Triangulation

Finding earthquake’s epicenter using seismograms!

In Class Activity (working with shoulder partner is A – OK!)

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

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

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

But 200 kms in what direction?

If a second station reports it as 75 kms away, they can draw a circle and see where it

intercepts the first circle.

But that still doesn’t pinpoint the location – it could have happened at either spot where the

two circles touch.

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

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

• Nomograph

Finding Magnitude

In class activity

• We’re going to find an epicentre of an earthquake in San Francisco

• Lesson 12 http://maccallum.wikispaces.com• Paper and Pencil activity• Let’s do 1st 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)