Seismic waves are detected by seismometers.  The pushes and pulls of the longitudinal 'P' waves arrive at parts of the Earth distant from the epicentre as horizontal movements of the Earth's crust.  The base and graph paper in this seismometer will move with the ground, but the heavy mass and connected pen stay still.   The resulting trace will therefore record all horizontal movements.

Click on the button to see how the seismometer responds to an an earthquake. Use the slider to change the magnitude of the earthquake.

Two seismometers are usually used to detect P waves. Why?