Seismic Profiles of Earth’s Interior Objective: Use seismic waves to determine the nature of Earth’s interior layers Scientists learn about Earth’s interior by using seismic waves or vibrations that travel through Earth’s layers during earthquakes. There are different types of seismic waves. P waves (Primary waves) are compressional vibrations that travel accordion style through solids and liquids. S waves (Secondary waves) are transverse. They travel in a snake-like fashion, but only through solid mediums. Surface waves travel in a variety of methods, but only along the solid crust. Seismic waves change speed and angle as they move through different mediums. In general, seismic waves travel faster through solids or rigid densely packed materials.
Radius = 6370 km Mantle Crust Liquid iron outer core Solid iron inner core http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html http://www.uh.edu/~jbutler/physical/chapter19.html Distance to core is ~1/2 the radius (2900 km depth)
Core Mantle Crust Sulfur (4%) Nickel (11%) Iron (85%) Density: ~3 g/cm3 Density: ~5 g/cm3 Density: ~11 g/cm3
Types of Seismic Waves 4-6 km/s 2-3 km/s P (primary) and S (secondary) waves travel through Earth’s interior info about the mantle and core (density, composition) Surfaces waves (Love, Rayleigh) travel through the crust info about the crust
Seismic Wave Velocity Density (composition) Physical State Which rock will seismic waves travel the fastest through: Sandstone or Periodotite? Give a reason why. Seismic waves travel faster through: solids or liquids?
Reflection and Refraction Refraction Reflection This is what happens to seismic waves in the Earth. Waves BEND or REFRACT when they move through different mediums.
Seismic Wave Reflection • Used to find depth to boundaries (density discontiniuites) P-Wave Reflections: P waves can travel through solids or liquids
P-Wave Shadow Zone S-Wave Shadow Zone Indicates depth to the core-mantle boundary Indicates that the outer core is liquid
Seismic Velocity Profile of Earth A • Which seismic waves travel faster: S waves or P waves? • Describe how velocity of seismic waves change from 0 – 670 km depth. • Give a reason for the change in speed in the first 670 km depth. • How does the velocity of seismic waves in the lower mantle compare to that of the upper mantle? (Where do they travel faster?) • Give a reason for the change is velocity between the upper and lower mantle. • What happens to waves between Dand E? Why? • Why are there no S waves in the outer core? • Which layer do waves travel fastest and why? B D E liquid F solid
The Crust & Mantle: Close Up A B . • 9. How does velocity of P wave change from A to B? Explain why. • 10. What change in composition or structure may account for the slow down of waves by C? (Hint: you may need to Google “low-velocity zone”) • What accounts for the gradual increase in speed from D to F? • Summarize how seismic waves allow us to “see” inside Earth. C D E F
Convection in the Mantle subducted slabs large-scale convection CMB mantle plumes