If the earth had no changes with depth
seismic waves would travel straight
paths. But the earth has composition,
density, and temperature changes that
cause the seismic rays to refract as
velocity in the mantle generally
increases with depth. We can zoom in to a
shallow part of the crust to watch how
waves behave when crossing into
boundaries of higher velocity. We will
track two paths from an earthquake. The
direct path travels to station X through
the slowest layer. The outer path
continually refracts as it crosses
boundaries of increased velocity until
the incident ray reaches a critical
angle and begins sending a head wave to
the surface. These head waves return to
the surface refracting back at the same
angles and velocities that they
descended with. This is because they
follows Snell's law at each boundary
with relative velocities now decreasing
upward. Some of the critically refracted
waves reach Station X and Beyond before
the blue direct wave which took the
shortcut. This is because the velocity
increases were great enough to overcome
the slower direct path. The dashed black
line emphasizes how much the wave is
refracted in each layer.
