1. Triangulation Finding earthquake’s epicenter using seismograms! In Class Activity (working with shoulder partner is A – OK!)

3. Focus – the place deep in the crust where the earthquake begins (source of primary and secondary waves)

4. Epicentre – the surface location directly above the focus (source of surface waves)

5. Let’s say station #1 knows the earthquake happened 200 kms away.

6. But 200 kms in what direction?

7. If a second station reports it as 75 kms away, they can draw a circle and see where it intercepts the first circle.

8. But that still doesn’t pinpoint the location – it could have happened at either spot where the two circles touch.

9. But a third station removes all doubt as to where it happened.

10. Three seismographs are needed to pin- point the source of an earthquake.

11. Nomograph Finding Magnitude

12. In class activity We’re going to find an epicentre of an earthquake in San Francisco Lesson 12 http://maccallum.wikispaces.comhttp://maccallum.wikispaces.com Paper and Pencil activity Let’s do 1 st one together! Walk through it. You and shoulder partner can quietly work on the last 2 seismograms What is the average magnitude of this San Francisco earthquake? Please raise hand when finished. If I saw your work, you can try the links at the bottom of wikispace (Virtual Earthquake)