3.2 Variations in diaster risks by earthquakes and volcanic eruptions across places

Question 1

Factors that affect extent of tectonic disaster risks across places

Answer 1

determined by:
1. nature of hazards
2. vulnerability
3. exposure to these hazards

Question 2

Factors affecting earthquake disaster risks

Answer 2

Nature of hazards:
- duration of shaking
- time of shaking
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Vulnerable conditions:
- quality of building design and construction
- soil and rock properties
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Exposure:
- population density
- distance from epicentre

Question 3

Duration of shaking

Answer 3

• The length of time of ground shaking can influence the extent of disaster risks.
• The longer the duration of ground shaking, the more damaging an earthquake will be.
• Structures such as buildings and bridges subjected to a long period of stress are more likely to collapse.
• People will more likely be trapped in collapsed infrastructure, leading to more injuries and loss of lives.
  —————
  Example:
• The 2011 Tohoku, Japan earthquake (Mw 9.0) lasted for 6 minutes, causing massive damage to buildings.

Question 4

Time of shaking

Answer 4

• The time of day influences the activities carried out by people and how they respond when the earthquake strikes.
• If people are asleep at night, they will be less alert and are unable to evacuate quickly.
• On the other hand, as people are at work or school during the day, they are more alert and able to evacuate quickly.
• hence, they will less likely be trapped, leading to less injuries and loss of lives.
  —————
  Example:
• The 1995 Kobe, Japan earthquake (Mw 6.9) occurred in the early morning at around 6am. As many people were asleep, they were trapped at home, leading to more than 6,000 lives lost.

Question 5

Quality of building design and construction

Answer 5

• In general, earthquakes do not kill people, buildings and their contents do.
• The poorer the quality of building design and construction, the more vulnerable the buildings are to collapsing, leading to more trapped people, injuries and loss of lives. Hence, the disaster risks are higher.
• Buildings and infrastructure are more vulnerable to collapsing when they:
  1. Are poorly built with low quality materials such as zinc sheets or materials that are rigid that are unable to withstand the shaking.
  2. Do not follow building codes set by the authorities.
  3. Lack earthquake-resistant features such as reinforced steel walls.
  —————
  Example:
• During the 2010 Mw 7.0 earthquake that occurred in Port-au-Prince, Haiti, many buildings and infrastructure collapsed.
• The buildings in Haiti are made of poor-quality materials, and the concrete pillars holding up the buildings are poorly reinforced.
• More than 90% of all buildings near to the epicentre were destroyed during the earthquake, contributing to more than 220,000 lives lost.

Question 6

Soil and rock properties

Answer 6

• It can potentially open the area up to other earthquake hazards like liquefaction.
• When the soil is saturated and loose, shaking may result in liquefaction.
• Buildings may be more vulnerable to collapse as they may sink into the liquefied soil and tip over.
• People will more likely be trapped in the collapsed buildings, leading to more injuries and loss of lives.
• The softer soil, the higher the disaster risks.
• As seismic waves travel from hard rocks to soft soil, the waves get amplified.
• Passing from rock to soil, the waves slow down but become bigger.
• Soft, loose soil will shake more intensely than hard rock, increasing the likelihood of buildings and bridges collapsing.
• People will more likely be trapped in buildings, leading to more injuries and loss of lives. On the other hand, solid rock is much more compact and will shake less intensely.
  —————
  Example:
• Large areas of Port-au-Prince, Haiti lie on layers of relatively soft soil.
• When the 2010 Mw 7.0 earthquake occurred, the seismic waves were amplified.
• This led to the collapse of many buildings and more than 220,000 people losing their lives.

Question 7

Population density

Answer 7

• The higher the population density, the greater the number of people and buildings that are exposed to earthquakes.
• When large numbers of people are located within buildings, more people will be trapped when the buildings collapse, causing more injuries and loss of lives.
  —————
  Example:
• In 1995, the densely populated Japanese industrial city of Kobe was severely affected by a Mw 6.9 earthquake.
• It killed over 6,000 people and injured about 40,000 people.
• The population density then was about 3,000 people per km².

Question 8

Distance from epicentre

Answer 8

• The nearer the city is to the epicentre, the greater the number of people and buildings exposed to the hazard, hence the greater the disaster risks.
• When the city is nearer to the epicentre, less energy is absorbed by the rocks before the seismic waves reach the city.
• The seismic waves reaching the city will be stronger, causing more violent shaking.
• As a result, buildings and bridges are more likely to collapse, and people will more likely be trapped, leading to more injuries and loss and of lives.
  —————
  Example:
• The 2010 Mw 7.0 earthquake in Port-au-Prince, Haiti resulted in more than 220,000 deaths as its epicentre was only about 25km west of the city.

Question 9

Factors influencing disaster risks of volcanic eruptions

Answer 9

Nature of hazards:
- chemical composition of magma
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Vulnerable conditions:
- availability of surface and groundwater facilitating development of lahars
- prevailing wind conditions affecting distribution of tephra
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Exposure:
- presence of human settlements

Question 10

Chemical composition of magma (Low silica magma)

Answer 10

Low silica magma:
- less viscous
- dissolved gas can escape from magma easily
- less pressure in magma when it is rising to the top of the volcano
- hence eruption is gentler and effusive
- lava can flow far from the volcano before cooling.
- damage infrastructure and properties over large areas within the geographic region of the volcano.
- rarely kill people as they can avoid the pathway of lava.
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Example: 2018 eruption of Kilauea in Hawaii
- 24 injuries.
- Destroyed 600 homes.
- USD$800 million in property damage.

Question 11

Chemical composition of magma (high silica magma)

Answer 11

High silica magma:
- more viscous
- it is harder for dissolved air trapped in magma to escape from the magma
- more pressure is built up in magma as magma rises to the top of the volcano
- causes eruption to be explosive as all the gas escape explosively
- may form highly destructive pyroclastic flows, causing widespread damage to infrastructure,
significant injuries and loss of lives.
- explosion of volcanic materials (e.g. volcanic bombs) may strike people and properties, resulting in injuries and loss of lives.
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Example: 2010 eruption of Mount Merapi in Indonesia.
- Pyroclastic flows travelled 3 km down the heavily populated mountain sides.
- Volcanic bombs spread over a distance of 10 km.
- 350,000 people evacuated.
- About 350 people were killed.

Question 12

Availability of surface and groundwater

Answer 12

• The greater the availability of surface and groundwater, the more likely lahars can develop.
• Increases the vulnerability of people and properties, increasing disaster risks as lahars can bury and destroy properties, increasing likelihood of injuries and loss of lives.
• Large quantities of water may come from a range of sources:
  1. Rapid melting of snow and ice on the volcano’s summit just before or during an eruption.
  2. Groundwater released through cracks and fractures during a volcanic eruption.
  3. Existing rivers or lakes nearby.
  4. Heavy rainfall.
  —————
  Example:
• Many lahars following the eruption of Mount Pinatubo, Philippines (1991) were triggered by heavy rain associated with the monsoon season, destroying more than 100,000 homes.

Question 13

Prevailing wind conditions

Answer 13

• The strength and direction of the prevailing winds affect the distribution of ashfall and tephra, influencing the extent of disaster
• Ashfall and tephra may be carried to human settlements, affecting the extent of disaster risks.
• The larger the areas affected, the higher the number of people and properties that become vulnerable.
• Heavier and larger particles are deposited close to the volcano.
• Finer ash particles, which are smaller and lighter, may be carried and deposited hundreds or thousands of kilometers away from the volcano.
• People will likely experience health, particularly respiratory, problems, like asthma
• Properties may be damaged when the weight of the ash accumulated on roofs causes the roofs to collapse.
• Farmland and crops may be destroyed.
  —————
  Example:
• In 1991, Mount Pinatubo, Philippines eruption resulted in ashfall and tephra spreading a large distance due to fast wind speed (more than 20m/s).
• More than 90, 000 hectares of damaged farmland.

Question 14

Presence of human settlements

Answer 14

• Presence of human settlements increases exposure of people and properties to volcanic hazards, increasing disaster risks caused by volcanic eruptions.
• However, many people choose to live near active volcanoes because:
  1. Volcanic soils are rich and fertile, which are ideal for farming.
  2. Geothermal energy can be harnessed to provide electricity.
  3. Valuable minerals such as sulphur can be mined.
  —————
  Example:
• Mount Sinabung is an active volcano in Indonesia. Despite several explosive eruptions of the volcano since 2010, many continue to live and work within the restricted zone 3km from the volcano. This is due to the presence of fertile soils for farming.