Surface waves form the longest and strongest portion of a seismic record
excited by explosions and shallow earthquakes. Traversing areas with
diverse geologic structures, they 'absorb' information on the properties
of these areas which is best retlected in dispersion, the dependence of
velocity on frequency. The other prop- erties of these waves -
polarization, frequency content, attenuation, azimuthal variation of the
amplitude and phase - arc also controlled by the medium between the
source and the recording station; some of these are affected by the
properties of the source itself and by the conditions around it. In
recent years surface wave seismology has become an indispensable part of
seismological practice. The maximum amplitude in the surface wave train
of virtually every earthquake or major explosion is being measured and
used by all national and international seismological surveys in the
determination of the most important energy parameter of a seismic
source, namely, the magnitude M, . The relationship between M, and the
body wave magnitude fI1t, is routinely employed in identification of
underground nuclear explosions. Surface waves of hundreds of earthquakes
recorded every year are being analysed to estimate the seismic moment
tensor of earthquake sources, to determine the periods of free
oscillations of the Earth, to construct regional dispersion curves from
which in turn the crustal and upper mantle structure in various areas is
derived, and to evaluate the dissipative parameters of the mantle
material.