Plate Tectonics » Earthquakes - causes, impacts, and management

What you'll learn this session

Study time: 30 minutes

The causes of earthquakes and their relationship to plate boundaries
How to measure and describe earthquake intensity and magnitude
Primary and secondary effects of earthquakes
How earthquake impacts vary between countries at different levels of development
Earthquake management strategies: prediction, preparation and response
Case studies of major earthquakes and their impacts

Understanding Earthquakes

Earthquakes are one of nature's most powerful and destructive forces. They happen when there's a sudden release of energy in the Earth's crust, creating seismic waves that shake the ground. But what actually causes them and why do they happen where they do?

Key Definitions:

Earthquake: A sudden shaking of the ground caused by the release of energy along fault lines in the Earth's crust.
Focus: The point beneath the Earth's surface where the earthquake originates.
Epicentre: The point on the Earth's surface directly above the focus.
Seismic waves: Vibrations that travel through the Earth carrying the energy released during an earthquake.
Fault: A fracture in the Earth's crust where movement has occurred.

Causes of Earthquakes

Most earthquakes are caused by the movement of tectonic plates. The Earth's crust is divided into large pieces (tectonic plates) that float on the semi-molten mantle beneath. These plates are constantly moving, albeit very slowly – about as fast as your fingernails grow!

⚠ Tectonic Causes

When plates move against each other, enormous pressure builds up. Eventually, this pressure is released suddenly, causing the ground to shake. This typically happens at plate boundaries where plates interact in different ways:

Destructive (convergent) boundaries: Where plates push together and one is forced beneath the other (subduction). Examples include the Pacific Ring of Fire.
Conservative boundaries: Where plates slide past each other. The San Andreas Fault in California is a famous example.
Constructive (divergent) boundaries: Where plates move apart, though these typically cause smaller earthquakes.

💡 Non-Tectonic Causes

Not all earthquakes are caused by plate movements. Other causes include:

Volcanic activity: Magma movement beneath volcanoes can trigger earthquakes.
Human activities: Mining, reservoir-induced seismicity (from large dams) and fracking can cause small earthquakes.
Collapse earthquakes: When underground caverns or mines collapse.

Measuring Earthquakes

Scientists use two main scales to measure earthquakes:

📊 Richter Scale

Measures the magnitude (energy released) of an earthquake. It's a logarithmic scale, meaning each whole number increase represents a tenfold increase in ground motion and about 32 times more energy release. For example, a magnitude 7 earthquake releases 32 times more energy than a magnitude 6.

Less than 3.5: Generally not felt
3.5-5.4: Often felt but causes minor damage
5.5-6.0: Slight damage to buildings
6.1-6.9: Can cause serious damage in populated areas
7.0-7.9: Major earthquake, causes serious damage
8.0+: Great earthquake, can destroy communities near epicentre

💭 Mercalli Scale

Measures the intensity of an earthquake based on observed effects. It uses Roman numerals from I to XII:

I-III: Barely noticeable
IV-V: Felt by many, minor damage
VI-VII: Felt by all, moderate damage
VIII-IX: Considerable damage, partial collapses
X-XII: Severe to total destruction

Unlike the Richter scale, Mercalli readings can vary within the affected area, as intensity depends on distance from epicentre, ground conditions and building quality.

Impacts of Earthquakes

Earthquakes can have devastating effects on people, property and the environment. These impacts are typically divided into primary (immediate) and secondary (follow-on) effects.

⚡ Primary Effects

Ground shaking causing building collapse
Ground rupture along fault lines
Deaths and injuries from falling debris
Damage to infrastructure (roads, bridges, power lines)
Landslides in mountainous areas

🌍 Secondary Effects

Tsunamis (if earthquake occurs under the ocean)
Fires from broken gas lines
Flooding from damaged dams or water mains
Disease outbreaks from contaminated water
Economic impacts (job losses, reconstruction costs)
Homelessness and displacement

Impacts in HICs vs LICs

The impact of earthquakes varies significantly between High-Income Countries (HICs) and Low-Income Countries (LICs):

🏛 High-Income Countries

Lower death tolls due to better building standards
Higher economic costs due to expensive infrastructure
Better emergency services and medical care
More effective early warning systems
Better insurance coverage for rebuilding
Example: 2011 Japan earthquake (magnitude 9.0) - 15,899 deaths

🏠 Low-Income Countries

Higher death tolls due to poor building standards
Lower economic costs but greater proportional impact on GDP
Limited emergency response capabilities
Slower recovery and reconstruction
Greater long-term impacts on development
Example: 2010 Haiti earthquake (magnitude 7.0) - over 220,000 deaths

Earthquake Management

Managing earthquake risk involves strategies before, during and after an event:

🔍 Prediction & Monitoring

Seismometers to detect ground movement
GPS to monitor plate movement
Studying animal behaviour
Monitoring radon gas emissions
Historical earthquake patterns

Note: Precise prediction remains challenging

🚧 Preparation

Building codes and earthquake-resistant design
Public education and drills
Emergency response planning
Hazard mapping to identify at-risk areas
Insurance schemes

🚑 Response & Recovery

Search and rescue operations
Emergency medical care
Temporary shelter provision
Infrastructure repair
Long-term rebuilding with improved standards

Case Study: 2011 Tōhoku Earthquake and Tsunami, Japan

Event details: On March 11, 2011, a magnitude 9.0 earthquake struck off the northeast coast of Japan, triggering a massive tsunami with waves up to 40 metres high.

Impacts:

15,899 deaths (majority from the tsunami)
6,157 injured and 2,529 missing
Nearly 400,000 buildings destroyed or damaged
Fukushima nuclear disaster - meltdown at nuclear power plant
Economic cost: approximately $235 billion (most expensive natural disaster in history)

Management:

Preparation: Japan had strict building codes, tsunami walls and regular drills
Immediate response: 100,000 troops deployed for rescue operations
International aid: 116 countries offered assistance
Recovery: Comprehensive rebuilding program with improved tsunami defences
Lessons learned: Revised nuclear safety standards, improved tsunami warning systems

Despite being one of the best-prepared countries for earthquakes, the unprecedented scale of this disaster overwhelmed many safeguards, highlighting that even HICs remain vulnerable to extreme events.

Case Study: 2010 Haiti Earthquake

Event details: On January 12, 2010, a magnitude 7.0 earthquake struck near Port-au-Prince, Haiti's capital.

Impacts:

Over 220,000 deaths
300,000+ injured
1.5 million people made homeless
250,000+ buildings destroyed or damaged
Economic cost: $7.8 billion (120% of Haiti's GDP)

Management challenges:

Pre-disaster: Poor building standards, no enforced building codes
Immediate response: Government severely affected, limited emergency services
International response: Massive international aid effort but coordination problems
Recovery: Slow rebuilding process hampered by political instability
Long-term impacts: Cholera outbreak, prolonged displacement in camps

The Haiti earthquake demonstrates how pre-existing vulnerabilities (poverty, poor infrastructure, weak governance) can dramatically amplify the impacts of natural disasters in LICs.

Reducing Earthquake Risk

While we cannot prevent earthquakes, we can reduce their impacts through:

Land-use planning: Avoiding building in high-risk areas
Building design: Using techniques like base isolation, cross-bracing and flexible structures
Education: Teaching people how to respond during an earthquake (Drop, Cover, Hold)
Early warning systems: Providing seconds to minutes of warning before shaking arrives
Community preparedness: Ensuring emergency supplies and plans are in place

Remember that earthquake management is not just about technology and infrastructure – it's also about social factors like poverty reduction, good governance and community resilience. The most effective approaches combine physical protections with social and economic development.