-related knowledge dissemination system. The previous Building Standards Law of 1950 was amended post Kobe earthquake to the Act for the Promotion of Earthquake Proof Retrofitting for Buildings (1995) to ensure better building

6


regulation and the national government expenditure for disaster risk reduction is now 5% of the general budget.

The government also set up the Disaster Risk Reduction Information Sharing Platform, which uses a range of different communications including radio, TV, Internet and mobile phones to keep the public informed of impending hazards. There is also a widespread education, awareness program that consists of mock drills and includes a National Prevention Day on Sept 1, which occurs during National Disaster Prevention Week.

Figure 6: Primary effects of the Kobe earthquake, 1995.

Source: Lewis (2007)

In comparison, the Haiti earthquake (2010) had a lower magnitude but much more devastating impacts as Haiti is the poorest country in the western hemisphere with more than 80% of its population lives below the poverty line (Engebawy, 2010) and no disaster mitigation or prevention strategy.

The island of Haiti was the site of a 7.0 magnitude earthquake on 12 th January 2010 at 16.53. The focus was very shallow at only 6.2 miles deep, whilst the epicentre was only 16 miles, from Port-au-Prince, Haiti’s capital and close to the town of Léogâne. The earthquake occurred at a conservative plate margin, where the North American and Caribbean plates were sliding past one another in an east-west direction. The event occurred on the Enriquillo-Plantain Garden fault system in southern Haiti, which is an east-west trending left-lateral strike slip fault (Engebawy, 2010) and was the first major earthquake since 1751, which meant many Haitians were previously unaware that they lived along a seismically active fault line.

7


Figure 7: Map of Haiti with the epicentre marked

Source: Mills (2010)

The earthquake claimed the lives of at least 233,000 people, making it the fifth most powerful earthquake on record, and more deadly than the Asian Tsunami. The International Federation of the Red Cross reported that one third of Haiti’s nine million population needed medical attention.

The densely populated capital city of Port-au-Prince suffered severe damage, largely due to the collapse of thousands of buildings (Figure 8), which, had been constructed in a haphazard fashion, as building regulations had not been enforced. Unreinforced masonry and nonductile reinforced concrete buildings were found in abundance among the debris, building practices that are banned in the wealthier state of California (Engebawy, 2010). Some of the newer buildings had been built to withstand the vertical force exerted by hurricanes, but the lateral force associated with earthquakes was neglected (Engebawy, 2010) so these too disintegrated or collapsed.

The Haitian government have taken certain measures to prepare for hurricanes as these hit the island frequently, but no efforts had been put into earthquake preparedness. Haiti should have been better prepared; especially considering Dixon et al (1998) acknowledged stress was building up fast and an earthquake of magnitude 7.0 could happen at any time and Mann et al (2008) stated at the Caribbean Geological conference that an earthquake was eminent. These observations came from GPS observations of plates and were presented to the Haitian authorities. However, the lack of mitigation strategies are due to the fact that earthquakes, are a rare event, the previous one being almost three hundred years prior, and most importantly the country does not have the financial resources to implement various mitigation schemes, particularly when money is desperately needed to fund more immediate social programs such as health care and education.

8


Figure 8: Building destruction on a hill slope in Haiti.

Source: Echo (2010)

From these examples, it has been shown that the magnitude of an earthquake is not paramount in understanding the social impacts it will cause. More importantly as geographers, we are aware that place, particularly in the context of hazards; define the physical impacts as well as any human or economic losses. While the physical processes involved in triggering an earthquake have been formulated to specific regions; the epicentre, and focus, and movements; body and surface waves, what cannot be translated across borders is the social impacts earthquakes have. Third world countries such as Haiti are more vulnerable due to their lack of investment in mitigation programs, in dealing with hazards during the aftermath and are more vulnerable to hazards themselves, as poor quality building collapse stems from underlying poverty and lack of investment. In this essay I have shown the physical theories that cause earthquakes as well as the component parts to this natural hazards. Through my investigation of this hazard I have identified its major consequences, in physical, human and economic impacts and highlighted this through two detailed case studies: Kobe and Haiti. Through their comparison the inequalities in global development has also been analysed as a cause for enhancing the impacts of this natural hazard.

Word Count: 2970

9

Documents

Earthquake Assignment