Seismic Waves
CH 8.2
NCSCoS 2.04
Warm-up
 How are earthquakes and plate tectonics
related?
 What do you know about waves?
 What do you remember about the major
tsunami in the Indian Ocean on
December 26, 2004?
Objectives
 Compare and contrast the three types of
seismic waves.
 Locate earthquake epicenters using
seismograms and a travel-time graph.
Sumatra Tsunami
December 26, 2004
“Earthquake triggers deadly tsunami.”
“The world's most powerful earthquake in more than 40 years struck deep
under the Indian Ocean on December 26, triggering massive tsunamis
that obliterated cities, seaside communities and holiday resorts, killing
tens of thousands of people in a dozen countries.”
-CNN News
http://www.time.com/time/photoessays/asia_earthquake/index.html
http://www.cnn.com/SPECIALS/2004/tsunami.disaster/
NOVA interactive site
http://www.pbs.org/wgbh/nova/tsunami/anat-flash.html
Seismic Waves radiate from focus
and transmit energy.
SURFACE WAVES
BODY WAVES
 Body waves
travel
through
earth.
 Surface
waves travel
on surface.
Three Types of Seismic Waves
http://www.gmi.edu/~drussell/Demos/waves/wavemotion.html
1. P-waves = primary waves



Body wave
push-pull = compression waves
Particles move parallel to wave direction.


Fastest
Travels through solids and liquids
2. S-waves = secondary waves
 body waves
 side-to-side = transverse waves
 Particles move perpendicular to wave
direction.
 intermediate speed
 cannot travel through liquids
3. Surface waves
 Particles at surface move in a circular
motion.
 Slowest
 Most destructive
Seismogram
Surface
P-wave
S-wave
Review/Preview:
 Sketch and label a seismogram with s-p
lag time.
 What does lag time tell you?
Seismograph – instrument used to
record seismic
waves
Heavy weight
keeps pen still
due to inertia.
Drum moves
beneath pen.

http://www.thetech.org/exhibits/online/quakes/seismo/
Travel Time
Graph
• S-P lag time –
time between P
and S wave
arrivals
•Greater S-P lag
time = greater
distance from
epicenter
P wave
S wave
S-P lag
time
Triangulation
 Use at least three
stations to locate
epicenter.
 Epicenter is where all
three lines meet.
Check For Understanding
Which wave is
an s-wave?
How does
material move
and change in
a p-wave?
Which wave is
faster?
 What is the
arrival time of
the p-wave?
 The s-wave?
 What is the
distance to
the epicenter
if the S-P lag
time is 5
minutes?
B
A
 Which station is the
closest to the
epicenter?
 Where is the
epicenter?
C
 Why can’t you just
use two stations?
Review/
Preview
Make your own
sheet this
week.
Write many
observations
of this
diagram. (also
on p. 236)
3) Earthquake destruction
l important factors:
Intensity & duration of shaking
Soil type (soft? hard rock?)
Building design
l other effects:
Liquifaction
Tsunamis
3) Earthquake destruction
l important factors:
Intensity & duration of shaking
Soil type (soft? hard rock?)
Building design
l other effects:
Liquifaction
Tsunamis
Bay Area, CA
1989
3) Earthquake destruction
l important factors:
Intensity & duration of shaking
Soil type (soft? hard rock?)
Building design
l other effects:
Liquifaction
Tsunamis
3) Earthquake destruction
l important factors:
Intensity & duration of shaking
Soil type (soft? hard rock?)
Building design
l other effects:
Liquifaction
Tsunamis
Intensity of
shaking
decreases
with distance
from
epicenter
(MM Intensity
scale).
M6.7, 1994
Northridge,
California
earthquake
Intensity of
shaking
decreases with
distance from
epicenter
(Peak
horizontal
acceleration).
M6.7, 1994
Northridge,
California
earthquake
Intensity of
shaking
decreases
with distance
from epicenter
(star; USGS
Shake Map).
M6.7, 1994
Northridge,
California
earthquake
Richter Scale
 Based on amplitude of the largest wave
 Logarithmic scale
 Ex) 3.0 is 10x greater than 2.0
Moment Magnitude
 Scientists use this now
 Based on several factors (more
complicated)
 But tries to measure displacement & energy
that is released
 >6 = significant
 >7 = major
 >8 = great
2) Earthquake Intensity and magnitude
l Mercalli intensity scale
Intensity of shaking & damage at a specific location
Depends on distance to earthquake
& strength of earthquake
l Magnitude
A measure of the energy released in an earthquake
Depends on size of fault that breaks
1) Earthquake “belts”
l Fault Types