Along a fault, often near a plate boundary, rocks on the two sides are pushed in different directions but stuck together. They bend slowly, storing energy like a drawn bow, until they snap and slip. The sudden slip sends out vibrations called seismic waves: an earthquake.
- The focus is the place underground where the rock first breaks. The epicenter is the point on the surface straight above it.
- P waves (primary) arrive first. They push and pull the rock in the direction they travel, and they can pass through solids, liquids and gases. In the crust they go about 6 km each second.
- S waves (secondary) arrive next. They shake the rock side to side and travel only through solids, at about 3.5 km/s in the crust.
- Surface waves arrive last and travel along the ground. They are slowest but often shake hardest and do the most damage.
Key idea
An earthquake is rock suddenly slipping along a fault. It sends out P waves first, then S waves, then slower surface waves. The break starts at the focus, underground; the epicenter is on the surface above it.
Why it works
Because P waves outrun S waves, the gap between their arrivals grows the farther you are from the quake, like the gap between two runners at different speeds. Over 84 km, a P wave at 6 km/s takes 14 s and an S wave at 3.5 km/s takes 24 s, a 10 s gap. Measure the gap, and you know the distance. Three stations together pin down the epicenter.
Worked example: Which arrives first?
A seismometer 300 km away records an earthquake. Using 6 km/s for P waves and 3 km/s for S waves, when does each arrive?
- P wave: $300 \div 6 = 50$ s after the quake.
- S wave: $300 \div 3 = 100$ s after the quake.
- The S wave arrives 50 s after the P wave.
Answer: P after 50 s, S after 100 s
Worked example: Through the core
Why don't S waves from a large earthquake reach stations on the far side of Earth?
- S waves travel only through solids.
- The outer core is liquid, so it blocks them, leaving a large "shadow zone" on the far side.
Answer: The liquid outer core stops them
Common mistake
Thinking the epicenter is where the rock breaks. The break starts at the focus, which can be a few kilometers or hundreds of kilometers underground. The epicenter is just the point on the map directly above the focus. Check: news reports give an epicenter and a depth; the depth is how far down the focus is.