Structure of the Earth
The Earth has a layered structure, including the core, mantle and crust. The crust and upper mantle are cracked into large pieces called tectonic plates. These plates move slowly, but can cause earthquakes and volcanoes where they meet. The seismic waves produced by an earthquake are monitored and tracked.
The Earth is almost a sphere. These are its main layers, starting with the outermost:
- crust - relatively thin and rocky
- mantle - has the properties of a solid, but can flow very slowly
- outer core - made from liquid nickel and iron
- inner core - made from solid nickel and iron
Note that the radius of the core is just over half the radius of the Earth. The core itself consists of a solid inner core and a liquid outer core.
P waves and S waves
There are three types of seismic waves – P waves, S waves and surface waves.
P waves are longitudinal waves, and their relative speed is faster than other waves. P waves can travel through solids and liquids.
S waves are transverse, and they are slower than P waves. They travel through solids only.
Surface waves are also longitudinal waves, and they are the slowest. They travel along surfaces (hence the name).