Tectonics topic 3.2 (variations in disaster risks caused by earthquakes and volcanic eruptions across places

factors influencing disaster risks

1. nature of hazard

2. vulnerable conditions

3. exposure

nature of hazards

physical characteristics of the hazard itself

vulnerable conditions

ability to cope, resist and recover

exposure

people and assets located iin hazard-prone areas

nature of hazards that influence disaster risks caused by earthquakes :

1. duration of shaking

2. time of shaking

3. magnitude of earthquake (not covered in tb)

earthquake nature of hazards : duration of shaking

• Longer duration, more damaging 

• Structures subjected to longer period of stress (structural fatigue)

• extended shaking can cause cumulative damage → collapse → more people will be trapped → increased injuries and death 

• Example : The 2011 Tohoku Japan earthquake (Mw 9.0) lasted for 6 minutes, causing massive damage and around 15,800 deaths and more than 2500 people still missing. 

earthquake nature of hazards : time of shaking

Daytime earthquakes : 

• Negative : occurs when people are at work or in school, increasing the likelihood of casualties in crowded buildings or transit systems. For example, schools and office buildings may collapse, trapping a large number of people. Crowding in urban areas can also lead to stampedes or injuries. 

• Positive : people are at work or school during the day, hence will be more alert and can evacuate quickly 

Nighttime earthquakes : 

• Negative : less alert and are unable to evacuate quickly as they may be disoriented 

• Positive : reduces risks or injuries from crowd panic

vulnerable conditions that can affect disaster risks caused by earthquakes :

1. quality of building design and construction

2. soil and rock properties

vulnerability earthquakes : quality of building design and construction

• The use of substandard materials (low quality materials), reduces a buildings strength, making it more prone to cracking or collapsing during shaking 

• Inadequate reinforcement such as steel rods in concrete structures are less capable of withstanding horizontal forces created by an earthquake (seismic waves causes the ground to shift back and forth, generating horizontal forces that can push or pull buildings horizontally) 

• Poor building design → harder to escape, leading to more trapped people, injuries and loss of lives 

• Example : During the 2010 Mw 7.0 earthquake in Haiti, many buildings and infrastructure collapsed. The buildings in Haiti are made of poor quality materialism and the concrete pillars holding up the buildings are poorly reinforced. More than 90% of all buildings near the epicenter were destroyed during earthquake → more than 220k lives lost

vulnerability earthquake : soil and rock properties

• Soft soils : loose, unconsolidated soils amplify seismic waves, increasing intensity of ground shaking compared to areas with solid bedrock 

• Saturated loose soils : intense shaking can cause soil liquefaction → buildings may be more vulnerable to collapse as they may sink into the liquefied soil and tip over. People will more likely to be trapped in the collapsed buildings, leading to more injuries and loss of lives 

• Taller buildings are more vulnerable in areas with soft soils that cause long-period ground motion 

• Hard rock : solid bedrock transmits seismic waves more efficiently, often resulting in less shaking compared to soft soils 

• Example : Large areas of Haiti lie on layers of relatively soft soil. When the 2010 earthquake occurred, seismic waves were amplified which led to the collapse of many buildings and more than 220k people losing their lives

exposure of people and their properties to earthquake may be influenced by

1. population density

2. distance from epicentre

exposure earthquake : population density

• Higher population density greater exposed to earthquakes

• Large number of people 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 which killed over 6000 people and injured about 40k people. The population density then was about 3000 people per km2.

exposure earthquake : distance from epicentre

• Closer to epicentre, greater the number of people and buildings exposed to earthquakes → seismic waves more concentrated, less energy absorbed by rocks before the seismic waves reach the city 

• Example : 2010 earthquake in Haiti resulted in more than 220k deaths as its epicentre was only about 25km west of the city 

nature of hazards volcanoes : low silica vs high silica magma

Low silica magma 

• Less viscous magma

• Gentle, effusive eruption 

• 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 pathway of lava

• Example : 2018 eruption of Kilauea in Hawaii caused 24 injuries, destroyed 600 homes, and USD$800 in property damage 

High silica magma 

• More viscous magma

• Explosive eruption 

• May form highly destructive pyroclastic flows, causing widespread damage to infrastructure, significant injuries and loss of lives

• Explosion of volcanic materials (eg volcanic bombs) may strike people and properties, resulting in injuries and loss of lives 

• Example : 2010 eruption of Mount Merapi in Indonesia. Pyroclastic flows travelled 3km down the heavily populated mountain sides. Volcanic bombs spread over a distance of 10km. 350k people evacuated and about 350 killed. 

disaster risks caused by volcanic eruptions can be influenced by vulnerability :

1. availability of surface and groundwater

2. prevailing wind conditions

vulnerability volcano : availability of surface and groundwater

• Greater availability of surface and groundwater, more likely lahars can develop 

• Increases 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

  • Rapid melting of snow and ice on the volcano’s summit just before or during an eruption

  • Groundwater released through cracks and fractures during a volcanic eruption

  • Existing rivers or lakes nearby

  • 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 100k homes. 

vulnerability volcano : prevailing wind conditions

• Strength and direction of the prevailing winds affect the distribution of ashfall and tephra, influencing the extent of disaster risks → ashfall and tephra may be carried to human settlements, affecting extent of disaster risks

• Larger the area affected, higher 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 likely to experience health, particularly respiratory problems. 

• Properties may be damaged when the weight of the ash accumulated on roofs caused the roofs to collapse. 

• Farmlands 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 90k hectares of damaged farmlands

exposure volcano

• Presence of human settlements increases exposure of people and properties to volcanic hazards, increasing disaster risks 

• Many people choose to live near volcanoes as volcanic soils are rich and fertile, which are ideal for farming. Geothermal energy can be harnessed to provide electricity and valuable minerals such as sulfur 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.