Primary and Secondary Impacts of Tectonic Hazards

How one event produces a chain of impacts

A tectonic hazard becomes a disaster when it harms people and damages the places and services they depend on. Its impacts include damage to property, such as homes, schools and roads, and effects on people, such as injury, homelessness and loss of income.

Primary impacts happen directly because of the earthquake or eruption. Earthquake shaking can collapse buildings, kill or injure occupants and break water pipes. During a volcanic eruption, lava can destroy buildings and crops, while ash can damage property.

Secondary impacts follow from those initial effects. A collapsed home leaves its occupants homeless. Broken water supplies can leave people without clean drinking water, increasing the risk of disease. Destroyed workplaces can cause unemployment and loss of income.

The distinction is causal, not simply about timing. An earthquake-triggered landslide is secondary because shaking sets the slope in motion, even if it happens almost immediately. That landslide may then block a road, isolating a community and making the consequences worse.

Nepal: severe impacts in a developing country

On 25 April 2015, a magnitude 7.8 earthquake struck Nepal. Its epicentre was about 80 km north-west of Kathmandu, in the Gorkha district, and its focus was approximately 15 km deep. More than 300 aftershocks followed.

Nepal is a mountainous, landlocked country between India and China. In 2015, around 80% of its population lived in rural communities, many of them remote. This matters because damaged roads and landslides could leave settlements difficult to reach.

The earthquake's primary impacts were extensive:

• Approximately 9,000 people died and more than 20,000 were injured.
• About 7,000 schools and 1,000 health facilities were damaged or destroyed.
• Electricity and water supplies were cut, and temples and UNESCO World Heritage sites were destroyed.

These direct losses created further problems. Almost 3.5 million people were made homeless. Damage to shops, factories and offices prevented people from earning a living, while tourism losses affected an important source of income. Lack of clean water contributed to the spread of disease.

Landslides blocked roads and rivers, and avalanches struck Mount Everest and the Langtang Valley. These were secondary hazards triggered by the earthquake. Their effects extended the disaster beyond buildings damaged by shaking.

Estimated damage was US$7–10 billion, approximately 35% of Nepal's GDP. This expresses the scale of the loss relative to the country's economy: replacing buildings and infrastructure represented a very large financial burden.

Italy: serious damage in a developed country

On 24 August 2016, a magnitude 6.2 earthquake struck central Italy. Its focus was shallow, at about 5.1 km, and more than 200 aftershocks followed over the next few days.

Italy is a high-income country, with GDP per person reported at over US$31,000 for this case study. However, national wealth did not prevent severe local damage.

Primary impacts included 296 deaths and about 400 injuries. More than half of all buildings in Amatrice were damaged or destroyed, while 80% of buildings in its old town were affected. The school collapsed and the hospital was badly damaged. In the immediate area, around 90% of livestock barns and stalls were destroyed, damaging farmers' property and livelihoods.

Secondary impacts included 4,454 people becoming homeless, landslides blocking roads, psychological effects on survivors and reduced tourism. Damage to the hospital also meant that a vital service was affected when people needed medical care.

Explaining the contrast without oversimplifying

Nepal experienced much greater losses of life and far more homelessness than this Italian event. Its earthquake was also larger in magnitude, so the contrast cannot be explained by development alone.

The severity of impacts depends on the physical event, the number of people exposed and their vulnerability. Magnitude, focus depth and location influence the hazard. Population density affects how many people and buildings may be exposed. Building quality and infrastructure affect how easily shaking becomes damage.

Limited financial resources can restrict investment in strong buildings and reliable infrastructure. Remote communities may also be isolated when transport routes fail. These factors help explain why developing countries can suffer severe human impacts and struggle with losses that are large relative to their economies.

Italy demonstrates an important qualification: a developed country can still experience a devastating local disaster. Assessing impact therefore means considering deaths, injuries, homelessness, damage to services and loss of livelihoods—not treating a country's income category as a prediction of the outcome.

Using social media to identify impacts

Social media can provide rapid reports from people near a disaster. A post might identify a damaged settlement, describe a blocked road or share a photograph of collapsed buildings. Such evidence can reveal local problems before a full assessment is available.

After Nepal's earthquake, social media reporting of a helicopter pilot's comments helped alert a NASA–USGS response team to the serious situation in Langtang Valley. Satellite teams were then alerted to investigate the area. This shows how a local report can help direct a wider assessment.

To interpret a report, establish who posted it, where it refers to and when it was recorded. Separate an observation, such as a blocked road, from an unverified estimate of deaths. Check whether other independent reports support it. Several reposts of the same photograph are not several independent observations, and places with disrupted communications may be under-represented.

Using satellite images to assess the extent of damage

Satellite images help assess the spatial extent of a disaster, including areas that are difficult to reach on the ground. A useful method is to compare images of the same place before and after the event.

First check the dates, orientation, scale and clarity of the images. Use unchanged features, such as river bends, to match locations. Then look for changes: debris covering previously visible land, a new landslide scar, interrupted transport routes or altered river channels. Link each change to a possible impact, while distinguishing what is visible from what is inferred.

For Nepal, scientists compared a largely cloud-free Landsat 8 image from 30 April 2015 with imagery from a year earlier. Their analysis identified a large rock-and-ice avalanche that had buried part of Langtang village. This supported an assessment of major damage from a secondary hazard, rather than shaking alone.

The analysis also suggested a river blockage, but there was no evidence at that stage of a lake forming behind it. A blockage does not automatically establish flooding. Scientists needed higher-resolution imagery and information from ground teams to investigate damage around smaller settlements.

Satellite images therefore show patterns and extent, but do not directly establish how many people died or whether every visible building remains usable. Cloud cover, image resolution and the interval between images can limit conclusions.

Combining evidence into an assessment

Socio-economic data adds information that images alone cannot provide: deaths, injuries, homelessness, damaged schools, lost income and the financial burden of destruction.

For example, Nepal's damaged health facilities indicate both physical destruction and reduced access to healthcare. Its estimated losses of about 35% of GDP show the burden relative to the economy, rather than just the monetary total. When comparing figures, check their date and definition: people made homeless are not necessarily the same group as people evacuated.

The strongest assessment brings the evidence together. Social media can identify an urgent local problem; satellite imagery can help establish its extent; socio-economic data can show the consequences for people and livelihoods. Agreement between independent sources strengthens a conclusion, while incomplete evidence should lead to a qualified judgement.

Primary or secondary?

• Primary: direct effects—deaths and injuries from shaking, collapsed buildings, broken water and electricity supplies.
• Secondary: resulting effects—homelessness, disease, lost income, landslides and avalanches triggered by shaking.

Contrasting earthquake cases

Nepal's estimated damage was US$7–10 billion, about 35% of GDP.

Explaining differences

Consider magnitude, depth, location, population exposure, building quality, infrastructure and wealth. Nepal's earthquake was larger; development alone does not explain the contrast. High income does not eliminate vulnerability.

Assessing impact from evidence

• Social media: rapid local reports; check author, location, date and independent confirmation.
• Satellite images: compare before and after; identify changed land cover, debris and blocked routes. Check cloud cover and resolution; images alone cannot establish casualty totals.
• Socio-economic data: assess deaths, homelessness, service disruption and economic losses. Check dates, units and definitions.

Langtang example: social media reports helped alert satellite teams; Landsat imagery identified avalanche burial. An apparent river blockage did not prove that a lake or flooding had developed.

Combine the sources to judge impacts on people, property and livelihoods, acknowledging uncertainty.