P-Wave and S-Wave Travel Times
Lesson notes
Overview
S-waves lag further behind P-waves with distance. This topic is rated beginner and is number 60 of 98 in Earth & Space. Core ideas and first intuitions. No prior knowledge needed.
What you are seeing
The panel above shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. 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
Because P-waves travel faster than S-waves, about 6 km/s against 3.5 km/s in the crust, the time of arrival, t = d/v, is earlier for P than S, and the gap grows in proportion to the distance from the earthquake. Plotting the S minus P times from three stations gives three circles whose intersection is the epicentre. Travel-time curves also reveal changes in speed at layer boundaries inside the Earth.
Reading the relationship
The expression "t = d/v" is a compact summary of the idea. It uses the symbols t, d, v; name what each one stands for before you start. Notice that it contains a ratio, so increasing the denominator makes the result smaller. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Set every slider to its middle value and note the shape of the curve. 2. Move one slider at a time and describe whether the curve shifts, stretches or turns. 3. Find the value where the behaviour changes sharply, then connect it to the formula.
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 “Richter Amplitude”, the topic before it in Earth & Space. Once you are comfortable, continue with “Geothermal Gradient”.
Why it matters
Earth and space science helps us understand our planet, its climate and the wider universe.
Where you might use it
- Revising “P-Wave and S-Wave Travel Times” 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 P-Wave and S-Wave Travel Times?
S-waves lag further behind P-waves with distance. It is a beginner topic in Earth & Space.
How do I use the P-Wave and S-Wave Travel Times simulation?
The panel at the top shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression t = d/v 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 “Richter Amplitude”, 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.