This document provides an overview of earthquakes and seismology. It discusses key concepts like epicenters, focuses, seismic waves, magnitudes, and locating earthquakes. Examples are given of calculating earthquake location using travel time curves. Factors that influence earthquake intensity like magnitude, distance, geology, and building design are covered. The role of resonance and different building designs for earthquake resistance are also summarized. Animations and activities are referenced to illustrate key concepts.

1. Part 2: Earthquakes & Seismology
(See Part 1: The Earth & Plate Tectonics for background)
Video lectures of this presentation can be viewed at:
www.iris.edu/hq/programs/education_and_outreach/videos
• “Normal” view has a Notes panel below with lecture notes and
links to video lectures, activities, or web sites.
•“Slide Show” view has elements that pop up on clicking
•Video & animations were removed to increase speed.
download information is in the Notes panel in Normal view
www.iris.edu/hq/programs/education_and_outreach/animations
• Modify slide show liberally for your own needs!
Slide show prepared by
Dr. Robert Butler, University of Portland, and Jenda Johnson.

2. Focus:
or hypocenter; point within Earth where the EQ occurred.
Epicenter—Location directly above EQ on Earth’s surface.
Epicenter & Focus of Earthquakes

3. Why are there earthquakes?
Brittle vs. ductile
Example:
Subduction-zone
earthquakes
occur in discrete
areas on and
between plates.
Why?
Watch the video lectures!

4. Body waves
(P and S) travel
inside Earth.
Surface waves
travel along Earth’s
surface.
Body Waves and Surface Waves
While P- and S- waves radiate outward in all directions, surface waves travel
along the surface of the earth and decrease in amplitude with depth.

5. P waves Fast
S waves Intermediate
Surface Slow
waves
Types of seismic waves

6. Body waves
P waves are compressional
Activity: “Seismic Slinky—Generating P & S waves”; see notes.
ACTIVITY:
Screen Grab
Animation link in notes

7. S waves are shear waves
Body wavesACTIVITY:
Screen Grab
Animation link in notes

8. Rayleigh & Love Waves
Surface waves

9. A seismograph detects
and records EQs.
A seismogram is the
EQ record.
How do scientists detect earthquakes?
When an earthquake occurs the seismic waves travel through the Earth to the seismic
station where the information is transmitted to distant computers.

10. Seismometers
Animations of horizontal and vertical seismographs and a
movie of an actual seismogram being recorded during an
earthquake are on the website:
http://www.iris.edu/hq/programs/education_and_outreach/animations
Screen Grab
Animation link in notes
Download one-pager,
“No. 7. How Does a Seismometer Work?”
http://www.iris.edu/hq/publications/brochures_and_
onepagers/edu

11. Narrated animations of basic effect of P, S, and
Surface waves on a seismic station are on:
http://www.iris.edu/hq/programs/education_and_outreach/animatio
ns
SeismicWaveBehavior—
Effect on Building: 3-Component Seismograph:
Screen Grabs
Animation link in notes

12. Download the impressive interactive P.C. program with other earthquakes from
Alan L. Jones’ website:
http://www.geol.binghamton.edu/faculty/jones/
If you don’t have a PC, you can watch
the progression (and how complicated
it can be) in an animated replication
of a part of the computer program:
http://www.iris.edu/hq/programs/education_and_outreach/animations
ACTIVITY:
Seismic waves from the 2004 Sumatran earthquake were
recorded at stations all over the globe.
Screen Grab
Animation link in notes

13. Travel-time curves
Seismic waves from
large earthquakes
arrive at seismic
stations around the
world at predictable
times.
The graph on the right (turned 90°
and includes only major direct waves)
shows the arrival times as continuous
lines for each of the waves reflecting
the pattern in the seismograms above.
(see animation “Travel-time curves”)
Animation link in notes

14. A seismogram is the record of an earthquake.
P waves travel about 1.7 times faster than S waves.
Surface waves take their time rolling along at shallow levels.
The difference between the arrival times of the seismic waves
indicates of how far away the earthquake was.
QUESTION:
If a P wave arrived at a station 5 minutes and 45 seconds
after the earthquake occurred; and the S wave arrived at
the station 10 min and 30 seconds minutes after the

15. How far away was the earthquake?
Calculate S-arrival time minus P-arrival time using
this graph of travel-time curves.
ACTIVITY:

16. How can you use this information to locate an earthquake?
ACTIVITY:
Graph of travel-time curves

17. Locating an Earthquake
1) Determine distance of EQ
from three seismic stations
by calculating the S minus P
arrival times.
2) Plot them on the travel-
time graph.
3) Intersection of the circles
gives the location.
1
23
ACTIVITY:
See: http://www.iris.edu/hq/programs/education_and_outreach/animations#N

18. How big was it? The Richter Scale
What is the Richter
magnitude of this EQ?

19. S — P = 26 sec
Amplitude
= 23mm
Magnitude = 5
How big was it? The Richter Scale
What is the Richter
magnitude of this EQ?

20. Magnitudes and Energy of Earthquakes
Download: How Often do Earthquakes Occur?
Annual Numbers of EQs
What’s the message?
MOST of the energy is released by around 20 magnitude-7 and larger EQs
every year.

21. Magnitudes and Energy of Earthquakes
Download: How Often do Earthquakes Occur?
Annual Numbers of EQs
What’s the message?
MOST of the energy is released by around 20 magnitude-7
and larger EQs every year.

22. The Earthquake Machine
Think forces, faults, and friction.
How could pulling a block of
wood with a string of rubber
bands have anything to do with
earthquakes?
There are no rubber bands in
the Earth, but all solids are
elastic, including the Earth’s
crust.
ACTIVITY:
ACTIVITY & ANIMATIONS are on http://www.iris.edu/hq/programs/education_and_outreach/animations/1

23. Earthquake Intensity is what you feel.
What Controls the Level of Shaking?
• Magnitude—More energy released
• Distance—Shaking decays with distance
• Geology—Local soils amplify the shaking
• Building style—Construction, not height
• Duration of shaking
Example:
1994 Northridge EQ, M=6.7

24. Seismic intensity is affected by rock type.
Amplitude of oscillation
How would you expect the houses to react during an EQ?

25. Seismic intensity is affected by rock type.
Amplitude of oscillation increasing

26. Liquefaction experiment
What happens to a structure built on a weak
foundation when an earthquake strikes?
Photos: Lily Rodriguez
From website below
For the description of this liquefaction activity:
http://www.exploratorium.edu/faultline/activezone/liquefaction.html
Niigata, Japan 1964
Source: National Geophysical Data Center

27. Building design:
If the resonant frequency of a building is equal to the frequency of
ground oscillation, then damage or collapse is likely.
What is resonant frequency?
Discussion: Why is building damage selective?

28. Resonance
Oscillating buildings demonstration—3 methods.
BOSS model
Video demonstrations by John Lahr are online:
http://www.iris.edu/hq/programs/education_and_outreach/videos
Two similar demonstrations use simpler methods:
Spaghetti & Manilla file
raisins……. “Boss lite”…
Screen Grabs

29. Building design is critical to earthquake safety
Resistance to
shear is
critical.
See the video lecture on Building Strength on:
http://www.iris.edu/hq/programs/education_and_outreach/videos