Six cases with meaningful contrasts

Why do geophysical hazards affect people differently from one local context to another? The most powerful event doesn't automatically cause the greatest human losses. Those losses also depend on where people live, their circumstances and how ready they are.

The class selects its own case studies: detailed investigations of particular events and their local contexts. The required set includes two volcanic events, two earthquake events and two mass movement events. Choosing different countries isn't enough. Each pair must show the specified contrast.

• Earthquakes: choose events of similar magnitude but substantially different human impacts. Comparing them helps separate earthquake size from the conditions that turn shaking into a disaster.
• Volcanic events: choose different plate-boundary settings. Establish how the tectonic settings differ before comparing the events and their consequences.
• Mass movements: choose contrasting physical characteristics, such as rapid versus slow movement or coherent rock versus loose material.

An illustrative set

These six cases show one suitable selection; none is compulsory. Their locations allow comparisons across different tectonic settings and local environments.

Earthquakes — Haiti, 2010, and Kumamoto, Japan, 2016. Both main events had moment magnitudes of about 7.0. Yet Haiti suffered vastly greater loss of life and destruction. This pair allows the class to investigate construction quality and settlement conditions, alongside public knowledge and preparedness, without assuming that earthquake magnitude alone explains the difference.

Volcanic events — Merapi, Indonesia, 2010, and Fagradalsfjall, Iceland, 2021. Merapi lies above a convergent plate boundary. Fagradalsfjall is in a divergent boundary setting influenced by Iceland's hotspot. At Merapi, explosive activity and pyroclastic flows threatened populated slopes; the relatively remote Icelandic fissure eruption mainly produced lava flows. Both eruption characteristics and exposure help explain the contrasting human impacts.

Mass movements — Oso, United States, 2014, and the active Slumgullion earthflow, United States. Oso involved a sudden, rapid movement of loose slope material. Slumgullion offers a contemporary, monitored example of slow, continuing movement. Specify the monitoring period the class uses for Slumgullion rather than treating its entire history as one sudden event. Comparing the two brings out differences in warning time and movement duration, as well as the opportunity to recognize danger.

What makes the comparison useful?

Give each case a clear location and date or observation period. Include an event profile, evidence of impacts and an explanation of unequal vulnerability. Wherever possible, compare equivalent evidence: deaths with deaths, displacement with displacement, and immediate effects with immediate effects.

Be explicit about the boundaries of the comparison. Earthquakes of similar magnitude do not produce identical shaking at every settlement. Different volcanic settings cannot, by themselves, explain casualties. Nor is slow movement necessarily harmless: damage can accumulate over time.

Event profiles: what happened and where?

An event profile gives a structured account of a hazard's location, timing, physical characteristics and development. The focus is on what people actually experienced, not on a general description of the hazard type.

For each chosen case, record the scale and affected area, along with the event's onset and duration. Include the physical characteristics that explain its impacts: earthquake magnitude and focus depth, volcanic eruption style, or mass movement material and speed. Follow any secondary hazards—additional hazardous processes triggered by the original event. Losses may extend beyond the initial shaking, eruption or slope failure.

In the illustrative earthquake pair, Haiti's shallow earthquake struck the densely settled capital region. Extensive building collapse there caused severe casualties. Kumamoto experienced a damaging earthquake sequence, with building failures and landslides. Its profile therefore needs to separate the main earthquake from the wider sequence and show where slope failures added to shaking damage.

Merapi's 2010 profile must cover explosive activity and pyroclastic flows moving down valleys. Rain-triggered lahars drew material from volcanic deposits, extending the danger beyond the eruption itself. At Fagradalsfjall in 2021, lava spread from fissure vents across a relatively remote landscape. Volcanic gases posed an additional health threat. Don't add a secondary hazard simply because it is possible at volcanoes; establish whether it occurred in the selected event.

At Oso, rapidly mobilized debris crossed the valley and buried homes. It also blocked the river, creating an additional flooding concern. The event killed 43 people. Slumgullion's active earthflow, by contrast, involves persistent, slow deformation. Use observations from a stated period to explain its continuing movement. Evidence must support any associated hazards; do not assume them.

Impacts on human well-being

Human well-being concerns people's health, security, material circumstances and ability to maintain a satisfactory life. A death total cannot capture the full consequences of an event.

Investigate several connected dimensions:

• Physical health: deaths, injuries, illness and interrupted access to treatment.
• Shelter and basic needs: damaged homes, displacement, unsafe water and disrupted food supplies.
• Livelihoods: lost employment, damaged farmland or businesses, and interrupted transport.
• Social and psychological conditions: bereavement, anxiety, separation from support networks and disrupted schooling.

Collapsing buildings in Haiti caused immediate casualties. Displacement and disrupted services then prolonged hardship. Kumamoto also suffered evacuation and housing losses. Deaths associated with evacuation and deteriorating health show why direct casualties need to be distinguished from later consequences.

Merapi caused deaths and evacuation, as well as damage to farming livelihoods. The relatively remote 2021 eruption at Fagradalsfjall had much lower direct impacts on residents, though gas exposure and access to the eruption site still created danger. Remoteness reduced exposure. It did not remove the physical hazard.

Oso brought sudden fatalities, destroyed homes and disrupted transport. A slow earthflow such as Slumgullion has a different impact profile. Continuing deformation can threaten land use and infrastructure without producing a comparable sudden casualty toll. Establish what damage actually occurred during the chosen monitoring period rather than inferring it from movement alone.

Why vulnerability differs between communities

Vulnerability is susceptibility to harm when a hazard occurs. It is different from exposure. Being in the affected area puts someone in contact with the hazard; their circumstances influence how severely it harms them.

Differences between communities in building strength and household resources help explain contrasting outcomes, as do access to services and preparedness. Haiti and Kumamoto show why similar earthquake magnitudes can have very different consequences. Construction and preparedness do not explain everything, though. The location of severe shaking relative to settlements matters too.

Vulnerability also varies within communities, from neighbourhood to neighbourhood and household to household. People living on unstable slopes or in poorly built housing may face greater danger than nearby residents on safer ground. Limited mobility may mean someone needs help to evacuate. Households with fewer resources may have less choice about leaving or finding temporary accommodation.

Perception, personal knowledge and preparedness

Hazard perception is how people interpret the likelihood and seriousness of a threat. Experience can shape that perception, as can trust in warnings and familiarity with the landscape. Even neighbours may perceive a threat differently.

Personal knowledge concerns what individuals understand about the hazard and appropriate actions. Preparedness concerns arrangements made before an event to reduce harm. These include knowing evacuation routes, receiving warnings and practising an agreed response.

The three factors are connected, but they are not interchangeable. Someone may recognize a threat yet have no transport to leave. Another person may know an evacuation route but underestimate the urgency of a warning. Long residence can build useful local knowledge. It can also encourage the mistaken belief that an apparently familiar slope or volcano will remain safe.

These factors vary spatially, and that variation matters. At Merapi, valley routes exposed some settlements more directly to pyroclastic flows and lahars. Access to warnings and evacuation arrangements affected people's ability to escape. Residents and visitors at Fagradalsfjall could have different knowledge of gas danger and safe access. At Oso, the rapid onset severely limited opportunities to act once movement began. Slumgullion's slow movement permits observation over time, but observation alone does not guarantee protection for exposed assets.

For every case-study pair, connect the evidence. Use the event profile to explain the physical threat and the impact evidence to show its consequences. The local comparison explains why those consequences were distributed unequally.