Question: What are seismic waves? Are they longitudinal or transverse and what is their source?

Lidunka Vocadlo answered on 16 Jun 2015:

@Chickenophile When there is an earthquake (the brittle failure of rock) energy is released in the form of seismic waves. The waves travel directly from the source of the earthquake (exactly where the rock has fractured, usually a few km below the surface, but could be up to 600 km below) around the earth (a bit like the ringing of a bell). There are four types of seismic weaves: two travel only over the surface, called surface waves, and they are Rayleigh waves and Love waves. Two more travel through the Earth, called body waves. These are called P-waves (or primary waves or compressional waves) and S-waves (or secondary waves or shear waves). The P-waves are longitudinal and the S-waves are shear waves. P-waves travel faster than S-waves so reach seismic stations (data recording centres) first. Both P and S waves bounce around the Earth several times, and depending on how many bounces they get called different things. For example, from the earthquake source to seismic station directly is a P wave, but if it bounces off the surface, goes back into the earth, comes back up and then reaches a seismic station, it’s called a PP wave, two bounces and it’s a PPP wave etc., and the same for S waves.