How do earthquake waves reveal distance?

An earthquake sends vibrations through Earth that can arrive at a recording station at different times. P waves alternately compress and stretch material along their direction of travel; S waves move material back and forth across that direction. Along the simplified equal-length route here, the faster P wave arrives first and the S wave follows. Their separation in time carries a clue to the distance traveled.

Travel time depends on distance and speed. This uniform teaching material assigns P waves a speed of 6 kilometers per second and S waves 3 kilometers per second. Over 60 kilometers, the P wave takes 10 seconds and the S wave 20 seconds. Their arrival-time gap is therefore 10 seconds.

The slower wave falls farther behind as the shared distance increases. For these particular speeds, the S–P gap equals distance divided by 6 when distance is in kilometers and time in seconds. A measured gap can therefore constrain distance in this model without requiring the earthquake's start time to be known.

Real seismologists use travel-time models that account for Earth's changing materials and wave paths, not these fixed speeds everywhere. A single station's distance does not identify direction or a unique source location. Arrival-time difference is also not earthquake magnitude: information about signal size and other factors is needed for that.