How Shallow Earth Structure Is Determined A Classroom Exercise Demonstrating Seismic Refraction Use in the Real World NSTA, Boston, 2008 Michael Hubenthal,

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Presentation transcript:

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

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.

Refraction of light

C B1.B1. B2B2 Layer 1 Layer 2 Explosion – or energy source B Prior Knowledge

Experiment Design?

Time Distance Direct wave: Slope is 1/v 1

Time Distance Direct wave: Slope is 1/v 1 1. Measure Direct Wave Slope and you get v 1 !!!!

Identify: Direct wave Head wave (Refracted wave, upper layer) Refracted wave Reflected wave

Direct Wave

Reflected Wave

Refracted Waves Direct Wave Reflected Wave

v1v1 v2v2 i1i1 i2i2

The first head wave is a reflected wave at the critical angle i c

The Head wave moves along the surface at the SPEED OF THE LOWER LAYER!!!

Time Distance Direct wave: Slope is 1/v 1 Head wave: Slope is 1/v 2

Time Distance Direct wave: Slope is 1/v 1 2. Measure Head Wave Slope and you get v 2 !!!! Head wave: Slope is 1/v 2

What about depth of lower layer?

Time Distance Direct wave: Slope is 1/v 1 The cross-over distance, x cross, is related to the depth to the boundary! Head wave: Slope is 1/v 2 x cross

Time Distance Direct wave: Slope is 1/v 1 The cross-over distance, x cross, is related to the depth to the boundary! Head wave: Slope is 1/v 2 x cross

Depth A little geometry and some algebra results in Measure v 1, v 2, and x cross, and you can get the depth to the boundary!!!!

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 x cross

Example: Step 3. Find the crossover distance. Step 4. Compute the depth to the boundary. x cross

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?

0100 km Mid-Atlantic Ridge off South America Computer model Ray trace diagram What about variable layers?