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Causes of Tectonic Hazards

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Douglas Greig

on 22 January 2013

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Transcript of Causes of Tectonic Hazards

The physical causes of tectonic hazards. Convergent/Destructive Boundaries There are a range of complex causes which vary depending on the precise location and circumstances of each tectonic event. Conclusion Subduction zones Divergent/Constructive Plate Boundaries Most occur as oceanic ridges where two plates move apart - eg Mid-Atlantic ridge. Transform Boundaries Transform boundaries occur where two plates slide past each other. Hot Spots Explore the physical causes, including those at different plate boundaries in contrasting places. Other Factors Fracking
http://www.huffingtonpost.com/2012/08/07/fracking-earthquake-conne_n_1752414.html Active Choice
(Behaviourist) Limits on Choice
(Structuralist) Climate Change
http://www.guardian.co.uk/environment/2012/feb/26/why-climate-change-shake-earth Often less dense and thinner oceanic plate forced down under more dense and thicker continental crust. Melting of the plate takes place releasing magma and creating magma chambers in the mantle creating the conditions for volcanic activity and development of stratovolcanoes. Friction between the plates creates heat and further melting and the creation of plutons (v large 'bubbles' of magma) which create magma chambers. The hotter magma is more buoyant than the mantle and rises into the lithosphere creating magma chambers which trigger volcanoes. Water pulled down into the mantle reduces the melting point and creates further magma chambers which are more buoyant and rise into the lithosphere. Oceanic trenches Mountain building Friction between the plates means they do not move smoothly and huge levels of pressure build up. This is released in ruptures in the crust with the two plates thrust in sudden movements along faultlines - these are earthquakes. Where they do occur within continents they produce rifting and eventually rift valleys eg African Rift Valley. Upwelling magma creates the conditions that lead to the development of volcanoes. As the magma is low viscosity, and erupts over long periods of time, shield volcanoes are the most common type of volcano found at these locations. Their eruptions are known as Hawaiian eruptions. Magnitude can arguably be seen to be a more influential factor during the 2010 Tohoku Japanese earthquake where by the convergent plate boundaries between the Philippines, North American and Pacific plates were subducted. This movement was dominated by thrust faulting on or near the subduction zone. This movement was driven by the complex convection currents deep within the core of our planet fuelled by the immense heat which dates back 4.5 billion years ago during the early days of our planet. (Power of the planet-2010) This heat was created from a process known as radioactive fusion, whereby radioactive materials collided to form the shape and dynamics of our planet. After the first three major layers of our planet formed, a cool crust formed surrounding these inner layers, which is commonly known as the lithosphere. This heat is what drives the movement of the lithosphere and is responsible for creating the plates and plate boundaries. As these plates come in and out of contact with each other, earthquakes result from a build up of these vast geological forces as friction and pressure from the plates rubbing together is released in massive ruptures causing the ground to shake in an earthquake. Furthermore, this particular hazard was somewhat interesting and unusual as it changed the way in which people understand how earthquakes occur. The earthquake's rupture behaviour was far more complex than any other. This is due to the fact that both the Pacific and North American plates were made up from oily and ‘velcro’ patches; the Velcro absorbing the strain of subduction, and the oily allowing the plates to slide past one another. Therefore, this dangerous mixture of oily and velcro-like patches allowed the plates to slip all over the place- accounting for the complex nature and abnormal magnitude of the quake. Furthermore, due to these oily patches, the plates slid past each other at its maximum slip of 60 meters. As a result of this, the rupture was noticed to travel in bursts, rather than linearly, as a subduction earthquake typically rips in one or two directions along a fault; on a north south fault line. These bursts of energy ripped four separate patches that have all individually generated quakes in the past. (New scientist-2011) More than any other factor, this rupture accounts for its magnitude. The rate at which this plate motion is taking place also plays a crucial role. In this instance the Pacific plate is moving relatively quickly under the Eurasian Plate at a rate of 9cm a year. This movement results pressure building up between the plates and along faultlines, as the friction between the plates creates resistance. This energy is released in the star burst of ruptures described in this report (New Scientist, 2011). Added to this, the eastern Japanese coastline is particularly vulnerable to tsunami waves due to it deep coastal embankments which amplify the tsunami waves. (BBC ‘How earthquakes cause tsunamis’ 2011) Therefore it is partly the geographical characteristics of this location that played a vital role in causing this secondary hazard. Furthermore, the tsunami fueled by the plate movement, more specifically, the Pacific plate subducting beneath northern Honshu releasing vast amounts of pressure. This build up of stress from the motion was after a lengthy period of time. As the plates buckle, they then slip releasing energy, which causes an uplift in the ocean floor. This release of geological forces displaces vast amounts of water causing a tsunami. Therefore, it is an interaction of duration, magnitude, proximity and geographical characteristics which dominated the causes of these seismic events. Spreading at divergent boundaries is generally not uniform, so where spreading rates adjacent ridge blocks away from a boundary are different, massive transform faults occur. These are the fracture zones that are a major source of earthquakes at these constructive boundaries. However, there are fewer earthquakes than at a subduction zone but they happen at a shallower depth (see map). Plates move relative to one another at about the rate that a fingernail grows and deform relatively little, except at their boundaries, where they slip past one another occasionally during earthquakes. Plate-bounding faults, such as the San Andreas fault, and the Enriquillo Plantain Garden Fault in Haiti, are frictionally locked between earthquakes: the friction between the plates hold them in place. During this locked phase, the Earth’s crust around plate boundaries slowly deforms elastically . This elastic strain slowly accumulates until it raises the associated stress sufficiently to overcome the friction holding the fault in place. The fault then slips, the deformed crust “rebounds” like a broken spring, and suddenly releases the stored strain energy as seismic waves. This cycle of strain accumulation and release, referred to as elastic rebound, represents how earthquakes work. At transform boundaries this rebound is characterised by a 'strike-slip' motion. In Haiti, the Caribbean plate is moving past the North American plate at rate of As the plates scrape past each other huge levels of pressure build up. This pressure is periodically released as earthquakes as rupture happen on certain sections of the fault. As a result earthquakes at such boundaries tend to be very large and happen at a shallow depth within the lithosphere. There is a complex link between divergent and transform boundaries. This video explain this, and also show s how earthquakes are created at both kinds of plate boundary. Some of the most active faultlines in the world are of this type - the North Anatolian faultline in Turkey. Volcanic activity is very rare along transform boundaries as the conditions for melting of and thinning of the lithosphere are not in place. http://www.earthds.info/pdfs/EDS_20.PDF Convection currents Magma plume - can drive divergence of plates, eg North Atlantic Oceanic ridge system. Leads to the formation of islands - and island chains where the plume exists under a moving plate. Active volcanism - with conditions encouraging the development of shield volcanoes. People choose to live in the coastal zone of Japan, in high population densities. In Haiti people's choices are limited by their level of economic development. Physical causes are central to the nature of tectonic hazards, however, hazards only come about when physical processes and events interact with people. Thus, physical causes are only one element that influence the nature of hazards. Another relevant factor that can influence the nature of tectonic hazards is level of development. Level of development can play a large role when determining the effects and nature of a tectonic hazard. It often accounts for how many prepared a country is and their capacity to cope post event. A relevant example to refer to is the Haitian strike slip earthquake; it occurred in 2010, 10 miles from the largest Haitian city, Port Au Prince. Haiti is categorised as a LEDC and is a relatively an undeveloped country. It falls in 145th out of 169 countries in the UN development index. If fact, over 60% of the population live in slums and 70% of the population lives on just $2 a day (OXFAM, 2012). As Haiti is a less developed country, they were extremely unprepared for this earthquake. This meant that very little infrastructure was earthquake proof which led to 70% of buildings in Port-au-Prince collapsing when the shaking occurred. This lead to a large death toll of 220,000 and over 1 million people being left homeless (BOSTON.COM, 2012). The damage in infrastructure also led to restricted clean water supplies, which in turn lead to the spread of many water borne diseases such as Cholera. If Haiti had a better, stronger infrastructure, many injuries and deaths could have been prevented. In addition, as a poorer country, Haiti has limited public and emergency services making rescue operations difficult. Response was very slow and people that were affected could not be reached in time again lowering Haiti’s capacity to cope. In fact, Haiti relied mainly on aid from NGOs such as Oxfam for recovery and rebuilding of the country. Communication, education and adult literacy rates are very low in Haiti, which is very common in less developed countries and some people cannot afford to go or send their children to school. This meant local knowledge about earthquakes and natural disasters was inadequate. People were unable to prepare for any natural disaster and cope afterward leading to an increased death rate. Level of development is ultimately the biggest factor in play regarding the impact of the earthquake Haiti and large cause of the huge death toll. In any location, level of development in its entirety, can change the nature of a tectonic process and is a significant factor to consider when considering the nature of the hazard that results from a physical event. Exam Tips
Underline the key command words and think about the hints in the question.
Plan your answer out in skeleton form - only write down important facts and figures.
Include sub-headings for each section.
Include regular references (author, date).
Include clear diagrams and refer to them.
Remember PEEL.
Use 5 Whys to structure your explanation.
Timing - 15 15 25 25 10 1515252510
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