Courtesy of USGS
Seismic Waves: Moving and Shaking During an Earthquake
During an earthquake, energy is released in waves that travel from the earthquake's focus or point of origin, in the form of seismic waves. The seismic waves radiate from the focus like ripples on the surface of a pond moving outward from where a stone is thrown into the water. Seismic waves cause intense shaking at the Earth surface that can cause buildings and roads to collapse.
Here's a description of what the shaking from seismic waves felt like during the 1906 earthquake that shook San Francisco, CA according to Police Lieutenant H.N. Powell, who was there at the time:
"Valencia Street not only began to dance and rear and roll in waves like a rough sea in a squall; but it sank in places and then vomited up its car tracks and the tunnels that carried the cables. These lifted themselves out of the pavement, then bent and snapped. It was impossible for a man to stand, or to realize just where he was trying to keep standing. Houses were cracking and bending and breaking the same as the street itself and the car tracks."
Not all seismic waves as the same! There are several different types. Each type of seismic wave has a unique way of moving. Even through there are several different types of seismic waves, accounts of what it was like to be in an earthquake, such as Powell's account, typically describe surface waves since those are most noticeable at the Earth's surface.
Surface waves travel over the Earth's surface.
- Rayleigh waves, also called ground roll, travel like ocean waves over the surface of the Earth, moving the ground surface up and down. Most of the shaking at the ground surface during an earthquake is typically caused by Rayleigh waves.
- Love waves move the ground from side to side. They are the fastest of the surface waves. (They are named after a mathematician named Love studied them.)
Body waves travel through the Earth.
- P-waves: These are the fastest type of seismic wave. As P-waves travel, the surrounding rock is repeatedly compressed and then stretched. P-waves can travel through both the solid rock and molten liquid layers of the Earth.
- S-waves: These seismic waves arrive after the P-waves because they travel more slowly. The rock is shifted up and down or side to side as it travels foreword. S-waves can only travel through solid rock.
