WEEK 13 - Earthquakes Flashcards
Why do natural disasters seem more destructive today?
More people are living in high-risk areas.
Media coverage has increased awareness.
Not because disasters are getting worse—just more visible and impactful due to population growth.
What is an earthquake?
Vibration of Earth caused by rapid release of energy
Happens along faults in Earth’s crust
What causes the shaking in an earthquake?
Energy is released as seismic waves
Waves radiate in all directions from the source (focus)
What is the focus of an earthquake?
The source point inside Earth where the energy is released
What is the epicentre of an earthquake?
The point on Earth’s surface directly above the focus
What happens to rocks near a fault under tectonic forces?
Rocks on both sides of a fault are deformed by tectonic forces
They bend and store elastic energy
What type of energy is stored in rocks before an earthquake?
Elastic energy is stored as the rocks bend and strain
What triggers an earthquake along a fault?
When frictional forces are overcome, slippage happens at the weakest point (called the focus)
What is elastic rebound?
When deformed rock springs back to its original shape
This release of energy causes earthquake vibrations
What are body waves in an earthquake?
Seismic waves that travel through Earth’s interior
Two types: Primary (P) waves and Secondary (S) waves
What are Primary (P) waves?
Fastest seismic waves
Move by push-pull motion (compress and expand)
Change the volume of material they pass through
What are Secondary (S) waves?
Move by side-to-side shaking
Travel slower than P waves
Shake material at right angles to wave direction
What are surface waves in an earthquake?
Travel along Earth’s surface
Have complex motion
Cause the most destruction
What are Rayleigh (R) waves?
Move in a rolling motion (like ocean waves)
Cause both vertical and horizontal ground movement
What are Love (L) waves?
Move with horizontal shearing motion
Can cause severe damage, especially to buildings
Example: landslide in El Salvador caused by L-waves
In what order do seismic waves arrive
- P waves arrive first
- Followed by S waves
- Then L waves (Love waves)
How is the distance to an earthquake’s epicentre determined?
Based on the arrival time difference between P and S waves
Larger time gap = greater distance from the epicentre
Used by recording stations to calculate location
What is a travel-time graph used for?
Used to find the distance to an earthquake’s epicentre
Based on the time gap between P and S wave arrivals
How does the P–S wave time difference relate to distance?
Longer time difference = greater distance from epicentre
The time gap can be converted into distance using the graph
How is the epicenter located using seismic stations?
Draw a circle around each station
Radius of the circle = distance to the epicenter
How do the circles help find the epicenter?
The point of intersection of all three circles marks the epicenter of the earthquake
Where is most earthquake energy concentrated?
About 95% of earthquake energy is concentrated along plate boundaries
Earthquakes occur as plates slide against each other at these boundaries
What is the Richter scale?
Introduced by Charles Richter in 1935
Measures earthquake magnitude
How does the Richter scale work?
ased on the amplitude (wave height) of the largest seismic waves recorded
Accounts for the decrease in wave amplitude with increased distance from the epicentre
Magnitude is determined using a chart based on amplitude and epicentral distance
