Earth's Internal Structure: Earth's Seismic Structure (2.2.4) Flashcards
How can seismic waves help us look inside Earth?
1) Seismic waves propagate radially away from focus.
2) Pass through Earth, detected by seismometer stations.
3) Velocity depends upon what material it passes through.
(Faster in peridotite, P-waves travel faster in solid than liquid, S-waves can’t travel through liquid).
What is Snell’s Law?
Waves are both reflected and refracted at boundary between materials of different velocities.
- Velocities in Earth normally increase with depth, so wave paths refracted back to surface.
- This refraction means can have multiple arrivals for same earthquake.
What is the Mohorovicic Discontinuity (Moho)?
- The boundary between Earth’s crust and mantle.
- Determining depth to Moho provides measure of crustal thickness.
- Moho has average depth ~7km beneath oceans (little variation), and ~32km under continents (much variation).
How was core-mantle boundary discovered?
Because of P-wave shadow zone. Depth at 2900km.
What degrees from epicentre do P-waves not arrive at seismometers?
103 degrees and 143 degrees.
Does refraction of P-waves increase or decrease their velocity?
Decreases.
S-wave shadow zone for all arrivals less than 103 degrees means?
The (outer) core must be liquid.
Is the inner core liquid or solid?
Solid.
What is the origin of discontinuities within the mantle?
- Step increases in Vp and Vs occur within mantle at 410km and 660km.
- Shown from high-pressure experiments to be crystal phase transitions of minerals in peridotite.
- Olivine lattice ‘repacks’ to higher density crystal structure.
What is the state-of-the-art investigations of mantle structure?
Seismic Tomography: combines data from thousands of ray paths to compute 3D models for subtle velocity variations throughout entire mantle.
Same principle as medical CAT scans.
