An Introduction to Surface Wave Analysis Michael E. Pasyanos - IRIS

Group Velocity vs. Phase Velocity
For typical earth profiles, the phase
velocity is generally faster than the
group velocity.
While the group velocity can increase or
decrease with increasing period, phase
velocities are monotonically increasing.
If the group velocities are constant over a
wide period range, then they can
produce a high-amplitude body-wave
looking pulse that is called an Airy
phase