Chapter 11 Earthquakes

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Chapter 11Earthquakes
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Chapter 11 Earthquakes

• What are earthquakes?
• Tremors or vibrations in the Earths crust
• How do earthquakes occur?
• Convergence, divergence and sliding of crustal
  plates create huge amounts of friction between
  rock surfaces
• The friction stresses the rocks
• The stress causes the rocks to bend and stored
  energy accumulates in the bend
• When the stress becomes too great, the rocks
  break and the stored energy is released as
  seismic waves which cause ground vibrations

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Chapter 11 Earthquakes

• Earthquake terminology
• Focus
• The place in the Earths crust where the
  earthquake originates
• Epicentre
• The point on the Earths surface directly
  above the focus

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Chapter 11 Earthquakes

• Magnitude
• The energy generated by an earthquake
• Recorded using a seismograph
• Measured on a moment magnitude scale (The
  well-known Richter scale is no longer in use in
  the Caribbean)
• Seismic waves are strongest at the epicentre
  and gradually weaken as they spread out

A seismograph
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Chapter 11 Earthquakes

• Measuring magnitude
• Three numbers are used to represent the following
  three measures
• The area that ruptures with the earthquake
• The amount of displacement during the earthquake
• The stiffness of the rocks that break

• A number called the seismic moment is
  obtained by multiplying the above three numbers
• The seismic moment is then converted into
  another number called the moment magnitude
  (magnitude)

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Chapter 11 Earthquakes

• Intensity
• Intensity scales attempt to describe the severity
  of an earthquake by describing the effects on
  people, structures and the physical landscape
• Each one-division increase in intensity
  represents a doubling in severity.

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Chapter 11 Earthquakes
Modified Mercalli Scale
Intensity Effects
I Not felt
II Felt by persons at rest and on upper floors
III Felt indoors. Hanging objects swing. Vibration like passing of a light truck.
IV Vibration like passing of heavy trucks. Stationary motor vehicles rock. Car alarms activated. Windows, dishes and doors rattle.
V Felt outdoors. Sleepers awakened. Liquids spill. Open doors swing.
VI Felt by all with many frightened and running outdoors. Persons walk unsteadily. Windows, dishes and glassware broken. Furniture moves and books fall off shelves. Masonry crack.
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Chapter 11 Earthquakes
Modified Mercalli Scale
Intensity Effects
VII Difficult to stand. Noticed by car drivers. Furniture broken. Damage to masonry and concrete culverts. Large bells ring.
VIII Steering of motor cars affected. Damage to masonry and partial collapse. Chimneys and elevated tanks fall. Branches broken from trees.
IX General panic. General damage to foundations. Serious damage to reservoirs. Cracks on ground.
X Most masonry and frame structures destroyed with their foundations. Some bridges collapse. Large landslides. Serious damage to dams.
XI Rails bend greatly. Ground shifts horizontally on flat land and beaches.
XII Damage nearly total. Large rock masses displaced. Objects thrown into the air.
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Chapter 11 Earthquakes

• Distribution of earthquakes
• Earthquakes coincide with areas of volcanic
  activity and plate boundaries

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Chapter 11 Earthquakes

• Earthquakes occur most often in the Pacific Ring
  of Fire which encircles the Pacific Ocean basin

• The Ring of Fire is located at the boundaries of
  major tectonic plates and is a zone of frequent
  earthquakes and volcanic eruptions

Pacific Ring of Fire
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Chapter 11 Earthquakes

• Damage of earthquakes
• Human settlements
• Buildings may collapse
• People are often killed
• or injured in such situations
• Transport infrastructure
• Roads and railways may
• buckle
• This hinders the
• transportation of food
• and other supplies to affected areas
• Evacuation also
• becomes more difficult

The Giddy House in Jamaica is a result of an
earthquake
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Chapter 11 Earthquakes

• Utility / infrastructure
• Water, gas and electricity supplies may be
  disrupted or cut off
• Fires and explosions may occur due to burst gas
  pipes
• There may be water contamination from sewage
  leakage
• Diseases may spread in such conditions
• Landslides
• The tremors can trigger landslides in unstable
  areas

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Chapter 11 Earthquakes
5. Floods and tsunamis ? Earthquakes near or
under the sea may generate tsunamis
which can devastate coastal areas ?
Flooding of coastal and low-lying areas may
occur

• The 2004 Indian Ocean tsunami, an
  earthquake-generated tsunami has just reached the
  coast

Tsunami that struck Malé in the Maldives in 2006
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Chapter 11 Earthquakes

• Factors that affect the extent of damage
• Magnitude
• The greater the magnitude, the greater the damage
• Location of epicentre and focus
• The closer an area is to the epicentre and focus,
  the greater the damage
• Population density
• The more densely populated an area, the greater
  the potential loss of lives and damage to property

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Chapter 11 Earthquakes

• Foundation of buildings
• Buildings sited on solid rock are less
  susceptible to damage
• The 1985 earthquake in Mexico City caused severe
  damage as the city was built on an old lake bed
• Liquefaction can occur, especially on reclaimed
  land
• During liquefaction, the vibrations cause
  loosely-packed sediments to settle, causing the
  buildings above to sink or shift, e.g. the Giddy
  House at Port Royal in Jamaica
• Prediction
• Effective prediction enables evacuation to be
  carried out before the earthquake occurs, thus
  reducing loss of lives
• In 1975, 90,000 people were evacuated before an
  earthquake of magnitude 7.3 struck Hai Cheng,
  China

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Chapter 11 Earthquakes

• 6. Occurrence of tsunamis
• There is greater damage if a tsunami accompanies
  an earthquake
• The total death toll of the 2004 Asian Tsunami
  has been estimated to be as high as 280,000
  people
• It was triggered by an undersea earthquake off
  the west coast of Sumatra, Indonesia, which
  measured between 9.1 and 9.3 on the Richter scale
• Waves of up to 30m high swept across South and
  Southeast Asia and reached as far as the east
  coast of Africa

Coastal infrastructures are more vulnerable to
tsunami waves than those built further and higher
inland
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Chapter 11 Earthquakes

• A village near the coast in Sumatra after the
  2004 Asian Tsunami

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Chapter 11 Earthquakes

• Ways to reduce the damage
• Emergency preparedness
• Earthquake drills should be carried out to
  familiarise the population with the procedures
  during an earthquake so as to reduce chaos and
  prevent loss of lives
• The government should prepare sufficient relief
  supplies, shelters and medication
• Evacuation plans and procedures should be put in
  place by the relevant authorities
• Strengthening buildings
• Buildings should be reinforced with steel pillars
  that are able to withstand earthquake vibrations
• Fireproof materials should be used in fittings in
  buildings to prevent fires in the event of an
  earthquake

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Chapter 11 Earthquakes

• Improving transport infrastructure
• Transport infrastructure like bridges and
  elevated highways should be retrofitted so that
  they will not give way during an earthquake
• Roads and other transport infrastructure should
  be designed such that they will facilitate
  evacuations, rescue operations and transportation
  of relief supplies

Part of a highway that collapsed due to failure
of its support columns
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Chapter 11 Earthquakes

• Controlling population density
• Population density in earthquake-prone areas
  should be kept low to minimise human casualties
  and damage to buildings
• Earthquake prediction
• Earthquakes cannot be predicted accurately.
• Prediction would enable advance evacuation to be
  carried out
• Scientists look for signs such as the rapid
  appearance or growth of irregular bulges on the
  Earths surface and changes in groundwater levels
• Earthquake control
• Injecting fluids such as water into fault zones
  can help reduce the stress build-up at major
  faults
• The fluids allow the rocks to move, triggering
  small earthquakes and preventing the sudden
  release of stress in the form of a major
  earthquake

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Chapter 11 Earthquakes

• Case study 2001 El Salvador earthquakes
• Occurred in January and February 2001
• Measured 7.6 and 6.6 respectively on the Richter
  scale
• Caused by the subduction of the Cocos Plate as it
  collided with the Caribbean Plate
• Effects
• Buildings collapsed, killing people who were
  trapped inside
• Hospitals were damaged
• Electricity, gas and water supplies were cut
  off
• Roads were damaged, hindering rescue
  operations
• Triggered 500 landslides in areas of unstable
  soil, burying properties, roads and people

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Chapter 11 Earthquakes

• Extent of damage
• 1.6 million people were affected
• 20,000 homes were destroyed
• Factors that affected the extent of damage
• Lack of resources to reduce risk such as by
  using better building materials that could
  withstand the tremors
• Poor planning and disaster management
• Lack of preparedness in the emergency
  services to cope with a major disaster

A landslide triggered by the earthquake