Seismic Waves

1
Seismic Waves

• CH 8.2
• NCSCoS 2.04

2
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?

3
Objectives

• Compare and contrast the three types of seismic
  waves.
• Locate earthquake epicenters using seismograms
  and a travel-time graph.

4
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/photoe
  ssays/asia_earthquake/index.html
• http//www.cnn.com/SPECIALS/2004/tsunami.di
  saster/
• NOVA interactive site
• http//www.pbs.org/wgbh/nova/tsunami/anat-flash.ht
  ml

5
Seismic Waves radiate from focus and transmit
energy.
6
SURFACE WAVES
BODY WAVES

• Body waves travel through earth.
• Surface waves travel on surface.

7
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

8

• 2. S-waves secondary waves
• body waves
• side-to-side transverse waves
• Particles move perpendicular to wave direction.
• intermediate speed
• cannot travel through liquids

9

• 3. Surface waves
• Particles at surface move in a circular motion.
• Slowest
• Most destructive

10
Seismogram
Surface
P-wave
S-wave
11
Review/Preview

• Sketch and label a seismogram with s-p lag time.
• What does lag time tell you?

12
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/seis
  mo/

13
Travel Time Graph
P wave
S wave

• S-P lag time time between P and S wave arrivals
• Greater S-P lag time greater distance from
  epicenter

S-P lag time
14
Triangulation

• Use at least three stations to locate epicenter.
• Epicenter is where all three lines meet.

15

Check For Understanding
Which wave is an s-wave? How does material move
and change in a p-wave? Which wave is faster?
16

• 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?

17

• Which station is the closest to the epicenter?
• Where is the epicenter?
• Why cant you just use two stations?

B
C
A
18
Review/Preview

• Make your own sheet this week.
• Write many observations of this diagram.
  (also on p. 236)

19
3) Earthquake destruction

• important factors

Intensity duration of shaking Soil type (soft?
hard rock?) Building design

• other effects

Liquifaction Tsunamis
20
3) Earthquake destruction

• important factors

Bay Area, CA 1989
Intensity duration of shaking Soil type (soft?
hard rock?) Building design

• other effects

Liquifaction Tsunamis
21
3) Earthquake destruction

• important factors

Intensity duration of shaking Soil type (soft?
hard rock?) Building design

• other effects

Liquifaction Tsunamis
22
3) Earthquake destruction

• important factors

Intensity duration of shaking Soil type (soft?
hard rock?) Building design

• other effects

Liquifaction Tsunamis
23
Intensity of shaking decreases with distance from
epicenter (MM Intensity scale). M6.7, 1994
Northridge, California earthquake
24
Intensity of shaking decreases with distance from
epicenter (Peak horizontal acceleration). M6.7,
1994 Northridge, California earthquake
25
Intensity of shaking decreases with distance from
epicenter (star USGS Shake Map). M6.7, 1994
Northridge, California earthquake
26
Richter Scale

• Based on amplitude of the largest wave
• Logarithmic scale
• Ex) 3.0 is 10x greater than 2.0

27
Moment Magnitude

• Scientists use this now
• Based on several factors (more complicated)
• But tries to measure displacement energy that
  is released
• gt6 significant
• gt7 major
• gt8 great

28
2) Earthquake Intensity and magnitude

• Mercalli intensity scale
• Magnitude

Intensity of shaking damage at a specific
location
Depends on distance to earthquake strength of
earthquake
A measure of the energy released in an earthquake
Depends on size of fault that breaks
29
1) Earthquake belts

• Fault Types