Travel-time curve

[1] Travel-time curve is a graph showing the relationship between the distance from the epicenter to the observation point and the travel time.

Seismic waves usually travel at a constant speed, but the travel-time curve does not become a straight line, but always bends somewhere.

The reason is that the speed at which seismic waves travel differs between the crust and the mantle.

Seismic waves travel slowly in the crust, but conversely fast in the mantle.

In other words, when the seismic wave reaches the Mohorovic discontinuity, the travel-time curve bends.