This document provides information about earthquakes, including what causes them, the different types of seismic waves, how the location and magnitude of earthquakes are determined, hazards associated with earthquakes such as shaking, ground displacement, tsunamis and fires, and challenges around predicting earthquakes. It describes how earthquakes occur due to the accumulation and sudden release of strain energy in rocks under stress. There are two main types of seismic waves - body waves that travel through the interior of the earth and surface waves that travel along the surface. The location of earthquakes is determined through measuring the time delay between arrival of P and S waves at multiple seismograph stations and triangulating the epicenter. Magnitude is a measure of the

1. Earthquakes

2. Global Earthquake Locations

3. Earthquakes
• Shaking of earth due to movement of rocks along a fault.
• Rocks under stress accumulate strain energy over time.
• When stress exceeds strength of rocks, rock breaks.
• Strain energy is released as seismic waves. The longer that
energy is stored up and is maintained without release, the more
likely that a strong earthquake will occur.
Types of seismic waves
1. Body waves -- travel through interior
2. Surface waves -- travel on surface of earth

4. Surface or "Love" (“L”) Waves
Cause vertical & horizontal shaking
Travel exclusively along surface of earth
Specific Body Waves
Primary or "P" Waves: Primary waves Highest velocity.
Causes compression and expansion in direction of wave travel.
Secondary or "S" Waves: Secondary or shear waves Slower than
P waves but faster than surface waves. Causes shearing of rock
perpendicular to direction of wave propagation. Cannot travel
through liquids

5. Primary or “P”
Wave
Secondary or
“S” Wave

7. Seismographs

9. Seismogram Printout

10. Determining the location of an earthquake
First, distance to earthquake is determined.
1. Seismographs record seismic waves
2. From seismograph record called the
seismogram, measure time delay
between P & S wave arrival
3. Use travel time curve to determine distance to
earthquake as function
of P-S time delay

11. Determining the location of an earthquake
Now we know distance waves traveled, but we don't know the direction from
which they came.
We must repeat the activity for each of at least three (3) stations to
triangulate a point (epicenter of quake).
Plot a circle around seismograph location; radius of circle is the distance to the
quake.
Quake occurred somewhere along that circle.
Do the same thing for at least 3 seismograph stations; circles intersect at
epicenter. Thus, point is triangulated and epicenter is located.

12. Focus and Epicenter of Earthquake

13. Time-Travel Curve

14. Triangulation
of 3 stations
to locate
earthquake
epicenter

15. Determining the magnitude of an earthquake
Magnitude -- measure of energy released during earthquake.
There are several different ways to measure magnitude.
Most common magnitude measure is Richter Magnitude, named for the renowned
seismologist, Charles Richter.
Richter Magnitude
• Measure amplitude of largest S wave on seismograph record.
• Take into account distance between seismograph & epicenter.
Richter Scale
• Logarithmic numerical (NOT a physical) scale
• Increasing one whole unit on Richter Scale represents 10 times greater
magnitude.
• Going up one whole unit on Richter Scale represents about a 30 times greater
release of energy.
Intensity
• Intensity refers to the amount of damage done in an earthquake
• Mercalli Scale is used to express damage

16. Hazards associated with Quakes
• Shaking:
Frequency of shaking differs for different seismic waves.
High frequency body waves shake low buildings more.
Low frequency surface waves shake high buildings more.
Intensity of shaking also depends on type of subsurface
material.
Unconsolidated materials amplify shaking more than rocks do.
Fine-grained, sensitive materials can lose strength when
shaken. They lose strength by liquefaction.
Buildings respond differently to shaking depending on
construction styles, materials
Wood -- more flexible, holds up well
Earthen materials -- very vulnerable to shaking.

17. Hazards associated with Quakes
• Ground displacement:
Ground surface may shift during an earthquake (esp. if focus
is shallow).
Vertical displacements of surface produce fault scarps.
• Tsunamis (NOT tidal waves)
Tsunamis are huge waves generated by earthquakes undersea
or below coastal areas.
If earthquake displaces sea surface, wave is generated that
can grow as it moves over sea surface.
• Fires
Usually occurs from shifting of subsurface utilities (gas lines)

18. Tsunami Movement

19. Tsunami Movement: ~600 mph in deep water
~250 mph in medium depth water
~35 mph in shallow water

20. Earthquake Prediction (?)
How can scientists predict an earthquake?
Currently, that is not possible.
Future technology will monitor subsurface seismic waves and
periodic shifting indicative of future slippage.
Tracking organic movement is also a source of future study.

21. Earthquake Hazard Potential Map
Parkfield, CA
“Earthquake Capital of the World”

22. World’s Largest Earthquake: 1964 Anchorage, Alaska
Registered 8.6 on Richter Scale