Seismic Waves — Locate the Epicentre

P-wave primary · fastest · solids & liquids

S-wave secondary · slower · solids only

Surface waves slowest · most damage

Time since quake—

Click anywhere on the globe to trigger an earthquake

Created by Mr Lau Chin Hang

🏫 @curiominds.inc · ▶ YouTube · 🌐 Website

Earthquake

Magnitude 7.0 major Focus depth 15 km

or click the globe

Show

Travel-time graph

Dots show when waves reached each station. The further away, the bigger the gap between P and S.

Learn

Earth's layers

Crust — thin solid rock, 5–70 km thick.
Mantle — solid rock that flows very slowly, down to 2,900 km.
Outer core — liquid iron and nickel, 2,900–5,150 km.
Inner core — solid iron and nickel, to the centre at 6,371 km.

P-waves vs S-waves

	P-wave	S-wave
Type	Longitudinal (push–pull)	Transverse (side to side)
Speed	Faster, 6–13 km/s	Slower, 3.5–7 km/s
Travels through	Solids and liquids	Solids only
Arrives	First	Second

Why do the waves curve?

Rock gets denser and stiffer with depth, so waves speed up as they go down. Changing speed makes them refract (change direction), so the paths curve gradually back up toward the surface. At a boundary between layers the speed changes suddenly, so the wave bends sharply.

Shadow zones — evidence for the core

S-wave shadow zone: no S-waves arrive more than about 104° from the epicentre. S-waves can't travel through liquid, so the outer core must be liquid.

P-wave shadow zone (about 104°–140°): P-waves refract sharply where they enter and leave the core, leaving a ring where almost none arrive.

In 1936 Inge Lehmann spotted faint P-waves inside the shadow zone. She worked out there must be an inner core that bends them back up. Later evidence showed the inner core is solid.

Surface waves

Surface waves travel round the outside of the Earth. They are the slowest, but they shake the ground most and cause most damage.

How to find an epicentre

On a seismogram, measure the time gap between the P-wave and S-wave arrivals.
Use a travel-time graph to turn the gap into a distance.
Draw a circle of that radius around the station.
Do this for 3 stations. The circles cross at the epicentre.