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?