Earthquakes and Seismic Waves
Lesson notes
Overview
P and S waves travel at different speeds; the delay between them locates the epicentre. This topic is rated beginner and is number 6 of 98 in Earth & Space. Core ideas and first intuitions. No prior knowledge needed.
What you are seeing
The panel above shows an interactive animation of the idea. Every slider changes one quantity so you can see its effect on the outcome. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
An earthquake occurs when accumulated stress in rock is suddenly released along a fault, sending out seismic waves. The fast P-waves compress the ground in the direction of travel, and the slower S-waves shake it sideways; surface waves cause most of the damage. The delay between the arrivals of P- and S-waves gives the distance to the source, and with three stations the epicentre can be found by triangulation. The focus is the point of rupture beneath the surface.
Reading the relationship
The expression "distance ∝ t_S − t_P" is a compact summary of the idea. It uses the symbols d, i, s, t, a, n; name what each one stands for before you start. Notice that it contains a trigonometric function, so the result repeats as the angle goes round, and a proportionality, so the two sides rise and fall together. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.
Try this
Start by only moving one slider at a time. Say aloud what you expect, then check.
Common pitfalls
Earth and space systems work on very different scales of time and distance. Check which scale a number refers to before comparing it with another.
How it connects
It builds on “The Rock Cycle”, the topic before it in Earth & Space. Once you are comfortable, continue with “How Volcanoes Erupt”.
Why it matters
Earth and space science helps us understand our planet, its climate and the wider universe.
Where you might use it
- Revising “Earthquakes and Seismic Waves” before a class, exam or certification
- Forecasting weather and understanding climate records and their uncertainty
- Planning for earthquakes, floods and other natural hazards in cities
- Interpreting satellite images, space missions and the age of rocks and stars
Frequently asked questions
What is Earthquakes and Seismic Waves?
P and S waves travel at different speeds; the delay between them locates the epicentre. It is a beginner topic in Earth & Space.
How do I use the Earthquakes and Seismic Waves simulation?
The panel at the top shows an interactive animation of the idea. Every slider changes one quantity so you can see its effect on the outcome. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression distance ∝ t_S − t_P packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Core ideas and first intuitions. No prior knowledge needed. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “The Rock Cycle”, the previous topic in Earth & Space. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.