Work out seismic wave travel time instantly with clear inputs, formula shown and shareable results.
P waves are compressional and travel at about 6 km/s in the crust; S waves are shear and manage about 3.5 km/s. At 400 km the P arrives in 66.7 s and the S in 114.3 s, a 47.6 second gap. That interval is what earthquake early warning systems exploit, and it also lets a single station estimate distance.
Travel time
t = distance / wave speed
Distance from the S-P interval
distance = (S - P interval) / (1/vs - 1/vp)
The S minus P interval gives distance, so three stations' distance circles intersect at the epicentre - the classic triangulation method.
Because shear waves need a medium that resists shear, and liquids do not. The S wave shadow zone is how the liquid outer core was discovered.