Note: at this point, the wave hits an “edge” from it’s perspective.

This is where the “diffracted” term comes from

Instrument response?

Seismic wave velocities• Wave equation:

– Part 1: • Force = stress x area

– Part 2: • Force = mass x

acceleration

• Also need:– Hooke’s law:

• F = -k x

• Deep breath (lower stress)…..

Deriving wave equation1. Go over some tools

2. Relate stress and displacement

3. Simplify as much as possible

4. Substitute into F=ma equation

5. Apply some vector identities

6. Show that there are only two solutions, which have the

velocities of the P- and S- waves

Stress & Strain

• Stress– Force/unit area

• Strain (1-dimension)– Here = L/L– Finite strain

• Whole history• >few %• Shape/relationships

change

– Infinitesimal strain• Less than few %• Allows some simplifications

(L’-L)/L = L/L = xx=dux/dx

Vector fields

• Divergence of a vector field:– No rotation– Just volume change– Which body wave?

Vector fields

• Curl:– No volume change– Just rotation– Which body wave?