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Seismic Waves
The wiggles on a seismogram are caused by seismic
waves which are generated by the movement of therocks along a fault.
The waves emanate from the “source” or earthquake, and travel:
through the body of the Earth, and
over the surface of Earth.
Waves in a pond
The idea is analogous to waves caused by tossing astone in a pond.
Sound Wave AnalogySeismic waves represent acoustic (sound) energy and so are analogous to speech:
(1)Vocal cords vibrate
(2) Sound waves propagatethrough atmosphere
(3) Ears record thesevibrations
(4) Brain processes the recordings
Speech Earthquakes(1)A locked fault segment
fails (ruptures)
(2) Sound waves propagate
through the Earth
(3) Seismometers record
these vibrations
(4) Seismologists process
these recordings (seismograms)
What is a Wave ?
A wave is a disturbance that transfers energy.
Waves are common in nature:Light is a waveSound is a wave
Waves are periodic in both space and time (theyhave wavelengths and periods)
Wave Terminology
Wavelength is a measure of the spatial width ofa wave. It has units of length (m).Period is a measure of the duration of avibration. Period has units of time (s)Frequency is one over the period. It has units ofone over time (s-1).Amplitude is a measure of the height of thewave. It has units of displacement (m).
Wavelength and PeriodA
mpl
itud
e
Distance from Source
Wavelength
• At a given instant in either time and space, thedisplacement is periodic in both space (distance) andtime. • Amplitude = maximum displacement from
equilibrium(ie to crest or to trough)• Wavelength or Period= crest-to-crest distance or
time
Am
plit
ude
Time
Period
Crest (High Points)
Through (Low Points)
Equilibrium(Middle)
Wave Speeds
The speed that a wave propagates at is not a dynamic quantity – it is a fixed material property.(like density) No matter how big an earthquake is, the seismic waves generated by earthquake will always travel atthe same speed. The seismic wave speed of a material depends mainly its upon:
Temperature
Pressure
Composition
Sources of Seismic Waves
Earthquakes generate seismic waves, but so do manyother processes:
Volcanic eruptions
Explosions
Wind
Sonic Booms (planes, shuttle, meteorites)
Humans
Jet and Earthquake
Multiple-Frequency Signals
Most interesting signals are composites of waves with many different frequencies. The range of frequency is sometimes called the “band” and we speak of bandwidth.
Light is usually a multiple frequency signal, and the
different frequencies correspond to what we call colors.
Sometimes we can use the
observed frequencies to
identify different sources of
vibrations.
Which has higher frequency
content, the sonic boom or
the earthquake?
Elastic Behavior
Elastic Deformation
Elastic/plastic Behavior:Asthenosphere
Body/Surface wave Prorogation
Bulk Modulus (Incompressibility) = (P/) … where = dilatation = V/V and P = pressure
Elastic Constants: Bulk Modules
shear stress = (F /A)
=shear stressshear strain
shear modulus (rigidity)
Elastic Constants: Shear Modules
shear strain = (l /L)
Poisson’s ratio = υ= - (yy / xx)
L L
Then,
W W
xx =
transverse strain =
•Under a stress (xx)along the x-axis,longitudinal strain
Elastic Constants: Poisson Ratio
yy =
From Lay & Wallace (1995)
Elastic Moduli and Densities of Some Common Materials
Poisson RatioBulk Module
Shear Module
Lame’s constant
•Typical Values of Elastic
Constants for Selected Materials
Particle Motions of Body Waves
3D Components of Waves
