applying occam’s razor to determine earth’s …...applying occam’s razor to determine...

Applying Occam’s razor to determine

Earth’s internal structure

V 3.1 - Prepared by Michael Hubenthal

Adapted from Braile, 2000, Hubenthal, 2006 & DLESE Teaching Boxes, 2008

http://www.iris.edu/hq/resource/determining_internal_structure

How do you know what is inside?

What is inside ?

What is inside

here?

Think, Write, Pair, Share

How do you know what is inside?

Occam’s Razor - All other things being equal, the simplest solution is the best

Based on your “evidence” what is beneath our feet?

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Could this help us test our ideas?

Theoreticians

Seismologist

Hypothesis: the simplest

solution to the question

“What is beneath our feet”

is a homogeneous Earth,

or one comprised entirely

of rock

Seismologist

Your task is to help test this hypothesis by analyzing a set

of seismograms from a real earthquake to determine how

long it takes for the seismic waves released from an

earthquake to arrive at various points on Earth’s surface.

Theoreticians

Your task is to help test this hypothesis by creating a model

of the Earth, using the known velocity of seismic waves in

rock ~ 11km/s to predict how long it should take seismic

waves to reach various distances around Earth.

Earth’s surface

Center of the Earth

Scale model of a homogeneous Earth

RightLeft

180o

140°

Seismologists

Theoreticians

How does this anomaly

in the data translate

back to Earth?

Measure the geocentric angle from the epicenter to the

anomaly in the data (108 degrees for example).

Use your ruler to connect the epicenter to the mark you just

drew on Earth’s surface.

Next, repeat this procedure to the bottom of the circle

and label the area inside the angles drawn as the “P-

wave shadow zone”

P-wave

shadow

zone

Answer the following questions on a separate sheet of

paper:

1. What sort of structure have we determined so far?

1. How has the seismic data helped us determine Earth’s

interior structure?

1. Look at the P-wave shadow zone. How is this “shadow

zone” similar to a person’s shadow on the ground?

Like a tree making a shadow, something is blocking the

energy. Can we tell what it is?

Cut out this shape from you model

Lay the cut out from Earth A on top of Earth B and trace

along its edges (indicated with orange)

Next, rotate the cut out and continue to trace its edges

Answer the following questions on a separate sheet

of paper:

4. As additional earthquake data is added, what shape is

being defined in the interior of Earth Model B?

4. What do you think this new inner circle represents?

4. Calculate the radius of the core of your Earth Model B,

and use the scale at the bottom to estimate the actual

radius of the core.

Current estimated radius of

outer core: ~ 3486 km

Answer the following questions on a separate sheet

of paper:

7. How well does you model radius of the outer core

match?

7. Where are there likely sources of error?

7. Considering the core was discovered only in 1906, is it

possible that the current model may have further

refinements?

http://www.seismo.com/msop/msop79/rec/nomen.htm

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