Transcript Seismic Waves

Making Waves: Seismic Waves
Activities and Demonstrations
Larry Braile, Purdue University
[email protected],
web.ics.purdue.edu/~braile
Sheryl Braile, Happy Hollow School
West Lafayette, IN
CSTA Conference, October 2009
Palm Springs, CA
This PowerPoint file: http://web.ics.purdue.edu/~braile/new/SeismicWaves.ppt
Seismic Waves
Slinky – P, S, Rayleigh, Love waves;
Reflection and transmission; energy carried by
waves; elastic rebound/plate motions and the
slinky; 5-slinky model – waves in all directions,
travel times to different distances.
 Human wave demo – P and S waves in solids and
liquids.
 Seismic wave animations – P, S, Rayleigh, Love
waves; wave motion; wave propagation activity.
 Seismograms – Viewing seismograms on your
computer (AmaSeis software).
 Seismic Waves software – Wave propagation
through the Earth.
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Why use several approaches for teaching
about seismic waves?
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Fundamental concept (worth spending time on)
Different approaches for different settings or size of
group
Different learning styles
Reinforce with more than one approach
Demonstrations, animations and hands-on activities
Use one or more approach for authentic
assessment
Elasticity – a property of materials that results
In wave propagation and earthquakes
Measuring Elasticity of a Spring
Added
Mass
(g)
Spring
Extension
(cm)*
(adding
masses)
Spring
Extension
(cm)*
(removing
masses)
0
0.0
0.3
100
3.7
3.6
200
7.7
7.5
300
11.4
11.4
400
15.3
15.1
Standard
Spring
PVC Pipe
Length
of
Spring
Mass
Wood
* Difference in length of spring before and after adding mass.
Stretching (length - original length, cm)
Elasticity of a Spring
16
14
Adding mass:
Removing mass:
12
10
8
6
1. Deformation (stretching) is
proportional to applied force (mass).
2. Spring returns to its original shape
(length) when force is removed.
4
2
0
0
50
100
150
200
250
Added Mass (grams)
300
350
400
Slinky and human wave demo and wave tank and
elasticity experiments:
http://web.ics.purdue.edu/~braile/edumod/slinky/slinky.htm
http://web.ics.purdue.edu/~braile/edumod/slinky/slinky.doc
http://web.ics.purdue.edu/~braile/edumod/slinky/slinky.pdf
Characteristics of Seismic Waves
Table 2: Seismic Waves
Type (and
names)
Particle Motion
Typical Velocity
Other Characteristics
P,Compressional
, Primary,
Longitudinal
Alternating
compressions
(“pushes”) and dilations
(“pulls”) which are
directed in the same
direction as the wave is
propagating (along the
raypath); and therefore,
perpendicular to the
wavefront
VP ~ 5 – 7 km/s in
typical Earth’s
crust;
>~ 8 km/s in
Earth’s mantle and
core; 1.5 km/s in
water; 0.3 km/s in
air
P motion travels fastest in materials,
so the P-wave is the first-arriving
energy on a seismogram. Generally
smaller and higher frequency than
the S and Surface-waves. P waves in
a liquid or gas are pressure waves,
including sound waves.
S, Shear,
Secondary,
Transverse
Alternating transverse
motions (perpendicular
to the direction of
propagation, and the
raypath); commonly
polarized such that
particle motion is in
vertical or horizontal
planes
VS ~ 3 – 4 km/s in
typical Earth’s
crust;
>~ 4.5 km/s in
Earth’s
mantle; ~ 2.5-3.0
km/s in (solid)
inner core
S-waves do not travel through fluids,
so do not exist in Earth’s outer core
(inferred to be primarily liquid iron)
or in air or water or molten rock
(magma). S waves travel slower
than P waves in a solid and,
therefore, arrive after the P wave.
Characteristics of Seismic Waves
L, Love,
Surface waves,
Long waves
Transverse horizontal
motion, perpendicular to
the direction of
propagation and
generally parallel to the
Earth’s surface
VL ~ 2.0 - 4.5 km/s
in the Earth
depending on
frequency of the
propagating wave
Love waves exist because of the
Earth’s surface. They are largest at
the surface and decrease in
amplitude with depth. Love waves
are dispersive, that is, the wave
velocity is dependent on frequency,
with low frequencies normally
propagating at higher
velocity. Depth of penetration of the
Love waves is also dependent on
frequency, with lower frequencies
penetrating to greater depth.
R, Rayleigh,
Surface waves,
Long waves,
Ground roll
Motion is both in the
direction of propagation
and perpendicular (in a
vertical plane),
and “phased” so that the
motion is generally
elliptical – either
prograde or retrograde
VR ~ 2.0 - 4.5 km/s
in the Earth
depending on
frequency of the
propagating wave
Rayleigh waves are also dispersive
and the amplitudes generally
decrease with depth in the
Earth. Appearance and particle
motion are similar to water waves.
A simple wave
tank experiment
– a ping pong
ball is dropped
onto the surface
of the water;
small floats aid
viewing of the
waves; distance
marks on the
bottom of the
container allow
calculation of
wave velocity.
Seismic waves and the slinky
(also, see the 4-page slinky write-up at:
http://web.ics.purdue.edu/~braile/edumod/slinky/slinky4.doc)
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P and S waves
Love and Rayleigh waves
Wave reflection and transmission
Elastic rebound
Waves carry energy
The five slinky model (waves in all directions
and different travel times to different locations
– the way that earthquakes are located)
Seismic waves
carry energy.
Observe the
shaking of the
model building
when P and S
waves are
propagated
along the slinky.
The 5-slinky model for demonstrating that seismic
waves propagate in all directions and the variation
of travel time with distance.
The human wave demonstration illustrating P and S
wave propagation in solids and liquids.
Wave animations
Animation courtesy of Dr. Dan Russell, Kettering University
http://www.kettering.edu/~drussell/demos.html
Seismic Wave
animations
(Developed by L. Braile)
http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm
Dan Russell animations – The
people wave
Animation courtesy of Dr. Dan Russell,
Kettering University
http://www.kettering.edu/~drussell/demos.html
Dan Russell animations – Rayleigh
wave Direction of propagation
Animation courtesy of Dr. Dan Russell,
Kettering University
http://www.kettering.edu/~drussell/demos.html
Compressional Wave (P-Wave) Animation
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.
Shear Wave (S-Wave) Animation
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. However, Earth’s layers tend to
cause mostly vertical (SV; in the vertical plane) or horizontal (SH) shear
motions. Material returns to its original shape after wave passes.
Rayleigh Wave (R-Wave) Animation
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.
Love Wave (L-Wave) Animation
Deformation propagates. Particle motion consists of alternating
transverse motions. Particle motion is horizontal and perpendicular to
the direction of propagation (transverse). To aid in seeing that the
particle motion is purely horizontal, focus on the Y axis (red line) as the
wave propagates through it. Amplitude decreases with depth. Material
returns to its original shape after wave passes.
You can download the animations separately to run more efficiently:
(http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm).
A complete PowerPoint presentation on the Seismic wave animations is
also available at:
http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.ppt
Demonstrate the AmaSeis software for displaying and analyzing
seismograms; software available at:
http://bingweb.binghamton.edu/~ajones/
A tutorial on AmaSeis and links to seismograms that can be downloaded and viewed
in AmaSeis available at:
http://web.ics.purdue.edu/~braile/edumod/as1lessons/UsingAmaSeis/UsingAmaSeis.htm
The IRIS Seismographs in Schools program:
http://www.iris.edu/hq/sis
IRIS AmaSeis
Software
24-Hour Screen Display
Extracted Seismogram
The AS-1 Seismometer
(developed by Alan Jones,
SUNY Binghamton, NY)
Teaching Modules and Tutorials:
http://web.ics.purdue.edu/~braile/edumod/as1lessons/as1lessons.htm
The Seismic Waves
program
From Alan Jones, SUNY, Binghamton
http://bingweb.binghamton.edu/~ajones/
Earthquake
Cross Section
Through Earth
Stations for
Seismograms
*
Wavefront
Ray Path
Seismograph
Ray Path is perpendicular
to wavefront