Tectonic Hazards » Earthquake Characteristics

What you'll learn this session

Study time: 30 minutes

The causes of earthquakes and tectonic plate movements
Key earthquake terminology and measurement scales
Primary and secondary effects of earthquakes
How earthquake waves travel and create damage
Case studies of major earthquakes and their impacts
Factors affecting earthquake severity and damage

Introduction to Earthquake Characteristics

Earthquakes are one of nature's most powerful and frightening phenomena. They can strike with little warning, shake the ground beneath our feet and cause widespread destruction in just seconds. But what exactly causes these earth-shaking events and how do scientists understand and measure them?

Key Definitions:

Earthquake: A sudden violent shaking of the ground caused by movement of tectonic plates along fault lines.
Tectonic plates: Large sections of the Earth's crust that float on the semi-molten mantle beneath.
Fault: A fracture in the Earth's crust where rocks have moved past each other.
Focus: The point underground where an earthquake originates.
Epicentre: The point on the Earth's surface directly above the focus.

🌎 Plate Tectonics and Earthquakes

The Earth's crust is divided into about 15 major tectonic plates that move very slowly (a few centimetres per year). Earthquakes occur at plate boundaries where plates interact in three main ways:

Convergent boundaries: Where plates push together, causing one to sink beneath the other (subduction).
Divergent boundaries: Where plates move apart, creating gaps that fill with magma.
Transform boundaries: Where plates slide past each other horizontally.

🔥 Why Earthquakes Happen

As tectonic plates move, they can get stuck against each other due to friction. Pressure builds up over time until it overcomes the friction. When the rocks suddenly slip past each other, energy is released in waves that travel through the Earth's crust - this is an earthquake!

This process is called elastic rebound theory - like stretching an elastic band until it snaps.

Measuring and Describing Earthquakes

Scientists use several methods to measure and describe earthquakes, helping us understand their power and potential for damage.

📊 Richter Scale

Measures the magnitude (energy released) of an earthquake on a logarithmic scale from 1-10. Each whole number represents a 10-fold increase in amplitude and about 32 times more energy. Earthquakes below 3 are barely felt, while those above 7 cause major damage.

🏢 Mercalli Scale

Measures the intensity of an earthquake based on observed effects on people, buildings and the environment. Uses Roman numerals from I (not felt) to XII (total destruction). Unlike the Richter scale, intensity varies with distance from the epicentre and local conditions.

🛠 Moment Magnitude Scale

The modern replacement for the Richter scale, more accurate for larger earthquakes. It measures the total energy released based on the area of fault rupture, the average displacement and the force required. Most commonly used by seismologists today.

Earthquake Waves

When an earthquake occurs, it releases energy in the form of seismic waves that travel through the Earth. Understanding these waves helps explain how earthquakes cause damage.

🔄 Primary (P) Waves

These are compression waves that push and pull the ground like an accordion. They:

Travel fastest (up to 6 km/second)
Can move through solids, liquids and gases
Are the first waves to arrive after an earthquake
Cause less damage than other wave types

🌊 Secondary (S) Waves

These are shear waves that move the ground up and down or side to side. They:

Travel more slowly than P-waves
Can only move through solids
Arrive second after an earthquake
Cause more damage than P-waves

🌋 Surface Waves

These waves travel along the Earth's surface rather than through it. They:

Are the slowest of all seismic waves
Arrive last but last the longest
Cause the most damage to buildings and infrastructure
Include Rayleigh waves (rolling motion) and Love waves (side-to-side motion)

🔩 Factors Affecting Earthquake Damage

The amount of damage caused by an earthquake depends on several factors:

Magnitude: How much energy is released
Depth of focus: Shallow earthquakes (< 70km) typically cause more damage
Distance from epicentre: Damage usually decreases with distance
Ground conditions: Soft soils amplify shaking (liquefaction)
Population density: More people = more potential casualties
Building standards: Well-built structures withstand shaking better
Time of day: Affects where people are (home, work, school)

Earthquake Effects

Earthquakes cause both immediate (primary) effects and longer-term (secondary) effects that can be equally devastating.

💥 Primary Effects

These are the immediate results of the earthquake itself:

Ground shaking and displacement
Building collapse and structural damage
Road and bridge destruction
Rupturing of gas and water pipes
Injuries and deaths from falling debris
Landslides and rockfalls
Tsunamis (if underwater)

🔁 Secondary Effects

These occur as a result of the primary effects:

Fires from broken gas lines
Flooding from broken dams or water mains
Disease outbreaks from contaminated water
Homelessness and displacement
Economic impacts (job losses, rebuilding costs)
Food and water shortages
Psychological trauma and social disruption

Case Study Focus: 2011 Tōhoku Earthquake and Tsunami, Japan

On 11 March 2011, a magnitude 9.0 earthquake struck off the northeast coast of Japan, triggering a devastating tsunami.

Key characteristics:

Occurred at a convergent plate boundary where the Pacific Plate subducts beneath the North American Plate
Focus was relatively shallow at 32km depth
Generated tsunami waves up to 40 metres high
Ground shaking lasted for approximately 6 minutes

Primary effects:

15,897 deaths (mostly from tsunami rather than the earthquake itself)
6,157 injuries
2,533 people missing
Buildings damaged or destroyed: 1.2 million
Fukushima nuclear power plant damaged, causing radiation leaks

Secondary effects:

Nuclear disaster led to evacuation of 470,000 people
Economic cost estimated at $235 billion (most expensive natural disaster in history)
Global supply chains disrupted (especially automotive and electronics industries)
Increased energy costs as Japan shut down nuclear power plants

Earthquake Distribution

Earthquakes aren't randomly distributed around the world - they occur in specific patterns that match plate boundaries.

🌏 The Ring of Fire

About 90% of all earthquakes occur along the "Ring of Fire" - a horseshoe-shaped zone around the Pacific Ocean where many tectonic plates meet. This includes:

The west coast of North and South America
Alaska and the Aleutian Islands
Japan, Philippines and Indonesia
New Zealand

The Ring of Fire contains about 75% of the world's active volcanoes and experiences frequent earthquakes due to subduction zones where oceanic plates dive beneath continental plates.

🗺 Other Major Earthquake Zones

Other significant earthquake zones include:

The Alpine-Himalayan Belt: Stretches from the Mediterranean through the Middle East and northern India to Southeast Asia
Mid-Atlantic Ridge: A divergent boundary running down the middle of the Atlantic Ocean
East African Rift Valley: Where the African continent is slowly splitting apart
Transform faults: Like the San Andreas Fault in California

Conclusion

Understanding earthquake characteristics is essential for predicting their occurrence, preparing for their impacts and designing structures that can withstand them. While we cannot prevent earthquakes, knowledge about how they work helps communities become more resilient and reduces the loss of life and property when these powerful natural events occur.

In future lessons, we'll explore how communities can prepare for earthquakes, how scientists monitor and predict them and examine more detailed case studies of significant earthquakes from around the world.

🧠 Test Your Knowledge!