Geology Reference
In-Depth Information
(a)
(b)
Fig. 3.4
Elastic deformations and ground
particle motions associated with the
passage of surface waves. (a) Rayleigh
wave. (b) Love wave. (From Bolt 1982.)
interior.Analysis of the observed pattern of dispersion of
earthquake waves is a powerful method of studying the
velocity structure of the lithosphere and asthenosphere
(Knopoff 1983). The same methodology, applied to the
surface waves generated by a sledgehammer, can be used
to examine the strength of near-surface materials for
civil engineering investigations.
If the surface is layered and the surface layer shear
wave velocity is lower than that of the underlying layer, a
second set of surface waves is generated.
Love waves
are
polarized shear waves with a particle motion parallel to
the free surface and perpendicular to the direction of
wave propagation (Fig. 3.4(b)). The velocity of Love
waves is intermediate between the shear wave velocity of
the surface layer and that of deeper layers, and Love
waves are inherently dispersive. The observed pattern
of Love wave dispersion can be used in a similar way
to Rayleigh wave dispersion to study the subsurface
structure.
Wavefront
Source
Ray path
Fig. 3.5
The relationship of a ray path to the associated
wavefront.
Rays have no physical significance but represent a useful
concept in discussing travel paths of seismic energy
through the ground.
It should be noted that the propagation velocity of a
seismic wave is the velocity with which the seismic en-
ergy travels through a medium.This is completely inde-
pendent of the velocity of a particle of the medium
perturbed by the passage of the wave. In the case of com-
pressional body waves, for example, their propagation
velocity through rocks is typically a few thousand metres
per second. The associated oscillatory ground motions
involve
particle velocities
that depend on the amplitude
of the wave. For the weak seismic events routinely
recorded in seismic surveys, particle velocities may be
as small as 10
-8
ms
-1
and involve ground displacements
3.3.3 Waves and rays
A seismic pulse propagates outwards from a seismic
source at a velocity determined by the physical proper-
ties of the surrounding rocks. If the pulse travels through
a homogeneous rock it will travel at the same velocity in
all directions away from the source so that at any subse-
quent time the
wavefront
, defined as the locus of all points
which the pulse has reached at a particular time, will be a
sphere.
Seismic rays
are defined as thin pencils of seismic
energy travelling along ray paths that, in isotropic media,
are everywhere perpendicular to wavefronts (Fig. 3.5).
