How Shallow Earth Structure Is Determined
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Transcript of 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, 2008Michael Hubenthal, IRISJohn 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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C
B1
.
B2
Layer 1
Layer 2
Explosion – or energy source
B
Prior Knowledge
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Experiment Design?
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Time
Distance
Direct wave: Slope is 1/v1
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Time
Distance
Direct wave: Slope is 1/v1
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 WaveReflected Wave
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Refracted Waves
Direct WaveReflected Wave
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v1
v2
i1
i2
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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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Time
Distance
Direct wave: Slope is 1/v1
Head wave: Slope is 1/v2
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Time
Distance
Direct wave: Slope is 1/v1
2. Measure Head Wave Slope and you get v2 !!!!
Head wave: Slope is 1/v2
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What about depth of lower layer?
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Time
Distance
Direct wave: Slope is 1/v1
The cross-over distance, xcross , is related to the depth to the boundary!
Head wave: Slope is 1/v2
xcross
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Time
Distance
Direct wave: Slope is 1/v1
The cross-over distance, xcross , is related to the depth to the boundary!
Head wave: Slope is 1/v2
xcross
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Depth
A little geometry and some algebra results in
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:
Step 3. Find the crossover distance
xcross
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Example:
Step 3. Find the crossover distance.
Step 4. Compute the depth to the boundary.
xcross
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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 -> 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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0 100 km
Mid-Atlantic Ridge off South America
Computer model
Ray trace diagram
What about variable layers?