Tectonic Hazards

🔍 How Tectonic Hazards Form

Figure 1: The three types of plate boundary. Arrows show the way each plate moves.

Constructive boundaries are also called divergent, destructive are convergent and conservative are transform. The blocks are not to scale.

The Earth's outermost layer, the crust and the rigid top of the mantle, is not a single solid shell. It is broken into roughly 12 large tectonic plates and several smaller ones, all moving slowly across the surface of the planet. This movement is driven by convection currents in the hot, slowly flowing rock of the mantle below: hot rock rises, spreads outward, cools, and sinks again in a slow churning cycle. The plates move at roughly 5 cm per year — about the speed your fingernails grow. Over millions of years, these tiny movements build mountains, open oceans, and generate the world's most destructive natural events.

Figure 2: Earth's layers. Heat from the core drives convection currents in the mantle, which move the plates.

The layers are drawn to scale except the crust, which is drawn a little thicker so you can see it. The crust is about 5 km thick under the oceans and up to about 70 km under mountains. The mantle is nearly 3,000 km thick.

Almost all earthquakes and volcanoes occur at or near plate margins — the boundaries where two plates meet. There are three types, each producing different hazards:

When an earthquake occurs, energy radiates outward from the focus — the underground point of origin — as seismic waves. The point on the surface directly above the focus is the epicentre, which usually experiences the most intense shaking. Shallow-focus earthquakes (less than 70 km deep) cause more surface damage than deep-focus ones because the energy has less distance to travel before reaching buildings and people.

🏔️ Chile 2010: When Preparation Saves Lives

Chile occupies one of the most seismically active locations on Earth. The Nazca Plate is being subducted beneath the South American Plate along the Peru-Chile Trench, generating earthquakes that regularly rank among the most powerful ever recorded. Chile has lived with this reality for centuries — and, crucially, has used that experience to build resilience into its society.

The Event

At 03:34 on Saturday 27 February 2010, a magnitude 8.8 Mw earthquake struck offshore in the Pacific Ocean, approximately 335 km south-west of Santiago. It lasted for approximately three minutes — an extraordinarily long duration. The epicentre was close to the coastal city of Concepción, Chile's second-largest city. A Pacific-wide tsunami warning was issued within minutes, and tsunami waves of up to 2 metres hit the Chilean coast within 30 minutes of the quake.

Primary Effects

• Approximately 550 people killed — a remarkably low figure for an earthquake of this magnitude
• Around 12,000 people injured
• Approximately 500,000 homes damaged or destroyed
• Around 2 million people displaced from their homes
• Major infrastructure destroyed: roads, bridges, the Biobío River bridge (a critical link between northern and southern Chile), and coastal ports
• Tsunami waves of 2–3 m hit coastal towns including Constitución and Dichato, where hundreds of homes were swept away
• Total economic damage estimated at $30 billion (approximately 18% of Chile's GDP)

Secondary Effects

• Fires broke out in Concepción as gas mains ruptured
• More than 300 aftershocks in the following weeks, the largest measuring 6.9 Mw, hampering rescue and causing additional building collapses
• Widespread looting in Concepción forced the government to impose a curfew and deploy police
• Power outages affected millions of homes across southern Chile
• Water supply disrupted to major cities including Santiago
• Economic disruption from port closures affecting Chile's copper exports (Chile is the world's largest copper producer)

Immediate Responses

• President Michelle Bachelet declared a state of catastrophe within hours, deploying 14,000 military personnel to maintain order and assist rescue efforts
• The Chilean Red Cross coordinated national relief, distributing food, water and emergency shelter
• International aid offers from over 50 countries; Chile accepted targeted assistance rather than general donations
• The Pacific Tsunami Warning Center issued alerts, giving some coastal communities 30 minutes' warning — though the warning system had gaps, and several communities were not evacuated in time
• Chilean hospitals — built to earthquake-resistant standards — remained functional throughout the crisis, allowing medical treatment to continue

Long-Term Responses

• The government launched an $8.4 billion national reconstruction plan, rebuilding roads, bridges, schools and housing within 2–3 years
• Building codes were reviewed and further strengthened — Chile's seismic construction standards are now among the strictest in the world
• The tsunami early warning network was significantly expanded and improved following gaps identified in the 2010 response
• Community resilience training was expanded to include coastal evacuation drills
• Recovery was largely complete within 3–4 years, demonstrating the effectiveness of coordinated national reconstruction

Why Deaths Were So Low

🏘️ Nepal 2015: When Vulnerability Turns Hazard into Disaster

Nepal sits between two enormous tectonic forces. The Indian Plate has been crashing into the Eurasian Plate for around 50 million years, pushing up the Himalayas — the world's highest mountain range — and generating earthquakes that have repeatedly devastated the region. Nepal is one of the poorest countries in Asia, with a GDP per capita of around $700 in 2015. It has very limited resources to prepare for or respond to the tectonic hazards that its geography makes inevitable.

The Event

At 11:56 on Saturday 25 April 2015, a magnitude 7.8 Mw earthquake struck with its epicentre 77 km north-west of Kathmandu, Nepal's capital and most densely populated city. The earthquake was shallow — just 15 km below the surface — which intensified the shaking at ground level. The timing, on a Saturday morning, meant that schools were empty (schools in Nepal run Monday–Friday), which reduced casualties among children. Had it struck on a school day, the death toll would have been significantly higher.

Primary Effects

• Approximately 9,000 people killed — 16 times more than Chile despite being 32 times less powerful
• Around 22,000 people injured
• Over 600,000 houses completely destroyed; a further 300,000 damaged — total 900,000 structures affected
• 2.8 million people displaced from their homes
• Entire villages in remote mountain districts such as Gorkha, Sindhupalchok and Nuwakot were completely destroyed
• Ancient heritage sites in the Kathmandu Valley — including the UNESCO World Heritage-listed Dharahara tower (built 1832) and temples in Durbar Square — collapsed entirely
• Total economic damage estimated at $10 billion — approximately 50% of Nepal's entire annual GDP
• Mount Everest base camp was struck by an avalanche triggered by the earthquake, killing 19 people and injuring dozens of mountaineers

Secondary Effects

• Hundreds of landslides across the mountain terrain, blocking roads and valleys and burying villages
• Landslides created natural dams across rivers, raising the risk of glacial lake outburst floods which could have caused additional mass casualties downstream
• Disease risk from contaminated water supplies in destroyed communities, particularly in remote areas without access to clean water
• Over 150 aftershocks in the first month, including a major 7.3 Mw aftershock on 12 May 2015 that killed a further 200 people and triggered new landslides
• Nepal's tourism industry — the country's largest source of foreign income — collapsed for the 2015 season, with climbers evacuated from Everest and trekking routes closed
• A mental health crisis emerged in the months following the earthquake, with large numbers of survivors — particularly children — experiencing trauma, anxiety and PTSD
• Long-term economic disruption as reconstruction stretched government resources and aid was slow to reach remote communities

Immediate Responses

• The Nepalese government declared a state of emergency and appealed immediately for international assistance
• India sent 16 aircraft and over 1,000 rescue workers within 24 hours — the fastest international response
• The UK pledged £73 million in emergency and long-term aid
• The UN coordinated a global humanitarian response; over $400 million was pledged within the first week
• However, Nepal's infrastructure severely limited the response: only one major road connects Kathmandu to the rest of the country; Kathmandu's airport could not handle the volume of incoming aid flights; and approximately 60% of Nepal's terrain is only accessible on foot
• Rescue teams could not reach many remote mountain villages for 72 hours or more after the earthquake — by which point the critical survival window of 72 hours had passed for many trapped survivors
• Military helicopters rescued over 4,000 climbers and trekkers stranded on mountain routes

Long-Term Responses

• The government established the National Reconstruction Authority (NRA) in December 2015 — seven months after the earthquake — to coordinate rebuilding
• However, progress was extremely slow: three years after the earthquake, hundreds of thousands of families were still living in temporary shelters
• Concerns were raised about corruption and mismanagement in aid distribution, with some communities reporting they had received little or no support
• A "build back better" approach was promoted, encouraging reconstruction using local stone (traditional material) but with improved earthquake-resistant foundations — yet enforcement was inconsistent
• Nepal's remote terrain meant that reconstruction costs were far higher per household than in an accessible country
• Heritage reconstruction began at Kathmandu's UNESCO sites, but the Dharahara tower was not fully rebuilt until 2021 — six years after its collapse

Why Deaths Were So High

🌋 Volcanic Hazards

Volcanoes form primarily at destructive margins, where the subducting oceanic plate melts in the hot mantle and produces magma. This magma is less dense than the surrounding rock, so it rises through fissures and weak points in the crust. When it erupts at the surface, it is called lava. But volcanoes pose multiple hazards beyond lava flows — and many of the most deadly are invisible until it is too late.

Volcanoes also form at hotspots — fixed plumes of exceptionally hot mantle that burn through the plate above them regardless of where the plate margin is. Hawaii sits in the middle of the Pacific Plate, far from any plate margin, yet is one of the most volcanically active places on Earth. Iceland sits on the Mid-Atlantic constructive margin, where the Eurasian and North American plates pull apart.

Types of Volcanic Hazard

• Pyroclastic flows — Superheated mixtures of gas, ash and rock fragments that travel at up to 700 km/h and reach temperatures of 1,000°C. These are the deadliest volcanic hazard — survivors are essentially impossible. The 79 AD eruption of Mount Vesuvius killed the residents of Pompeii and Herculaneum via pyroclastic flows.
• Lahars — Volcanic mudflows created when volcanic material mixes with water (from melted snow, crater lakes, or heavy rain). Lahars can travel at 60 km/h and bury entire towns. The 1985 Nevado del Ruiz eruption in Colombia produced a lahar that buried the town of Armero, killing 23,000 people.
• Ash fall — Fine volcanic ash can collapse roofs under its weight, contaminate water supplies, damage aircraft engines, and disrupt agriculture for years across huge areas. The 1991 eruption of Mount Pinatubo, Philippines, spread ash across South-East Asia.
• Lava flows — Molten rock moving across the surface, destroying everything in its path. However, most lava flows are slow enough that people can be evacuated — property is lost, but lives can be saved.
• Volcanic gases — Eruptions release sulphur dioxide, carbon dioxide, and other toxic gases. In 1986, a sudden release of CO₂ from Lake Nyos in Cameroon (a volcanic crater lake) killed 1,800 people in surrounding villages.

Case Study: Mount Pinatubo, Philippines, 1991

The eruption of Mount Pinatubo on 15 June 1991 was one of the largest volcanic eruptions of the 20th century. The volcano is located on the destructive margin where the Philippine Sea Plate subducts beneath the Eurasian Plate. The eruption ejected 10 km³ of material into the atmosphere and produced pyroclastic flows that destroyed everything within 10 km of the volcano.

• ~800 people died — but scientists estimated 50,000 people's lives were saved by successful evacuation
• The Philippine Institute of Volcanology and Seismology (PHIVOLCS) monitored seismic activity in the weeks before the eruption and correctly predicted the timing and scale
• Around 50,000 people were evacuated from the danger zone before the main eruption
• The eruption injected 20 million tonnes of sulphur dioxide into the stratosphere, temporarily reducing global temperatures by 0.5°C — demonstrating how major eruptions can affect global climate
• Lahars caused by heavy monsoon rain mixing with volcanic ash continued for years after the eruption, repeatedly affecting communities in river valleys below the volcano

Pinatubo demonstrates a critical geographical principle: prediction and evacuation can dramatically reduce volcanic fatalities, even when the physical event is enormous. The comparison with Nepal (where building design and access were the limiting factors) shows that the nature of the hazard determines what type of preparation is most effective.