Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 184

P -waves SV-waves

(a) (b)

Reference point

Reference point

Time 1 Time 1

Time 2 Time 2

Time 3 Time 3

Time 4 Time 4

t1 t2 t3 t4 t1 t2 t3 t4

Fig. 4.132 (a) P-waves; rarefraction-compression elastic deformation shown by the evolving size of the black rectangular element for successive
times at the reference point; (b) SV-waves: vertical shear elastic deformation.

P -wave first arrival S -wave first arrival

E

Love waves

S

N

Rayleigh waves

P

Z

Fig. 4.133 These are seismograms from a moderate earthquake (magnitude 5.1) in the Norwegian Sea recorded in Germany. Each seismic
trace shows the component waveforms recorded by seismometers of different orientation. The top two record horizontal ground motions in
E-W and N-S directions, respectively, while the lower records the vertical “up-down” ground motion. In these cases the E-W seismometer
has nicely picked out the horizontal E-W motion of the Love wave package. The N-S seismometer has a very clear S-wave first arrival. The
vertical ground motions induced by the steeply inclined P -wave signal are well captured by the third seismometer, as is the vertical Rayleigh
wave train. The records illustrate the long duration of the damaging surface wave signals compared to P - and S -waves.

S -waves different relationships must apply. The inverse
approach determines the gradient of travel time with
distance at the point of interest and relates this to the ratio
of radial distance and velocity (Fig. 4.137).
The pioneers of seismology made the simplifying
assumption that seismic energy was transported as linear
rays moving with constant average velocity characteristic
of a particular rock medium, that is, they envisaged a
complete analogy between seismic rays and light rays.
Subsequent research proved that seismic velocities gener-

ally increased with depth (Box 4.2) and that the rays were
curved, concave side up (Fig. 4.138). Not only that, but
seismic wave energy is reflected and refracted in complex
ways across internal discontinuities (Fig. 4.136), a phe-
nomenon that leads not only to elucidation of the internal
planetary structure of Earth using accurate travel time data,
but also to the location of subsurface geological structures
containing vast economic reserves of oil and gas.
The speed of travel of seismic waves is controlled by
elastic properties, in particular the bulk modulus, K , and