Dig Earth Studio Lab · Case 01

Find the earthquake.

An earthquake has been recorded at three seismic stations. Use the gap between the P and S waves to work out how far away it was — then triangulate the epicenter in this interactive seismology and Earth science investigation.

Before you start

Meet the waves

P waves — the fast ones

P stands for primary. These waves compress and expand rock in the same direction the wave travels. They are the fastest seismic waves, so they arrive first. Their first arrival is often relatively subtle on a seismogram.

S waves — the slower ones

S stands for secondary. These waves move rock perpendicular to the direction the wave travels. They are slower than P waves, so they arrive later and often produce stronger shaking, so their arrival may appear as a larger burst on the seismogram.

The important clue: P waves always win the race. The farther the waves travel, the more time the faster P wave has to pull ahead of the S wave. That is why the P–S arrival-time gap tells us how far away the earthquake was.

1. Read the seismograms

Time runs from left to right on a true 0–120 second scale. Look for the first clear arrival: this is usually the faster P wave. The S wave arrives later and is often stronger. Mark the point where each wave first begins.

Station AALP

0 s306090120 s

P: ?S: ?P–S gap: ?

Station BBRK

0 s306090120 s

P: ?S: ?P–S gap: ?

Station CCRS

0 s306090120 s

P: ?S: ?P–S gap: ?

Detective shortcutFor this simplified case, every second of P–S separation represents about 8 km of distance from the earthquake.

2. Turn the P–S gap into distance

P waves are faster than S waves, so they pull farther ahead as they travel. A small P–S gap means the earthquake was nearby; a larger gap means it was farther away.

Real seismologists use travel-time curves based on measured seismic-wave speeds through Earth. For this simplified investigation, we use 1 second of P–S difference ≈ 8 km.

Small gap → closerLarge gap → farther away

3. Draw the circles

One station gives you a distance, not a direction. Three distances can reveal where the earthquake happened.

Station A

Station B

Station C

Convert the three P–S gaps to draw the distance circles.

4. Pick the epicenter

Once the three distance circles are drawn, click where you think they meet.

Station A?

Station B?

Station C?

Epicenter?

Start by turning the three P–S gaps into distances.