9.1 Hazardous Environments resulting from tectonic movement

• Global Distribution of Earthquakes:

  → Over 80% of large earthquakes occur around the edges of the Pacific Ocean, in the area known as "Pacific Ring of Fire", this is where the Pacific Plate is sub-ducted beneath the surrounding plate.

  
  

  →Most powerful earthquakes are associated with convergent or conservative boundaries.

  
  

  →Geologists believe that if a fault shows evidence of having moved at least once in the past 100,000 years, it should be regarded as a potential source of earthquakes.

  
  

  →If it has moved at least once in the past 5000 years, then it should be considered a potential source of damaging earthquakes to any settlement within a radius of 50km.

  
  

  →Several Earthquakes occur away from plate margins - they are often known as 'mid-plate boundaries
• The Relationship between the Type of Plate Margins and the Depth of the Earthquakes:

  →There is a greater depth of earthquake focus on convergent plate margins, compared to divergent plate margins.

  
  

  →Convergent plate margins between the Pacific plate margin and the Eurasian plate and between the Pacific plate margin and the Indo-Australian plate margin have a significant depth of earthquake focus (as deep as 800km)

  
  

  →The longer the length and the wider the width of the fault area, the larger the magnitude of the earthquake.

  
  

  →Large Faults tend to accumulate greater stress over-time, hence they produce stronger earthquakes.
• Environmental Unity:


  One event or action can cause a chain of events or actions.
• Reid's Elastic Rebound Theory:

  →The Reid Elastic Theory by Henry Field Reid (Professor Geology at John Hopkins University) concluded that the earthquake must have included an "elastic rebound" of previously stored elastic energy.

  
  

  →If a stretched rubber band is broken or cut; elastic energy stored in the rubber band during the stretching will suddenly be released. Similarly, the crust of the earth can gradually store elastic stress that is released suddenly during an earthquake.

  
  

  →This gradual accumulation and release of stress and strain is now referred to as the "elastic rebound theory" of earthquakes. Most earthquakes are the result of the sudden elastic rebound of previously stored energy.
• What is Asperity?

  Asperities are rough or irregular parts of the fault surface that can resist movement and increase frictional resistance.
• Strike Slip Fault (Conservative Boundaries Body Waves P/S)


  →A type of fault where rocks on either side move past each other (parallel), in the opposite direction. (Shear Stress)

  
  

  →Convection Currents cause the plates to move, which results in stress being applied to the rock. As the plates move, they can become stuck and unable to pass one another, at a fault due to irregularities on the fault surface.

  
  

  →The two sides of the fault becomes locked in place due to frictional resistance, until the stress on the fault surface reaches a critical point where the shear stress exceeds the frictional resistance of the asperity.

  
  

  →The build up of stress due to the motion, causes elastic energy to build up in rocks, and this continues until the stress sufficiently breaks through the asperity (frictional resistance)

  
  

  →The stuck portion of the fault suddenly slides, when the stress over-comes the friction.

  
  

  →The elastic strain energy (Rebound) causes the rock to return to the normal (unstrained) position.

  
  

  →The sudden movement of the rock back into an unstrained position creates seismic waves, which are waves of energy that propagate through the earth's crust.

  
  

  →The seismic waves radiate outwards from the fault surface and cause shaking, which we perceive as an earthquake.

  
  

  Points:

  
  

  →Within the fault zone, there are often areas of "asperities", can concentrate stress, making them more likely to fail and causing the earthquake to originate from there.
• Reverse Fault:

  A type of fault where the hanging wall slides upward; caused by compression in the crust.

  
  

  In a reverse fault, the block above the fault (the hanging wall) moves up relative to the block below the fault (footwall). This type of faulting is caused by compressional forces, which push the rock layers together and cause them to shorten and thicken.

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  Reverse faults are particularly effective at generating earthquakes because the upward movement of the hanging wall produces a more sudden and dramatic displacement of the rocks than other types of faults. As a result, reverse faults are responsible for some of the largest and most destructive earthquakes in history, including the 2011 Tohoku earthquake in Japan and the 2004 Sumatra-Andaman earthquake that caused the Indian Ocean tsunami.

  
  

  Earthquakes on thrust-reverse faults can be particularly devastating because they can result in the uplift of large areas of land, leading to tsunamis and other hazards.

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  A reverse fault can cause an earthquake when the accumulated stress along the fault plane is suddenly released, causing the rock layers to rupture and move rapidly along the fault. This sudden movement generates seismic waves that radiate outward from the epicenter of the earthquake. These waves can cause the ground to shake violently, resulting in damage to buildings and other structures, landslides, and other types of ground failure.
• Normal Fault: (Divergent Boundaries)

  A normal fault, also called tension fault and gravity fault, is formed when there is tension and the rock is being pulled apart from itself. One rock face slips down past the other rock face due to gravity.

  
  

  
  

  This type of fault occurs when the hanging wall (the block of rock above the fault) moves downward relative to the footwall (the block of rock below the fault).

  
  

  When the tensional forces exceed the force, there will be a sudden break/slip releasing energy in the form of seismic waves.

  
  

  The tensional forces that create normal faults are typically caused by the movement of tectonic plates, which are large pieces of the Earth's crust that are constantly in motion. As the plates move apart, the crust is stretched and thinned, creating a zone of tensional stress that can lead to the formation of normal faults.

  
  

  Once the tensional stress exceeds the strength of the rock, the rock will begin to deform and break, forming a fault. The fault plane represents a surface of weakness along which the rock can move, allowing the hanging wall to drop down relative to the footwall.

  
  

  When the rocks on either side of a normal fault move, they release energy in the form of seismic waves, which can cause an earthquake. The sudden movement along the fault generates vibrations that travel through the Earth's crust, causing the ground to shake.

  
• Thrust Fault: (Convergent Boundaries- Subduction)

  • Thrust faults occur at convergent plate boundaries, where two tectonic plates are moving towards each other.

  
  

  • In subduction zones, an oceanic plate is forced beneath a continental or another oceanic plate, creating a deep ocean trench.

  
  

  • The subducting plate sinks into the mantle along the "Benioff Zone", generating friction and heat that causes the plate to deform and stress the surrounding rock.

  
  

  • Over time, the stress builds up, and the rocks on one side of the fault are pushed up and over the rocks on the other side, creating a reverse fault or thrust fault.

  
  

  • Thrust faults occur when the angle of the fault plane is less than 45 degrees, causing one block of rock to move up and over the other block.

  
  

  • The subducting plate's downward motion causes it to bend and generate additional stress in the rocks. This bending can cause the stress to build up more rapidly, leading to larger earthquakes with higher magnitudes.

  
  

  • Thrust faults can cause significant deformation and displacement of rock layers, leading to the formation of mountains, such as the Himalayas, which were formed by the collision of the Indian and Eurasian plates.
• Key Terms

  Shaking:

  
  

  Þ Faulting : A crack in the earth's crust, typically found at tectonic plate boundaries.

  Þ Focus- Where the earthquake starts

  Þ Epicentre- The point on the surface of the earth directly above the focus.

  Þ Only buildings that lie on the fault line, are affected by an earthquake movement.
• What Factors affect the Severity of an Earthquake?

  -The amount of movement on the fault: A larger amount of movement on the fault will generally result in a larger magnitude earthquake.

  
  

  -The length and width of the fault: A longer and wider fault is more likely to generate a larger magnitude earthquake.

  
  

  -The properties of the rock:

  Harder and more brittle rock can generate larger magnitude earthquakes than softer rock. One example is granite, which is a hard and strong rock, tends to create more energy during an earthquake than a softer rock like shale. This is because granite can store more energy before breaking and releasing it as an earthquake. Similarly, a rock like basalt, which is typically denser and stronger than granite, can create more energy than granite in an earthquake. (SOFT ROCK LESS ENERGY IS CREATED)

  
  

  -The type of fault: Different types of faults can release different amounts of energy when they move. For example, thrust faults, which are caused by one piece of rock moving over another, are more likely to generate larger magnitude earthquakes than strike-slip faults, which are caused by two pieces of rock sliding past each other. This happens because thrust faults have greater potential energy stored in them and they tend to release more energy than strike-slip faults.

  
  

  -The depth of the focus: Earthquakes that occur at shallower depths are closer to the surface of the Earth, which means that the seismic waves they generate have a greater effect on the surface. Therefore, earthquakes that occur at shallower depths tend to have larger magnitudes than those that occur at greater depths.

  
  

  -The geology of the area: The type of rock and the presence of underground structures can affect the transmission and amplification of seismic waves. This can influence the magnitude of the earthquake in a positive or negative way.

  
  

  -The shape and size of rock formations can also affect the behaviour of an earthquake. For example, a valley or basin filled with loose sediment (Mud Clay) can amplify the shaking during an earthquake because the sediment can act as a natural amplifier of seismic waves.

  
  

  -The presence of underground structures, such as underground cavities, can also affect the behaviour of an earthquake. These structures can cause the ground to shift and behave differently during an earthquake, leading to more intense shaking in some areas and less intense shaking in others.

  
  

  - Rupture of the Earthquake

  
  

  Building Structural (Vulnerability): "Earthquakes don't kill people, buildings do" - Cameron Sinclair 'Design Architect'

  
  

  Population Density: A natural event such as an earthquake only becomes 'hazard' when it impacts human activity.

  
  

  Nature of the Bedrock: Some materials are vulnerable to becoming "jelly-like" i.e. slit and clay, which can make it prone to liquefaction.
• Richter Scale:


  →The Richter Scale measures the strength (magnitude) of an Earthquake according to the amount of energy released.

  
  

  →1 Point Up the Scale; 32x Energy is released.

  
  

  →Measured by the Magnitude of an Earthquake; Each Increase in +1 magnitude represents a 10x increase in Amplitude.

  
  

  The Richter scale provides a single numerical value that represents the size of an earthquake, while the Mercalli scale provides a qualitative description of the effects of an earthquake, ranges from I (not felt) to XII (destruction) and is considered an "intensity" scale.

  The Richter scale is more widely used and accepted by seismologists and is considered more objective as it is based on measurements of seismic waves, whereas the Mercalli isn't as widely accepted/used but offers a better qualitative information
• Merali Scale:


  →The Merali Scale fails to account for the development of infrastructure or population (Population density) within the area near the epicentre.

  
  

  →The Merali scale is measured by "intensity"

  →It measures the intensity of the earthquake but doesn't account for the timing

  
  

  Subjectivity: The Mercalli scale is based on observations of the effects of an earthquake, rather than on measurements of the seismic waves themselves, this can lead to inconsistencies in how earthquakes are rated on the scale.

  
  

  Limited information: The Mercalli scale does not provide a numerical value that represents the size of the earthquake, which makes it difficult to compare the magnitude of the earthquake between different areas and times.

  
  

  Not standardized: Different versions of the Mercalli scale are used in different countries, this can lead to confusion in communication.

  
  

  Limited applicability: The Mercalli scale is not applicable to all types of earthquakes, such as those that occur in the deep ocean or in very remote areas, where there are no structures or people to observe the effects.
• HIndu- Kush Mountains 25th March 2002


  Case Study 1: Hindu Kush, Afghanistan, 25th March 2002

  
  

  A series of earthquakes lasting 10 hours killed 800 - 1,000 people, injured 4,000 and left approximately 20,000 homeless in a remote mountain region some 150km north of Kabul (Fig. 3). Entire towns were flattened by the earthquake which measured only a moderate 6.1 on the Richter scale. There were several reasons why such a moderate earthquake caused such widespread destruction:

  
  

  The region is remote, war-torn and very poor. Afghanistan is one of the poorest countries in the world and recent droughts and wars have left it without the resources necessary to cope with the after math of an earthquake.

  
  

  The houses were generally inappropriate to withstand ground shaking, many with heavy roofs for insulation which simply collapsed burying their occupants.

  
  

  Although the earthquake was not especially powerful, it was a shallow earthquake occurring at a depth of 8km close to the boundary of the Eurasian and Indian plates, which are converging at a rate of about 4.5cm per year. A report published by the National Earthquake Information Centre in the USA states that "the earthquake of March 25 is another tragic example that shallow earthquakes cause more casualties and damage than intermediate depth ones".
• Direct Volcanic Hazards

  Pyroclastic Flows

  Volcanic Bombs

  Lava Flows

  Ash fallout

  Volcanic Gases

  Nuee Ardente

  Earthquakes
• Indirect Hazards

  Atmospheric ash fall out

  Landslides

  Tsunamis

  Acid Rainfall

  Lahars
• Socio-economic impacts of Volcanic Hazards:

  Destruction of Settlement

  Loss of Life

  Loss of farmlands and forests

  Destruction of infrastructure - roads, airstrips and port facilities

  Disruption of communication
• Retrofitting buildings


  Older buildings can be strengthened by adding new technology to them to make them earthquake proof.
• Cross bracing


  This gives strength to the building prevent it twisting during an earthquake. An example is the Pearl River Tower in China.
• Pyramidal Shape


  The taller a structure and the broader the base, the more flexible it is. The more flexible it is, the less energy is required to keep it from toppling or collapsing when the earth's shaking makes it sway.

  An example is the Trans America Tower in San Francisco.
• Earthquake Drills

  The Great California Shakeout is an annual programme which helps residents to prepare for 'The Big One'. More than 13.6 million people took part in 2015.
• Tsunami warning

  Deep-ocean Assessment and Reporting of Tsunami (DART) system uses buoys and sensors stationed far out to sea. This is done in the Indian Ocean.
• Appropriate Technology


  technology which is designed with consideration for the community it is intended for.
• Bamboo houses

  This is being used in Costa Rica as it is both strong as well as pliable and when a 7.6 magnitude earthquake hit in January 2009 almost all of the bamboo houses at the epicentre survived
• Earthquake early warning


  This quickly announces to the public

  that an earthquake has occurred and to inform them of the estimated seismic

  intensity several seconds or more before the arrival of strong tremors caused by

  the quake. This is used in Japan by the Japan Meteorological Agency.