Exploring Earth’s structure
using Occam’s Razor
Adapted from Braile, 2000 & DLESE Teaching Boxes, 2008
Prepared by
Michael Hubenthal, IRIS Consortium
What is inside ?
How do you know what is inside?
What is inside here?
How do you know what is inside?
Think, Write, Pair, Share
How do you
know?
Occam’s Razor - All other things being equal, the simplest
solution is the best
Based on your “evidence” what is beneath our feet?
Calvin and Hobbes by Bill Watterson
Could this help us test our model?
Theoreticians
Seismologist
Theoreticians
Seismologist
Earth’s surface
Center of the Earth
Scale model of a homogeneous Earth
140°
Seismologists
Theoreticians
Creating a travel-time plot
•Work in teams of ~ 2
•Each pair calculates 3 travel
times
•Each team makes 1 travel time
plot, 0-180 degrees
•Discuss results
Use velocity=11 km/sec, distance
scale 320 km/cm
10
30
40
50
70
80
100
115
130
150
Ti m
e (
Min
)
Earthquake Wave Fi rst A rri val s
D i stance From Epi center (D eg)
What is the cause of model and observations mismatch?
Shadow zone
Something changes
beyond ~110 deg.
Combine results
from multiple
earthquakes
Assumptions?
?
What has your data
established so far?
Have we found any layers?
First Arrivals
1st: 1 deg - .5 min
2nd: 16 deg - 3.5 min
http://rev.seis.sc.edu/
EarthΥs Interior Structure Seismic Travel Times in a
Constant Velocity Sphere
Receiver
EarthХs
Surface
Raypath
(chord of length S)
S/2
R = 6371 km
/

(L. Braile , December, 2000)
Source
2
R = 6371 km
http://web.ics.purdue.edu/~braile
Updated Version
http://rev.seis.sc.edu/ > DLESE Teaching Boxes!