chapter 11 earthquakes. what are earthquakes? tremors or vibrations in the earth’s crust how do...

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


• 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


Measuring magnitude• Three numbers are used to represent the following

three measures1. 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)


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.


Modified Mercalli Scale

IntensityIntensity EffectsEffectsI 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.


Modified Mercalli Scale

IntensityIntensity EffectsEffectsVII 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.

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

plate boundaries


Volcanic belt

Volcano0 2,000km

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


Pacific Ring of Fire

• 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


Damage of earthquakes1. 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


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

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


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

Tsunami that struck Malé in the Maldives in 2006

http://upload.wikimedia.org/wikipedia/commons/2/2b/2004_Indian_Ocean_earthquake_Maldives_tsunami_wave.jpg


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


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


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

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



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


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


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

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


• 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

chapter 11 earthquakes. what are earthquakes? tremors or vibrations in the earth’s crust how do...

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