applying occam’s razor to determine earth’s …...applying occam’s razor to determine...
TRANSCRIPT
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?
Cal
vin
and H
obbes
by
Bil
l W
atte
rson
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?
