Earthquakes release energy in the form of several types of seismic waves, including P-waves (primary), S-waves (secondary), and L-waves (surface/Love waves).
PRO TIP
Re-check your working by substituting the answer back into the original question.
During a thunderstorm, the sound of thunder is heard 3 seconds after the lightning flash is seen. If the speed of sound in air is 340 m/s, how far away did the lightning strike? (Ignore the time taken by light.)