Global Patterns of Earthquakes

Published Date: 23 Nov 2017

Achievement Standard Geography 91432: Analyse aspects of a geographic topic at a global scale. (Parshotam)

Task 1: Describe the global pattern of earthquakes, including maps in your answer.

An earthquake is defined by GNS Science, as a “sudden motion or trembling in the crust caused by the abrupt release of accumulated stress along a fault, a break in the Earth’s crust.”[1] As seen below from the map representing data between 2000 and 2008 there is a major peripheral pattern of ‘Global earthquake activity’ focused around the exterior of the earth’s tectonic plates. This is evident from the similarities in comparison to the second map below showing the ‘Global plate boundaries of the earth’.[2] These plate boundaries accurately follow the same shape, size and location of the thin red and bright multi-coloured linear outlining in the first, validating this claim. The most intense locations of earthquake activity are concentrated in a linear pattern in locations of the earth where continental plates interact with oceanic plates. This is illustrated by the multi-coloured bright regions indicating heavy earthquake activity of ranging depth in the earth’s crust in the Western regions of South America, the South East Asian islands, Japan, the far Eastern boarders of Russia, the South Western edges of Alaska, New Zealand and Central America. The information provided here is validated with background research ranking Japan, Indonesia and Chile as first, second and third in the world in terms of countries with the most 6.5+ magnitude earthquake events in 2014.[3] Japan accumulated 72 occurrences, Indonesia 52 and Chile 43. Other noteworthy countries include Papa New Guinea ranked 4^th at 40, Mexico ranked 6^th at 32, Alaska ranked 9^th at 25, Peru ranked 10^th at 23 Russia ranked 12^th at 20 and New Zealand ranked 18^th at 14; all countries included in the heavy earthquake activity regions. Also seen from the ‘Global earthquake activity’ map, there are scattered occurrences of shallow earthquake activity in the middle of the tectonic plates shown from the small distribution of red dots over Eastern Africa for example. This is due to the global pattern of fault lines. The map seen on the next page shows the layout of earth’s major fault lines which explains this smaller scattered pattern of earthquake activity. The location of scatter follows the same alignment as the fault outlines on the ‘Major global fault lines’ map.[4] This is exemplified from the red scatter, on the “Global earthquake activity’ map, throughout Ethiopia, Sudan, Kenya and South Sudan for example which corresponds with the fault line of the Central African Shear Zone fault system illustrated running through Eastern Africa.

Task 2: Fully explain the factors and/or processes contributing to the global pattern of earthquakes.

As defined by GSI Science earthquakes are releases of stress through the interaction of tectonic plates. These tectonic plates are compacted into a patchwork like arrangement to form the Earth’s lithosphere, its outer shell which is comprised of the crust and uppermost mantle (the individual layer of upper mantle which is closest to the surface of the earth). The plates are made up of several large plates being, the Eurasian plate, the North American plate, the African plate, South American plate, the Antarctic plate and the Pacific plate with a few other minor plates in between. Most plates range from 4 to 40 miles in width and consist of both continental crust and oceanic crust. Beneath the lithosphere of the Earth’s tectonic plates is a much hotter and softer layer, called the asthenosphere. A process occurs where due to the amount of pressure and extreme temperatures the rock of the asthenosphere is able to morph and therefore it moves and flows, creating convection currents from radiating hotspots centring from the inner earth which affect the movement of the plates, termed continental drift[5]. The diagram to the left illustrates the relation of levels which contribute to the movement of tectonic plates. This is very slow though, only up to 100mm per year. Due to the heat of the asthenosphere the lithosphere lying above is brittle and therefore susceptible to breaks in the form of faults.

Earthquakes are created through three different forms of plate interaction:

Convergence is one such interaction where two plates collide, and this region of meeting in the Earth’s crust is called a subduction zone or convergence boundary. When the two plates collide, one is forced to slowly scrape over, under or alongside the other. Resulting from the pressure of these movements, the crust gives way and a fault ruptures releasing an earthquake. During the collision of tectonic plates, the denser plate with more mass forces the lighter plate underneath it. This process is called subduction, hence subduction zone. The plate that is forced down is destroyed and is completely melted as it submerges into magma of the asthenosphere underneath the crust. This is how plate boundaries are altered over time. Volcanic activity is produced from subduction where, as the plate is forced down and melted due to the pressure and heat it turns into magma. Pockets of the magma created finds channels to the surface in the form of volcanoes. On December 26^th 2004 an earthquake was generated off the coast of Sumatra when the India and Burma plate converged. The India plate was subducted and the India Ocean tsunami was triggered.[6]

There are multiple situations where Convergence operates, where ocean crust interacts with oceanic crust, where oceanic crust interacts with continental crust and where continental crust interacts with continental crust seen in the following diagrams.[7]

Seen from the diagram to the left where oceanic crust meets oceanic crust island arcs and deep marine trenched are formed which are examples of visible creases in the Earth’s crust as a result of subduction. Island arcs and oceanic trenches occur when both of the plates are made of oceanic crust. The Mariana Trench in Honolulu Hawaii is an example of a result from this process.

Where continental crust meets continental crust spectacular mountain ranges are formed as both plates are too light to be sub ducted so the collision forces the crust upwards. The Himalayas mountain range and the Tibetan Plateau were results of this process when the Australian-India Plate collided with the Eurasian Plate north of India.

Divergence is another type of plate interaction. This is a process where two plates separate from each other and a rift, a gap, is produced and widens eventually creating a rift valley, a larger linear rift. Divergent boundaries that occur between oceanic plates produce mid-oceanic ridges. In areas of rifts molten lava can rise and fill which cools reacting with the water and create new crust material, forming new landmasses potentially altering the landscape. Oceans can be formed when plates diverge and water fills the rift valley between the two land masses. This process is called sea floor spreading where the Red Sea exemplifies this process.

Transformation is the third type of plate interaction which occurs when two plates slide past each other. From this movement the crust of both plates remains unchanged. However stress is stored in the plates from the friction of moving past and aggregates until the amount of stress surpasses the threshold. Energy is released suddenly in a shift of the plates in opposite directions relative to each other and an earthquake occurs. The earthquake impact is focused on an epicentre location. An example of this is the San Andreas Fault running through California resulting from the friction created from the Pacific Plate and North American. This fault is responsible for the linear pattern of red on the South West of Coast of the United States seen on the ‘Global earthquake activity’ map.

As previously stated earthquakes are not only active on tectonic plate boundaries but also fault lines. Fault lines are created due to stresses in the earth’s crust and fractures occur where rock has been disturbed, which can range in length from metres or thousands of kilometres. Fault lines are considered active “if a fault shows evidence of having moved at least once in the past 100,000 years.”[8] With this the fault is hazardous and a risk of earthquake activity. Once a fault line has been formed future earthquakes are conducted along it. With the exception of earthquakes which take place at a depth of 600km+ all earthquakes are channelled along fault lines. Spectacular mountain ranges such as the Southern Alps can be created also as a result of this process where thousands or millions of years of repeated earthquake activity along one major fault line, the Alpine Fault, build up into extreme dislocations of crust. Different to the earthquake activity generated along plate boundaries seismic waves are produced along fault lines through the rapid interaction of the opposing parallel crust.[9] A process is a sequence of related actions which modify or maintain an environment. There are several processes which crust along fault lines carry out to generate seismic waves and subsequent earthquake activity.[10] [11]

Where the two crusts of a fault line are being pulled apart the interaction is termed ‘Normal faulting’. In this situation the hanging wall, which is the crust positioned higher shifts downward, lower than the footwall, the crust which is initially positioned lower.

‘Reverse faulting’ occurs on horizontal ground when the two crusts are instead being compressed together resulting in the hanging wall shifting upwards, over the footwall. Where reverse faulting occurs on sloped land, it is known as thrust faulting.

Crusts on ‘Strike slip’ faults shift sideways, laterally past to each other. This type of faulting is unique in comparison to Normal and Reverse as there are no hanging walls or footwalls due to this being on a strike, horizontal ground, instead of a dip so the fault is on vertical ground.

All three types of fault processes can be seen in an ‘Oblique slip’ where a combination of these shifts occurs. Strike slips are both right lateral and left lateral.

Task 3: Explain in detail and evaluate the social and economic significance for people affected by earthquakes.

Over time, earthquakes have had a range of impacts on the cultural and natural landscape of the world with both positive and negative consequences. The people most affected by these impacts are the people living in the countries which lie on the plate boundaries and fault lines.

New Zealand’s natural landscape has been largely affected by earthquakes and is a prime example to showcase the long term results. Beautiful scenery such, as the Southern Alps, can be created and be of great economic significance for a country by contributing to the tourism industry as a popular feature. This is specifically seen in Queenstown, a location with much mountainous relief where snow sports like snowboarding and skiing is available and popular. Where these spectacles are created they can be of major significance to the country as this is a long term effect which has the potential to earn large amounts of income for the country’s GDP and be a permanent anchor for the tourism industry which it has proven to be in New Zealand. This effect is beneficial socially also as mountains such as these are available for use for the world to enjoy, however only the countries which the mountains are in will prosper economically.

Earthquakes are also capable of causing large scale death and injuries when occurring in an region of high population density. This was demonstrated by the 2010 earthquake in Haiti on January 12^th where there were over 300,000 people injured and an estimated 220,000 casualties.[12] Depending on the magnitude of the earthquake and the location of where it happened to be able to cause the necessary destruction in order for such wide spread death and injury to occur. But when the full effects of an earthquake are felt the social impacts are extreme, seen in the figures previously mentioned. This is a long term effect as the pain suffered from losing a loved one will stay with the victims of the earthquake for the rest of their lives. The physical pain endured can also be intense when the case is severe so because of this the victims will be much more affected by death and injury than the other people of the world in unaffected countries. I believe this effect to have greater significance over that of a hindered economy where as the statistics have shown, regions can improve their economy and standard of living in a reasonably short time and the effects will be reversed, however death and serious injury such as amputation can never be reversed.

Earthquakes have proven to also be detrimental to the economy of the country affected by destroying valuable exports or urban features and environments which cost to be rebuilt and replaced. An example of this effect is seen where as a result of the demolition of the February 2011 earthquake in Christchurch the New Zealand economy shrunk by an estimated 6%-8% which “is equivalent to the economic impact from severe recession.”[13] It is also noted that, coupled with the economic impacts of the September 2010 earthquake the fall in New Zealand’s GDP was around 8%-11%.Similar to the effect of death and injury this impact focuses mainly on the people living in the affected country. It varies in terms of being a major impact or not seen in the paragraph below it can be short term and is possible to reverse if the economy is strong enough to be resilient. From the effects a recession as the economic consequences were compared to, there will be falls in employment, disposable income and savings etc. but this also depends on the magnitude of the earthquake and the harshness of the destruction. Surely economies can be crippled as a result of the destruction of an earthquake, which Christchurch’s was not and therefore it did not remain at that level of severity due to redevelopment. It can be a major impact to the extent of death and injury but in more economically developed countries it is not.

As previously mentioned strong earthquakes which occur in dense urban areas, like cities, wreak much destruction where the seismic waves generated are able to collapse the buildings and infrastructure. This is exemplified from the results of the Haiti earthquake where “over 188,383 houses were badly damaged and 105,000 were destroyed.”¹² Mental and emotional damages such as trauma, fear of another earthquake and the stress of rebuilding and carrying on with life ensue as consequentially1.5 million people were left without a home. This meant many families were faced with the struggle to survive and eventual relocation of their lives. This with the additional loss of incomes for people who lost their business and places of work and hindered of opportunity for kid’s education, where it is recorded 4000 schools were destroyed or damaged, shows earthquake pose as severe threats to the social wellbeing of the world. This impact is major also and again limited to the victims of the event. People’s livelihoods are being endangered and therefore this effect is as serious as death and injury. Homelessness, job losses, lack of education are all risking the futures of the victims where it is a possibility, if the earthquake occurred in an less economically developed country the economy may not be as resilient as that of an more economically developed country. People in LEDC’s are more vulnerable to fall into poverty after a major earthquake.

Redevelopment, however, is a beneficial economic effect that earthquakes can make. Multiple factors of a city or country’s economy can be positively impacted such as the availability of new job opportunities. This is as workers will be employed to clean up and clear the debris and rubble from the left over destruction and construction/development firms will be in heavy demand in order to rebuild the damaged and levelled areas. Where employment rises there will be an increased demand for goods and services as more households are willing and able. From this more income can be taxed tax giving the government revenue and ability to aid in the redevelopment and expand the economy which is seen as $16 billion has been generated as a result of Christchurch’s redevelopment increasing New Zealand’s Real GDP. The economic growth made possible by earthquake redevelopment is validated by the statistic that 9 months after the February earthquake the Christchurch economy was growing between 1-3% every 3 months, more than the national economy growth. Rejuvenation of an economy is positive and major impact. As seen in Christchurch it can lift a city and it’s people out of despair and it will soon be restored as a major New Zealand city. However as mentioned before redevelopment has varying levels of success and is not as effective in some cases as others.

Along with economic growth, resulting from mass redevelopment, locations affected by earthquake activity can become more modern and beautiful which will in turn attract high levels of population, increasing population density and again employment.

Landmarks of historical significance can be lost as a result of earthquakes such as bridges of churches. This loss can cause much grief to those who valued the landmark’s significance and it can also be detrimental to the location’s sense of identity as such features can play a large factor in the region’s history and significance as a place. An example of this is Christchurch’s cathedral which was more than a century old, built in the second half of the 19^th century was damaged beyond safety and therefore was demolished. This is not a major impact when considering the horrific possibilities of widespread poverty and mass death. Also to some history may not be as important as it is to others and if the landmark is not a key point in the country’s GDP, loss of it will not create serious negative economic consequences. This however is long term as history can never be rebuilt or recreated entirely.

[1] http://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/Earthquakes-at-a-Plate-Boundary

[2] http://www.globalchange.umich.edu/globalchange1/current/lectures/evolving_earth/evolving_earth.html

[3] http://www.world-earthquakes.com/index.php?option=ethq_statistics

[4] http://www.gsi.ie/Education/Geology+for+Everyone/Plate+Tectonics.htm

[5] https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Asthenosphere.html

[6] http://www.gsi.ie/Education/Geology+for+Everyone/Plate+Tectonics.htm

[7] http://en.wikipedia.org/wiki/Convergent_boundary

[8] http://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/Earthquakes-and-Faults/When-is-a-Fault-Active

[9] http://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/Earthquakes-and-Faults

[10] http://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/Earthquakes-and-Faults/Different-types-of-Faults

[11] http://geology.about.com/library/bl/blnutshell_fault-type.htm

[12] http://www.dec.org.uk/haiti-earthquake-facts-and-figures

[13] http://ccdu.govt.nz/faq/economy