1. Earthquakes!!

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

3. 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.)

4. 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.

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

6. 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

7. 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

8. 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. Measure 5 minutes here Then move it to where it fits exactly between the lines. And read the distance here as 2900 km.

9. 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.

10. 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.

11. Finding Magnitude 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.

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

13. 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.here

14. The shadow zone 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.

15. Shadow zone 2 So, match the correct seismogram below with its location 1234512345 DAECBDAECB

16. 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.

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

18. Tsunami

19. Old Tsunami Photo

20. The Big Tsunami Dec 26, 2004

21. 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.

22. How many earthquakes/how bigHow many earthquakes/how big? How many earthquakes/how big

23. Historical Earthquakes

24. Intensity – damage done