Download Chapter 11: Earthquakes

Chapter 11
Earthquakes
Chapter 11: Earthquakes
What are earthquakes?
• Tremors or vibrations in the Earth’s 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
Chapter 11: Earthquakes
Earthquake terminology
• Focus
– The place in the Earth’s crust where the earthquake originates
• Epicentre
– The point on the Earth’s surface directly above the focus
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
Chapter 11: Earthquakes
Measuring magnitude
•
Three numbers are used to represent the following
three measures
1. The area that ruptures with the earthquake
2. The amount of displacement during the earthquake
3. 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)
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.
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.
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.
Chapter 11: Earthquakes
Distribution of earthquakes
• Earthquakes coincide with areas of volcanic activity and
plate boundaries
Volcanic
belt
0
Volcano
2,000km
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
0
2,000km
Pacific Ring of Fire
Chapter 11: Earthquakes
Damage of earthquakes
1. Human settlements
•
•
Buildings may collapse
People are often killed
or injured in such situations
2. 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
Chapter 11: Earthquakes
3. 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
4. Landslides
•
The tremors can trigger landslides in unstable areas
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 earthquakegenerated tsunami has just reached the coast
Tsunami that struck Malé in the Maldives
in 2006
Chapter 11: Earthquakes
Factors that affect the extent of damage
1. Magnitude
•
The greater the magnitude, the greater the damage
2. Location of epicentre and focus
•
The closer an area is to the epicentre and focus, the greater
the damage
3. Population density
•
The more densely populated an area, the greater the
potential loss of lives and damage to property
Chapter 11: Earthquakes
4. 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
5. 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
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
Chapter 11: Earthquakes
A village near the coast in Sumatra after the 2004 Asian Tsunami
Chapter 11: Earthquakes
Ways to reduce the damage
1. 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
2. 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
Chapter 11: Earthquakes
3. 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
Chapter 11: Earthquakes
4. Controlling population density
•
Population density in earthquake-prone areas should be kept
low to minimise human casualties and damage to buildings
5. 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 Earth’s surface and changes
in groundwater levels
6. 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
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
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