The propagation of waves along and across the boundary between two media
with different characteristic velocities is much more complicated when
the source is on or near the boundary than when it is far away and the
incident waves are plane. Examples of waves generated by localized
sources near a boundary are the electromagnetic waves from the currents
in a dipole on the surface of the earth and the seismic waves from a
slip event in a fault in the earth's crust like the San Andreas fault in
California. Both involve a type of surface wave that is called a lateral
wave in electro- magnetics and a head wave in seismology. Since the two
are analogous and the latter is more easily visualized, it is
conveniently used here to introduce and describe this important type of
surface wave using the data of Y. Ben- Zion and P. Malin ("San Andreas
Fault Zone Head Waves Near Parkfield, CA," Science 251, 1592-1594, 29
March 1991).