This introduction to seismic wave behavior describes Snell's Law and how it applies to layers in the earth. It addresses reflected, refracted, critically refracted, and head waves. And ends with a quiz about which path is the fastest to seismic stations 30-100 meters away. Seismic waves travel at different speeds through different materials. In this 2-layer model two wave fronts leave an impact at the same time but the lower layer is faster.