1. Measuring Earthquakes
Seismology

2. Learning Target
I will explain how earthquakes are measured and how scientists locate the epicenter of an earthquake.

3. Earthquakes generate waves that travel through the earth
Seismic Waves
Earthquakes generate waves that travel through the earth

4. Earthquake Detection
Seismograph - instrument that records earthquake waves.
a weight is suspended from a support that is attached to bedrock.
When the bedrock vibrates, a pen records the earthquake on a rotating drum.

5. How Seismographs Work
Weight is suspended from an object on bedrock, vibration causes it move.

6. Seismic Records Seismograms - records of the ground motion
Sheets of paper removed from the rotating drum of the seismograph.

7. Calculating Earthquakes
Earthquake - the vibration of the Earth caused by a rapid release of energy
Focus - the point inside the Earth where the earthquake begins.
Epicenter - the location on the surface directly above the focus

8. Locating Earthquakes
The difference in speeds of P and S waves helps us locate the epicenter.
The more time between the 1st arriving P and S waves, the greater the distance the earthquake is away.
Example: an interval of 5 minutes indicates the earth-quake was 3,400 km away from the recorder.

9. Seismic Waves in the Earth

10. Locating Earthquakes How can we find an earthquake’s direction?
Location can be found if three seismic stations recorded the earthquake.
Draw circles around seismograph station
Radius = distance from the seismograph to the epicenter (using time-distance graph)
the epicenter is where the 3 circles intersect (triangulation)

11. Drawing circles around seismograph stations to find the epicenter
Locating Earthquakes
Drawing circles around seismograph stations to find the epicenter

12. Drawing circles around seismograph stations to find the epicenter
Locating Earthquakes
Drawing circles around seismograph stations to find the epicenter

13. Locating Earthquakes

14. Magnitude Magnitude - measures energy released (earthquake’s size)
Richter Scale - measures magnitude using seismic records of energy release
- No upper or lower limits
- Largest quakes about Magnitude 9.5 (Chile 2010)

15. Intensity
Intensity - measures how strong an earthquake feels to the observer
Depends on:
Distance to quake
Geology
Type of building (or outside?)
Person
Varies from place to place

16. Mercalli Scale
Mercalli scale – rates earthquakes according to the level of damage at a given place.
Mercalli Scale: 1 to 12
1 is not felt at all
12 is catastrophic

17. Think-Pair-Share
How are the Mercalli scale and Richter scale similar? …Different?
On which scale would an earthquake’s scale vary from one place to another, and why?

18. Comparing Magnitudes
One point increase in magnitude = 32 times more energy
Ex. Magnitude 6 earthquake releases about 32 x more energy than a magnitude 5 earthquake

19. Magnitude and Energy Magnitude Energy Explosive Power Example 9 8 7 6
U.S. Energy Use for a month
Alaska 1964 Indonesia 2004 8
U.S. Energy Use for a day
San Francisco, 1906 7
One Megaton
World Series Earthquake, 1989 6
U.S. Energy Use for a minute
Large Thunderstorm 5
One Kiloton 4 3
One ton of explosives
World Trade Center Collapse

20. Magnitude and Energy Magnitude Energy Explosive Power Example 3 2 1 -1
One ton of Explosives
World Trade Center Collapse 2 1
Topple 50-meter tree
One kilogram of explosives
Head-on collision at 60 mph
Drop a car 10 meters
Half stick of dynamite
Very bad day skydiving -1
Impact of bullet
One gram of explosives -2
Hammer blow -3
Dribbling a basketball