The Impacts of Natural Hazards » Secondary Hazards from Tectonic Events

What you'll learn this session

Study time: 30 minutes

The concept of secondary hazards resulting from tectonic events
Types of secondary hazards including tsunamis, landslides and fires
How volcanic eruptions trigger additional environmental hazards
Case studies of significant secondary hazards from real events
Impacts of secondary hazards on people and the environment
Methods to predict and manage secondary hazard risks

Introduction to Secondary Hazards from Tectonic Events

When we think about earthquakes and volcanic eruptions, we often focus on the immediate shaking or lava flows. However, these primary tectonic events frequently trigger a chain reaction of additional hazards that can cause even more damage and loss of life. These are called secondary hazards.

Key Definitions:

Secondary hazards: Dangerous events triggered by a primary natural hazard, often extending the impact zone and timeframe of the disaster.
Tectonic events: Natural processes related to the movement and deformation of the Earth's crust, primarily earthquakes and volcanic eruptions.
Cascading disasters: A sequence of hazards where one event triggers another, potentially creating a domino effect of multiple disasters.

🌋 Primary vs Secondary Hazards

Primary hazards are the direct results of tectonic activity:

Ground shaking during earthquakes
Lava flows from volcanic eruptions
Pyroclastic flows (fast-moving clouds of hot gas and volcanic matter)
Ash fall from volcanic eruptions

💥 Why Secondary Hazards Matter

Secondary hazards often:

Affect larger areas than the primary event
Cause more deaths and destruction
Last longer and complicate recovery efforts
Are harder to predict and prepare for
Create long-term environmental changes

Major Secondary Hazards from Earthquakes

Tsunamis

Tsunamis are among the most devastating secondary hazards from underwater earthquakes. They occur when seafloor movement displaces large volumes of water, creating massive waves.

🌊 Tsunami Formation

Tsunamis typically form when:

An earthquake with magnitude >7.0 occurs under the ocean
Vertical displacement of the seafloor pushes water upward
Energy transfers through the entire water column
Waves travel outward at speeds of 500-800 km/h in deep water
Waves slow down but grow taller as they approach shore

⚠ Tsunami Impacts

Tsunamis can cause:

Coastal flooding extending kilometres inland
Destruction of buildings and infrastructure
Erosion of beaches and coastal defences
Contamination of freshwater with saltwater
Loss of agricultural land due to salt deposition
Long-term economic damage to coastal communities

Case Study Focus: 2011 Tōhoku Earthquake and Tsunami, Japan

On 11 March 2011, a magnitude 9.0 earthquake off Japan's coast triggered a massive tsunami with waves reaching heights of up to 40 metres. The tsunami travelled up to 10 km inland, causing:

Over 15,000 deaths (most from the tsunami, not the earthquake)
Destruction of 130,000+ buildings
A nuclear disaster at the Fukushima Daiichi power plant when tsunami waves overwhelmed protective seawalls
Estimated economic losses of $360 billion
Long-term environmental contamination from the nuclear accident

This event demonstrates how a secondary hazard can cause far more damage than the primary earthquake itself.

Landslides and Ground Failures

Earthquakes can destabilise slopes and trigger various forms of mass movement, especially in mountainous areas or regions with loose soil.

⛰ Earthquake-Induced Landslides

Ground shaking loosens soil and rock on slopes, causing them to slide downhill. These can bury communities, block roads and dam rivers.

💦 Liquefaction

Occurs when water-saturated soil temporarily loses strength during shaking and behaves like a liquid. Buildings can sink or tilt as if built on quicksand.

🕳 Subsidence

The sudden sinking of ground due to underground material movement. Can damage infrastructure and change drainage patterns, leading to flooding.

Fires

Post-earthquake fires are a major secondary hazard in urban areas. They occur when gas lines rupture, electrical systems short-circuit, or heating systems overturn during shaking.

The 1906 San Francisco earthquake is a classic example where fires caused approximately 90% of the total damage. Modern cities remain vulnerable because:

Water mains often break during earthquakes, limiting firefighting capabilities
Roads may be blocked by debris, preventing fire engine access
Multiple simultaneous fires can overwhelm emergency services
Building collapse can trap people who are then threatened by approaching fires

Secondary Hazards from Volcanic Eruptions

Lahars and Mudflows

Lahars are destructive mudflows containing volcanic material, water and debris that flow down the slopes of volcanoes. They can travel at speeds of up to 100 km/h and move objects as large as houses.

How lahars form:

When volcanic heat melts snow or ice on a volcano
When heavy rainfall mixes with loose volcanic ash
When crater lakes or dammed rivers suddenly release
Even years after an eruption when heavy rain mobilises ash deposits

Case Study Focus: 1985 Nevado del Ruiz Lahar, Colombia

On 13 November 1985, a relatively small eruption of Nevado del Ruiz volcano melted the mountain's ice cap, generating massive lahars that flowed down river valleys.

The town of Armero, 74 km from the volcano, was buried under 5 metres of mud, killing approximately 23,000 people (about 75% of the town's population). This disaster highlights how secondary hazards can strike far from the volcano itself and cause more casualties than the primary eruption.

Volcanic Gases and Acid Rain

Volcanoes release various gases that can create environmental and health hazards:

☁ Volcanic Gas Impacts

Sulphur dioxide (SO₂): Irritates respiratory systems and combines with water to form acid rain
Carbon dioxide (CO₂): Can accumulate in low-lying areas, causing asphyxiation
Hydrogen sulphide (H₂S): Toxic gas with a rotten egg smell
Hydrofluoric acid (HF): Can damage vegetation and contaminate water sources

💧 Acid Rain Effects

Damages plant leaves and leaches nutrients from soil
Increases acidity in lakes and rivers, harming aquatic life
Corrodes buildings, statues and infrastructure
Contaminates drinking water supplies
Can affect areas hundreds of kilometres from the volcano

Climate Effects

Large volcanic eruptions can inject ash and gases into the upper atmosphere, creating temporary but significant climate impacts:

Global cooling: Sulphate aerosols in the stratosphere reflect sunlight, reducing temperatures worldwide
Altered rainfall patterns: Weakened monsoons and disrupted precipitation in tropical regions
Ozone depletion: Volcanic gases can react with and destroy ozone molecules

The 1815 eruption of Mount Tambora in Indonesia led to the "Year Without a Summer" in 1816, causing crop failures and food shortages across North America and Europe. Global temperatures dropped by about 0.5°C for several years.

Predicting and Managing Secondary Hazard Risks

Predicting secondary hazards is challenging but critical for effective disaster management. Modern approaches include:

📊 Risk Assessment

Creating hazard maps that identify areas vulnerable to secondary hazards based on topography, soil conditions and infrastructure.

🔍 Monitoring Systems

Tsunami warning networks, gas monitoring stations and satellite observation of volcanic activity to provide early warnings.

💻 Computer Modelling

Simulation of potential secondary hazard scenarios to predict impact zones, timing and severity.

Mitigation Strategies

Communities can reduce secondary hazard risks through:

Land-use planning: Restricting development in high-risk areas like tsunami zones or lahar paths
Engineering solutions: Building tsunami walls, landslide barriers and earthquake-resistant infrastructure
Ecosystem protection: Maintaining natural buffers like mangroves that can reduce tsunami impacts
Education and drills: Teaching communities to recognise warning signs and practise evacuation procedures
Building codes: Requiring automatic shut-off valves for gas lines to prevent post-earthquake fires

Conclusion

Secondary hazards often cause more damage and casualties than the primary tectonic events that trigger them. Understanding the complex relationships between primary and secondary hazards is essential for effective disaster risk reduction. By implementing comprehensive monitoring systems, appropriate land-use planning and community education, the impacts of these cascading disasters can be significantly reduced.

As our climate changes and populations in hazard-prone areas grow, managing secondary hazard risks becomes increasingly important. The lessons from past disasters like the 2011 Japan tsunami and the Nevado del Ruiz lahar demonstrate that preparing for secondary hazards must be a priority in disaster management planning.

🧠 Test Your Knowledge!