1. Earthquakes!!

2. Videos

3. Focus and Epicenter Focus – where the earthquake happens
Epicenter – the spot on the surface above the focus

5. Elastic Rebound
Stress causes the ground begins to be deformed. An earthquake occurs when a rock passes its point of maximum elasticity. After it breaks, it regains its original shape (but not position.)

6. Seismograph and inertia
Notice that it is the mass that is moving, not the drum. A heavy mass is used (more inertia) and is suspended so that it is less effected by whatever it is attached to. See the animation next.

7. Seismographs
To view this animation, click "View" and then "Slide Show" on the top navigation bar.

8. Seismic Waves P(Primary) – body wave S(Secondary) – body wave
L(Long) – surface wave

9. Earthquake Waves
To view this animation, click "View" and then "Slide Show" on the top navigation bar.

10. Seismogram- recording from seismograph
P wave arrives first
S wave then arrives
The time between them tells you how far away they came from. Notice that it is 5 minutes from P to S wave.
The graph on the next page shows how to do this
5 minutes

11. Different speeds of P and S waves
First, look at an example at the times for P and S wave travel. How long does it take for a P wave to travel 5000 km? An S wave?
P wave – 8 minutes
S wave – 14 minutes and 40 seconds

12. Finding distance from seismogram
The earlier seismogram has an S-P travel time difference of 5 minutes. So, we must find the place on the graph where the difference between the P and S curves is 5 minutes and find the corresponding distance, as shown here. Start by using your y axis to measure 5 minutes, as shown.
Then move it to where it fits exactly between the lines.
Measure 5 minutes here
And read the distance here as 2900 km.

13. Distance Graph
Another graph allows you to find the distance to the epicenter using a line that shows the difference in arrival time between P and S waves. The one shown can only be used for distances from 300 – 750 km. Simply read the distance off the line, as shown.
Let us see how far an epicenter is if the difference between P and S waves is 60 seconds.
About 580 km.

14. Finding the Epicenter Three seismograph stations are needed. Why?
Each location gives us a distance, so we can draw a circle there. Then the intersection of 3 circles gives us the epicenter.

15. Finding Magnitude
Use the distance you calculated from the seismogram and graph. Then find the amplitude of the S-wave on the seismogram. Put these two quantities on the opposite sides of this special graph called a nomogram. Connect them with a line, and this gives you the magnitude.

16. Let’s try one here.
Use the time difference of 35 seconds to find the distance to the epicenter.
The difference in arrival times between the P and S wave here is about 35 sec. (These graphs are all in seconds.)
The amplitude of the S wave is about 350 mm.
About 340 km

17. Finding the Magnitude
Now mark the distance and amplitude on the graph here - called a nomogram. (Watch the graph carefully as we click here)
And then connect them with a line.
Where the line intersects the magnitude line, you have found the magnitude. This one is about 6.7, right?
We will do some for practice here.

18. The shadow zone
Notice in this diagram that S waves are absorbed by the liquid outer core. P waves are refracted by the outer core, because waves refract (bend) when they enter a new medium at an angle. This is how geologists found out that the outer core is liquid.

19. Shadow zone 2 So, match the correct seismogram below with its location1 2 3 4 5 D A E C B

20. Changing earthquake depth at subduction zones
Why do the earthquakes vary in depth?
Which way is the plate subducting at the Tonga Trench?
To the west
Which way is the plate subducting at the Peru-Chile Trench?
To the east
We will do this in an activity later.

21. Let’s look at the Tonga Trench
Deepening earthquakes at a subduction zone

22. Tsunami

23. Tsunami travel times to Hawaii

24. Old Tsunami Photo

25. Video

26. The Big Tsunami Dec 26, 2004

27. One of the greatest disasters
Safety tip. If you see the ocean floor, and the ocean has retreated, head for high ground as fast as possible.

28. How many earthquakes/how big?

29. Historical Earthquakes

30. Magnitude How much energy is released
Moment Scale Mw replaced the Richter Scale ML

31. Moment Scale Mo = seismic moment
Seismic moment take in account area, displacement, and type of rock

32. Intensity – damage done

33. Effects of Earthquakes
Shaking and Ground Rupture
Liquefaction
Change in land elevation
Landslides
Fires
Disease