Types of Seismic Activity and Associated Risks
- Imagine standing on solid ground when, without warning, it begins to shake violently.
- Buildings sway, roads crack, and chaos unfolds.
- This is the reality of an earthquake, a sudden release of energy within the Earth.
Earthquake
Earthquake is a sudden releases of energy in the Earth’s crust, causing ground shaking.
Earthquake Features
Depth of Focus: Shallow vs. Deep
Focus
The focus is the point within the Earth where an earthquake originates.
Epicenter
The epicenter is the point on the Earth’s surface directly above the focus.
Shallow Focus (0–70 km)
- Closer to the Surface: Energy travels a shorter distance, causing more intense shaking.
- Higher Destruction Potential: These earthquakes often cause significant damage to infrastructure and pose greater risks to human life.
The 2010 Haiti earthquake had a shallow focus, leading to widespread devastation.
Deep Focus (300-700 km)
- Farther from the Surface: Energy dissipates as it travels upward, reducing the intensity of shaking.
- Less Destructive: While still powerful, deep-focus earthquakes generally cause less surface damage.
The 2013 Okhotsk Sea earthquake had a focus over 600 km deep, resulting in minimal surface impact despite its magnitude.
NoteShallow-focus earthquakes are more common near tectonic plate boundaries, while deep-focus earthquakes occur in subduction zones where one plate sinks beneath another.
Types of Seismic Waves
- 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)
- Fastest Waves: Travel at speeds of 6–8 km/s.
- Compressional Motion: Push and pull the ground in the direction of wave travel.
- Travel Through Solids and Liquids: Can move through the Earth’s core.
Think of P-waves like a slinky toy being compressed and released.
S-Waves (Secondary Waves)
- Slower than P-Waves: Travel at speeds of 3–4 km/s.
- Shear Motion: Move the ground perpendicular to the direction of wave travel.
- Travel Only Through Solids: Cannot pass through the Earth’s liquid outer core.
Imagine shaking a rope side to side, this is how S-waves move.
Surface Waves
- Slowest but Most Destructive: Travel along the Earth’s surface.
- High Damage Potential: Responsible for most of the destruction during earthquakes.
Two Types:
- Love Waves: Cause horizontal, side-to-side motion.
- Rayleigh Waves: Cause a rolling, up-and-down motion.
Don’t confuse wave speed with destructiveness. Surface waves are slower but cause more damage than P-waves or S-waves.
