How Shallow Earth Structure Is Determined
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Transcript of How Shallow Earth Structure Is Determined
How Shallow Earth Structure Is Determined
A Classroom Exercise Demonstrating Seismic Refraction Use in the Real World
NSTA, Boston, 2008Michael Hubenthal, IRISJohn Taber, IRIS
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
C
B1
.
B2
Layer 1
Layer 2
Explosion – or energy source
B
Prior Knowledge
Experiment Design?
Time
Distance
Direct wave: Slope is 1/v1
Time
Distance
Direct wave: Slope is 1/v1
1. Measure Direct Wave Slope and you get v1 !!!!
Identify: Direct wave
Head wave (Refracted wave, upper layer)
Refracted wave
Reflected wave
Direct Wave
Direct WaveReflected Wave
Refracted Waves
Direct WaveReflected Wave
v1
v2
i1
i2
The first head wave is a reflected wave at the critical angle
ic ic
ic ic
The Head wave moves along the surface at the SPEED OF THE LOWER LAYER!!!
Time
Distance
Direct wave: Slope is 1/v1
Head wave: Slope is 1/v2
Time
Distance
Direct wave: Slope is 1/v1
2. Measure Head Wave Slope and you get v2 !!!!
Head wave: Slope is 1/v2
What about depth of lower layer?
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
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
Depth
A little geometry and some algebra results in
Measure v1 , v2 , and xcross , and you can get the depth to the boundary!!!!
Example:
Example:
Step 1. Find the slope of the Direct wave.
Example:
Step 2. Find the slope of the Head wave.
Example:
Step 3. Find the crossover distance
xcross
Example:
Step 3. Find the crossover distance.
Step 4. Compute the depth to the boundary.
xcross
Extensions to simple case
EarthScope seismic stations with a good recording of the Magnitude 5.6 earthquake in central California on 10/31/07
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”
What about multiple layers?
What about dipping layers?
0 100 km
Mid-Atlantic Ridge off South America
Computer model
Ray trace diagram
What about variable layers?