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How Shallow Earth Structure Is Determined

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## 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**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.**Explosion – or energy source**C B B1. Layer 1 B2 Layer 2 Prior Knowledge**Distance**Direct wave: Slope is 1/v1 Time**Distance**Direct wave: Slope is 1/v1 Time 1. Measure Direct Wave Slope and you get v1 !!!!**Identify: Direct wave**Head wave (Refracted wave, upper layer) Refracted wave Reflected wave**Direct Wave**Reflected Wave**Direct Wave**Reflected Wave Refracted Waves**i1**v1 i2 v2**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!!!**Distance**Direct wave: Slope is 1/v1 Time Head wave: Slope is 1/v2**Distance**Direct wave: Slope is 1/v1 Time Head wave: Slope is 1/v2 2. Measure Head Wave Slope and you get v2 !!!!**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!**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!**A little geometry and some algebra results in**Depth Measure v1 , v2 , and xcross , 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:**xcross Step 3. Find the crossover distance**Example:**xcross Step 3. Find the crossover distance. Step 4. Compute the depth to the boundary.**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 variable layers?**Mid-Atlantic Ridge off South America Computer model Ray trace diagram 0 100 km