The apparent wave propagation velocity is an important parameter in calculating the strain
in pipe induced by seismic waves. The apparent wave propagation velocity of shear waves with
respect to ground surface is many times higher than the shear wave velocity of the near surface
material. The seismic energy originating at depth passes through increasing layers of softer
materials and refraction causes a concave travel path. Hence, the net result being the body waves
arriving at the ground surface with small incident angle θ with respect to vertical. Whereas in case
the R-waves which travel parallel to the ground surface, the apparent propagation velocity is same
as its phase velocity