The hypocentre and epicentre of an earthquake are calculated because these seismic waves are propagated at very wide velocities (see the questions "What is a hypocentre?", "What is an epicentre?" and "How is earthquake's place of origin determined?").
The hypocentre and epicentre of an earthquake can be calculated because these seismic waves are propagated at different velocities (see the questions "What is a hypocentre?", "What is an epicentre?" and "How is an earthquake's place of origin determined?").