1. Seismic Waves P waves (primary waves) compress and expand the ground
first waves/fastest moving
travel through solid, liquid, and gas

2. Seismic Waves S waves (secondary waves)
travel at half the speed of P waves and arrive second
travel through solids only
strong enough to shake structures

3. Seismic Waves

4. Seismic Waves Surface waves move most slowly
produce severe ground movements; can cause ground to roll or buildings to shake back and forth.

5. Measuring Earthquakes
Seismograph: instrument that records and measures the seismic waves.

6. Measuring Earthquakes

7. Measuring Earthquakes
Modified Mercalli Scale
Rates the amount of shaking
Rated by observations; useful in areas where there aren’t instruments available.

8. Measuring Earthquakes

9. Measuring Earthquakes
Richter Scale
Finds the magnitude; a number assigned to an earthquake based on size.
Depend on the size of the waves recorded by a seismograph.

10. Earthquake Magnitude
Bill Nye!

11. Measuring Earthquakes
Moment Magnitude Scale
Rates the total energy an earthquake releases.
Geologists use data from seismographs and other sources.

12. Measuring Earthquakes

13. Comparing Magnitudes
A 1-point increase on a scale represents an increase of 32-times more energy.
The higher an earthquake is rated, the more energy it has and the more damage it can do.

14. Locating an Earthquake
You need seismogram info from 3 places:
Find the difference of arrival times between the P and the S waves.
The difference is used to find the distance to the epicenter – the longer it takes, the farther away the epicenter.
A circle is drawn around each point (location). The point where they all cross is the epicenter.

15. Locating an Earthquake

16. 4.3 Monitoring Earthquakes
Chapter 4: Earthquakes
4.3 Monitoring Earthquakes

17. Earthquake Risk
Largely depends on how close a given location is to a plate boundary.
In N. America, there are plate boundaries in California, Washington, and Alaska.
Around the world, the Ring of Fire contains boundaries where several volcanoes and earthquakes occur.