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Geophysics/Tectonics GLY 325 The Wave Equation Modeled Even for models with no noise, identifying phases may be challenging. However, you can train your eye/brain to learn how to do this. The Wave Equation Modeled You will learn to do this... The Wave Equation Modeled So, at any interface, some energy is reflected (at the angle of incidence) and some is refracted (according to Snell’s Law). Let’s look at a simple model and just watch what happens to the P-wave energy... Refraction Interpretation We’ll no look specifically at refractions and discuss interpretation. First, let’s look at something beyond the two-layer case. Multilayer Model Distance Direct Wave Time 1st Layer Refraction 2nd Refraction 1st Reflection 2nd Reflection Multilayer Model Prior to the first arrivals, only background noise interferes. After the first arrivals, background noise plus many other phases are recorded. Distance Direct Wave Time 1st Layer Refraction 2nd Refraction Case Studies Why is determining depths to interfaces and seismic propagation velocities important? (1) Depth to the Moho (crust/mantle boundary) often relates to tectonic history: • Thin crust at rift zones and mid-ocean ridges • Thick crust at mountain ranges •Thin to thick transition at continental margins (2) Velocity of upper mantle (lower lithosphere) suggests asthenospheric upwelling: • Low seismic velocity means hot asthenosphere is shallow and partial melting occurs (continental rifts & mid-ocean ridges) • Higher seismic velocity means no asthenospheric upwelling. Case Studies Global Crustal Thickness (km) Case Studies Case Studies Case Studies Global Geologic Province Case Studies Case Studies Case Studies Case Studies Case Studies Case Studies Case Studies Case Studies Case Studies Case Studies Case Studies Case Studies Case Studies Case Studies Case Studies Case Studies
