Concepts
- Some earthquake (seismic) waves travel THROUGH the Earth, as well as along its surface.
- Seismic waves are a type of mechnical (rather than electromagnetic) wave and thus require an intervening medium to propagate (move through).
- Mechanical waves such as earthquake waves travel through materials at different velocities depending on properties of materials.
- At boundaries between materials of different properties, earthquake waves bend and change direction of travel.
- The more time elapsed, the further away the front of an earthquake wave gets from its source.
- P (primary or compressional)and S (secondary or shear) waves generally travel faster through solids that are more dense.
- S (shear) waves cannot travel through liquids.
- Scientists use earthquake (seismic) waves to look inside the Earth to determine the depth boundaries between different layers of rock.
- Scientists occasionally get to look at the composition of materials that escape from the interior of the planet when it comes out of volcanoes.
- Most material that erupts out of a volcano does not come from very deep inside the Earth.
- In rare places, we can see pieces of the upper mantle in person where they have been pushed to the Earth's surface by tectonic forces in unusual circumstances.
- Scientists make "educated" guesses about the composition and existence of layers by considering multiple lines of evidence - such as the strength of the Earth's gravitational field, and the composition of the Earth compared with the composition of other objects in our solar system such as meteorites.
- Knowing what lies underneath the surface of the Earth helps us figure out why the plates move at the surface and why volcanoes erupt in certain places.
- Knowing how to use seismic waves to image the interior of the Earth helps us similar technologies to buried natural resources such as oil and gas.
- Knowing how to use seismic waves to image the interior of the Earth helps us spot hazards such as faults where earthquakes might occur.
- Knowing about the interior of our own planet lets us compare the Earth with other planets and understand the special conditions that make it our home.