Seismic Wave Travel Times

We use travel times computed in Earth models to estimate the arrival time of seismic waves around the planet. After decades of experience, we have a pretty good idea of the waves that are commonly identifiable on seismograms. Each curve on the chart below shows the travel time of the wave as a function of distance from a shallow earthquake.

You generally do not see all of these waves for each earthquake, and the pattern changes with the depth of the earthquake (the change in pattern is one way to constrain the depth of an earthquake).

The letters P and K represent P waves (outside and inside the core); the letter S represents shear waves. Some of the arrivals travel part of the distance as P waves and another part as S waves, so the letters are mixed. The arrangement of the letters starts at the source and ends at the receiver. So any wave that ends with P arrives at the seismic station as a P wave. The letter "c" represents reflection of Earth's core, so PcP is a P wave that travels down to and bounces off the core-mantle boundary. PcS leaves the earthquakes as a P wave, but converts to an S wave when it bounces of the core. The "diff" phases are diffracted, which are more clearly seen only at long periods. The gray region shows the arrival time "window" of short-arc Rayleigh waves, which we call R1. 

The slope of the travel-time curve for waves traveling the short distance from the earthquake to the seismic station have a positive slope (towards the upper right); the slope of the travel-time curve for waves traveling the other direction (that went the longer distance from the earthquake to the seismic station have a negative slope (towards the lower right).

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.