How Shallow Earth Structure Is Determined

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How Shallow Earth Structure Is Determined

• A Classroom Exercise Demonstrating Seismic
  Refraction Use in the Real World
• NSTA, Boston, 2008
• Michael Hubenthal, IRIS
• John Taber, IRIS

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Objectives

• Explore how refracted seismic waves are used to
  determine earth structure.
• Reinforce the concepts of reflection and
  refraction of waves through a real world example.
  

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Refraction of light
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Prior Knowledge
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Experiment Design?
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Distance
Direct wave Slope is 1/v1
Time
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Distance
Direct wave Slope is 1/v1
Time
1. Measure Direct Wave Slope and you get v1 !!!!
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Identify Direct wave Head wave (Refracted
wave, upper layer) Refracted wave Reflected
wave
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Direct Wave
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Direct Wave
Reflected Wave
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Direct Wave
Reflected Wave
Refracted Waves
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i1
v1
i2
v2
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The first head wave is a reflected wave at the
critical angle
ic ic
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ic ic
The Head wave moves along the surface at the
SPEED OF THE LOWER LAYER!!!
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Distance
Direct wave Slope is 1/v1
Time
Head wave Slope is 1/v2
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Distance
Direct wave Slope is 1/v1
Time
Head wave Slope is 1/v2
2. Measure Head Wave Slope and you get v2 !!!!
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What about depth of lower layer?
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Distance
xcross
Direct wave Slope is 1/v1
Time
Head wave Slope is 1/v2
The cross-over distance, xcross , is related to
the depth to the boundary!
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Distance
xcross
Direct wave Slope is 1/v1
Time
Head wave Slope is 1/v2
The cross-over distance, xcross , is related to
the depth to the boundary!
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A little geometry and some algebra results in
Depth
Measure v1 , v2 , and xcross , and you can get
the depth to the boundary!!!!
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Example
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Example
Step 1. Find the slope of the Direct wave.
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Example
Step 2. Find the slope of the Head wave.
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Example
xcross
Step 3. Find the crossover distance
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Example
xcross
Step 3. Find the crossover distance. Step 4.
Compute the depth to the boundary.
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Extensions to simple case
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EarthScope seismic stations with a good recording
of the Magnitude 5.6 earthquake in central
California on 10/31/07
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Depth
Upper layer velocity 6.3 km/s Lower layer
velocity 8.2 km/s Cross over distance 220
km Depth 40 km -gt Depth to the base of the
crust Moho
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What about multiple layers?
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What about dipping layers?
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What about variable layers?
Mid-Atlantic Ridge off South America
Computer model
Ray trace diagram
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100 km