1. Introduction to Seismology
Geology 5640/6640
Introduction to Seismology
13 Jan 2017
Last time: Course overview
Seismology is used as a tool to investigate:
• Seismic sources: Earthquake physics, magma transfer
in volcanic systems, “icequakes”, storms, nuclear test
verification, drill bit location…
• Seismic velocity & impedance structure: Seismic
reflection imaging (oil & gas industry), site investigation
(construction, environmental, hydro resources), virtually
all fundamental research into processes in the Earth’s
interior
Read for Wed 18 Jan: S&W (§ )
© A.R. Lowry 2017

2. How to use these powerpoints:
• Review them often: Before each class, while doing
exercises, and in preparing your projects
• The most important points of each lecture (in my opinion)
are summarized on the first slide of the next lecture
• Note the notation!
 Arial Black, italic means Important, pay
attention…
 Arial Black, italic, red font means
Critically Important concept or terminology
that I expect you to understand intimately
for exercises and projects
 Times New Roman, italic, black font means this is an
equation or an algebraic variable
 A Red Box with Grey Background means this is an
especially important concept or equation

3. A seismogram is a time-record of motion of an inertial mass…
Source
Pulse
Seismogram
(After S&W
Fig )
Receiver
Source
Medium
Origin
time
Travel
time
Arrival
time
… that contains information about the source,
Earth response, and seismometer response.

4. Most of what we know about the Earth’s interior comes from seismology.
8-14 km/s
8-10 km/s
11 km/s

5. And not just
Earth!
Apollo-era
seismic data
demonstrate
the lunar
interior has
similarities
to Earth’s…

6. } Seismology (A brief review of things you “already know”)
Four 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 hours or days
“standing waves”
Body
Waves

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

8. Seismic waves are strain waves that propagate in a medium…
Common analogies use ripples in a pond, or light. There are
similarities in that all three are described by the wave equation.
Ripples & seismic waves similarly 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.

9. Despite differences, similarities inherent in the
wave equation  many important principles can
be borrowed from optics.
One of these is Huygen’s Principle:
Every point on a wavefront can be treated as a point source
for the next generation of wavelets. The wavefront at a time
t later is a surface
tangent to the
furthest point on
each of these
wavelets. This is
because extremal
points of propagation
have the greatest
constructive
interference…