Earthquakes are easier to identify than other signals because they have a characteristic signal. When an earthquake occurs, energy is released in all directions.

This energy travels as vibrations through the Earth. Earthquake energy vibrates in multiple ways. The P wave (primary or compressional) behaves like a pulse radiating outward. It arrives at the seismometer first (red arrow). The S wave (shear or secondary) energy vibrates horizontally as it radiates outward. This wave arrives second (blue arrow).