Seismic Wave Propagation
What are Seismic Waves
• Seismic waves are the vibrations from
earthquakes that travel through the Earth
• They are the waves of energy suddenly created
by rock fracture within the earth or an explosion.
• They are the energy that travels through the
earth and is recorded on seismographs
History
Seismology - the Study of Earthquakes and
Seismic Waves
Dates back almost 2000 years
• Around 132 AD, Chinese scientist Chang Heng invented
the first seismoscope, an instrument that could register the
occurrence of an earthquake.
• Seismographs record a zigzag trace that shows the varying
amplitude of ground oscillations beneath the instrument.
• Sensitive seismographs, which greatly magnify these
ground motions, can detect strong earthquakes from
sources anywhere in the world.
• The time, location and magnitude of an earthquake can
be determined from the data recorded by seismograph
stations.
Seismometers and
Seismographs
• Seismometers are instruments for detecting
ground motions
• Seismographs are instruments for recording
seismic waves from earthquakes.
• Seismometers are based on the principal of an
“inertial mass”
• Seismographs amplify, record, and display the
seismic waves
• Recordings are called seismograms
Types of Seismic Waves
• Body waves
Travel through the earth's interior
• Surface Waves
Travel along the earth's surface similar to ocean waves
Seismic Waves
Body waves
P
S
Surface Waves
Love
“Ground Roll”
Rayleigh
P-Wave(Body Wave)
Primary or compressional (P) waves

The first kind of body wave is the P wave or primary
wave. This is the fastest kind of seismic wave.

The P wave can move through solid rock and fluids, like
water or the liquid layers of the earth.

It pushes and pulls the rock it moves through just like
sound waves push and pull the air.

Highest velocity (6 km/sec in the crust)
P-Wave
Particle motion
Deformation
propagates
Particle motion consists of alternating compression and dilation.
Particle motion is parallel to the direction of propagation
(longitudinal). Material returns to its original shape after wave passes.
Secondary Wave (S Wave)
Secondary or shear (S) waves
The second type of body wave is the S wave or
secondary wave, which is the second wave you
feel in an earthquake.
An S wave is slower than a P wave and can only
move through solid rock. (3.6 km/sec in the crust)
This wave moves rock up and down, or side-toside.
S-Wave
Particle
motion
Deformation
propagates
Particle motion consists of alternating transverse motion. Particle motion
is perpendicular to the direction of propagation (transverse). Transverse
particle motion shown here is vertical but can be in any direction.
Material returns to its original shape after wave passes.
L-Wave
Love Waves
• The first kind of surface wave is called a
Love wave, named after A.E.H. Love, a
British mathematician who worked out
the mathematical model for this kind of
wave in 1911.
• It's the fastest surface wave and moves
the ground from side-to-side.
L-Wave
Particle
motion
Deformation
propagates
Particle motion consists of alternating transverse motions. Particle
motion is horizontal and perpendicular to the direction of propagation
(transverse). Particle motion is purely horizontal, focus on the Y axis
(black lines) as the wave propagates through it. Amplitude decreases
with depth (yellow lines). Material returns to its original shape after
wave passes.
Rayleigh Waves
Rayleigh Waves
• The other kind of surface wave is the Rayleigh wave, named
for John William Strutt, Lord Rayleigh, who mathematically
predicted the existence of this kind of wave in 1885.
• A Rayleigh wave rolls along the ground just like a wave rolls
across a lake or an ocean.
• Because it rolls, it moves the ground up and down, and sideto-side in the same direction that the wave is moving.
•
Most of the shaking felt from an earthquake is due to the
Rayleigh wave, which can be much larger than the other
waves.
Rayleigh Waves
Particle
motion
Deformation
propagates
Particle motion consists of elliptical motions (generally retrograde
elliptical) in the vertical plane and parallel to the direction of
propagation. Amplitude decreases with depth. Material returns to its
original shape after wave passes.
Seismic Wave Speeds
The bulk modulus (K) of a substance
essentially measures the substance's
resistance to uniform compression.
It is defined as the pressure increase
needed to effect a given relative
decrease in volume.
Shear modulus, μ, sometimes referred
to as the modulus of rigidity, is the
ratio of shear stress to the shear strain.
= shear modulus
 = density
K = modulus of compressibility (bulk modulus)
Elastic Materials
Rand quartzite
L
stress
F
DL
F = E * DL/L (Hooke’s Law)
E = Young’s modulus
strain
Young's modulus (E) is a measure of the
stiffness of a given material.
E predicts the amount a wire will extend
under tension, or to predict the load at
which a thin column will buckle under
compression
Seismic Wave Speeds
= modulus of rigidity
Material
P wave Velocity (m/s)
Air
332
Water
1400-1500
Petroleum
1300-1400
Steel
6100
3500
Concrete
3600
2000
Granite
5500-5900
2800-3000
Basalt
6400
3200
Sandstone
1400-4300
700-2800
Limestone
5900-6100
2800-3000
Sand (Unsaturated)
200-1000
80-400
Sand (Saturated)
800-2200
320-880
Clay
1000-2500
400-1000
Glacial Till (Saturated)
1500-2500
600-1000
 = density
K = modulus of compressibility (bulk modulus)
S wave Velocity (m/s)
Seismic
Velocities
Why are seismic waves important?
Some things seismic waves are good for include
·
Mapping the Interior of the Earth
·
Monitoring the Compliance of the Comprehensive Test
Ban Treaty
·
Detection of Contaminated Aquifers
·
Finding Prospective Oil and Natural Gas Locations
IRIS Earth’s Interior Structure Poster –
Seismic waves through the Earth
Earth’s interior
structure and
seismic
raypaths that
are used to
determine
the Earth
structure.