Answer:
They measure the difference between arrival of P waves and S waves.
They look at data from at least three seismographs.
They find the point where circles intersect one another.
Explanation:
Earthquakes produce P and S waves.
These waves travel at different speeds and through different materials.
Geologists at seismic stations measure the difference between their arrival times.
This tells them how far the seismograph is from the epicentre.
They do this at three or more seismographic centres.
They draw on a map circles with radii equal to their distances from the epicentre.
The circles intersect at the epicentre of the earthquake.
The figure below shows the circles from three seismic stations. They intersect at the epicentre of an earthquake in southwest Mexico.
