Seismic Waves: Moving and Shaking During an Earthquake
During an earthquake, energy is released in seismic waves that travel from where the earthquake starts, a place called the focus. The seismic waves radiate from the focus. Seismic waves cause intense shaking at the Earth surface that can cause buildings and roads to collapse.
Police Lieutenant H.N. Powell described his experience witnessing the impact of the seismic waves of the 1906 San Francisco, CA earthquake like this:
"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 and each type has a unique way of moving. Accounts of what it was like to be in an earthquake usually 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. They cause most of the shaking at the ground surface during an earthquake.
- Love waves are fast and move the ground from side to side.
Body waves travel through the Earth.
- P-waves are the fastest type of seismic wave. As P-waves travel, the surrounding rock is repeatedly compressed and then stretched.
- S-waves arrive after P-waves because they travel more slowly. The rock is shifted up and down or side to side as the wave travels through it.