1. Geology 5660/6660 Applied Geophysics 13 Jan 2014
Last time: Seismic Thought Exercise
• Seismic Concepts:
 Velocity is distance traveled per unit time, V = x/t
 Different materials have different seismic velocities!
 Introduced Wavefronts & Rays
 Huygen’s Principle: Every point on a wavefront
can be treated as a point source for the next generation
of wavelets. The wavefront at time t = x/V later is a
surface tangent to the furthest point on each of these.
• Thought exercise: What do wavefronts look like for a slow
layer over a fast layer? For fast over slow?
Read for Wed 13 Jan: Burger 1-21 (Ch 1–2.1)
© A.R. Lowry 2014

2. } Seismology (A Brief Introduction)
Four Important Types of Seismic Waves:
(1) P (primary) wave (Velocity VP = 4 to 14 km/s)
(2) S (secondary) wave (VS = 2/5 to 3/5 VP, or 0)
(3) Surface Waves (Love, Rayleigh) V slightly < VS
(4) Normal Modes (Resonant “Tones”, like a bell…)
 continue for months after largest earthquakes
 periods of minutes to a few hours
“standing waves” }
Body
Waves
Animation 0S3 from Lucien Saviot
0S3: (25.7 minutes) 2

3. P S
Surface (Love)
Surface (Rayleigh)

4. Seismic waves are strain waves that propagate in a medium…
The text begins with an analogy to ripples in a pond. There is
similarity in that both are described by the wave equation;
both involve stress & displacements that propagate as
individual particles in the medium oscillate between potential
and kinetic energy states… But,
A major difference is rheology. Stress, displacement &
strain in a solid continuum are governed by Hooke’s Law.

5. Elastic Rheology (Hooke’s Law):
Stress (= force per unit area) 
and Strain (= change in shape) 
are linearly related via
 = c 
where c is an elastic coefficient (a material property).
Strain is a spatial derivative of displacement u
described by (in 1-D),

6. A quick “review” of various strains and their elastic constants:
Uniaxial compression:
Elongation
(change in
length l)
(strain)
xx
yy
Young’s modulus E:
 = E
applied 
(elastic
constant)
Poisson’s ratio :
(elastic
constant)
(0 <  < 0.5)
For dilatation  (change in volume V/V0):
Bulk modulus K = P/(where P is pressure)
(strain)
(elastic
constant)
applied s
Rigidity modulus  = s/ 
 = tan
(elastic
constant)
(strain)