D.1.3 Earthquake Characteristics and Hazards Notes

Types of Seismic Activity and Associated Risks

1. Imagine standing on solid ground when, without warning, it begins to shake violently.
2. Buildings sway, roads crack, and chaos unfolds.
3. This is the reality of an earthquake, a sudden release of energy within the Earth.

Earthquake Features

Depth of Focus: Shallow vs. Deep

Shallow Focus (0–70 km)

1. Closer to the Surface: Energy travels a shorter distance, causing more intense shaking.
2. Higher Destruction Potential: These earthquakes often cause significant damage to infrastructure and pose greater risks to human life.

Example

The 2010 Haiti earthquake had a shallow focus, leading to widespread devastation.

Deep Focus (300-700 km)

1. Farther from the Surface: Energy dissipates as it travels upward, reducing the intensity of shaking.
2. Less Destructive: While still powerful, deep-focus earthquakes generally cause less surface damage.

Types of Seismic Waves

1. When an earthquake occurs, it generates seismic waves that travel through the Earth. These waves are classified into three main types:

P-Waves (Primary Waves)

1. Fastest Waves: Travel at speeds of 6–8 km/s.
2. Compressional Motion: Push and pull the ground in the direction of wave travel.
3. Travel Through Solids and Liquids: Can move through the Earth’s core.

S-Waves (Secondary Waves)

1. Slower than P-Waves: Travel at speeds of 3–4 km/s.
2. Shear Motion: Move the ground perpendicular to the direction of wave travel.
3. Travel Only Through Solids: Cannot pass through the Earth’s liquid outer core.

Surface Waves

1. Slowest but Most Destructive: Travel along the Earth’s surface.
2. High Damage Potential: Responsible for most of the destruction during earthquakes.

Two Types:

1. Love Waves: Cause horizontal, side-to-side motion.
2. Rayleigh Waves: Cause a rolling, up-and-down motion.

Human-Triggered Earthquakes

While most earthquakes are natural, some are caused by human activities, including:

1. Dam Building: The immense weight of water in reservoirs can increase pressure on faults, triggering earthquakes.
2. Fracking: Injecting high-pressure fluids into the ground to extract oil or gas can induce seismic activity.
3. Resource Extraction: Mining and groundwater extraction can destabilize the Earth’s crust.

Hazards Associated with Earthquakes

Primary Hazards

1. Ground Shaking: The most immediate and destructive effect, causing buildings to collapse and infrastructure to fail.
2. Surface Rupture: The ground cracks or shifts along a fault line, damaging roads, pipelines, and other structures.

Secondary Hazards

1. Tsunamis: Undersea earthquakes displace large volumes of water, generating massive waves that can devastate coastal areas.

How Do Earthquakes Cause Tsunamis?

1. Tsunamis are often caused by undersea earthquakes, particularly those occurring at subduction zones where one tectonic plate is forced beneath another.
2. When the Earth's crust shifts suddenly along a fault line, it displaces a massive volume of water, generating waves that radiate outward across the ocean.
3. These waves, while barely noticeable in deep water, grow dramatically in height as they approach shallow coastal areas, causing catastrophic flooding.
4. The process can be broken down into three key stages:

• Initiation: An earthquake causes a sudden vertical displacement of the seafloor, pushing water upward and creating a series of waves.
• Propagation: The waves travel across the ocean at speeds of up to 800 km/h. In deep water, their low amplitude and long wavelength make them difficult to detect.
• Inundation: As the waves reach shallow coastal areas, their energy is compressed, causing them to grow in height and crash onto the shore with devastating force.

Example

The 2004 Indian Ocean tsunami, triggered by a 9.1-magnitude earthquake, killed over 230,000 people.

1. Landslides: Earthquakes can destabilize slopes, causing rocks and soil to slide downhill.

1. Liquefaction: Saturated soils lose their strength during shaking, causing buildings to sink or tilt.

Why Understanding Seismic Activity Matters

1. Earthquakes are unpredictable, but understanding their features and associated risks can save lives.
2. By studying the depth of focus, types of seismic waves, and potential hazards, scientists and policymakers can develop better strategies for earthquake preparedness and risk reduction.