Seismic Waves

P waves and S waves

Learning from Earthquakes

When an earthquake lets loose, it sends ripples of energy through Earth called seismic waves. There are two main types of seismic waves that travel from the site of an earthquake: P waves (for “pressure”) and “S” waves (for “secondary” or “surface”).

P waves are faster and therefore arrive first. They are compression waves, moving the ground back and forth sort of like a Slinky. S waves are slower, but can cause much more destruction. They cause the ground to ripple up and down, like shaking a rug. The closer you are to an earthquake, the closer together the P and S waves arrive.

To study seismic waves, seismologists use sensitive instruments called seismometers. By analyzing the way these waves travel through and across Earth, how fast they arrive, and how close together, seismologists learn a great deal. They can determine the earthquake’s magnitude, and pinpoint the epicenter, or exact location, of the earthquake on Earth’s surface. They can locate the place deep underground where the fault that caused the earthquake slipped, the hypocenter. They also use seismic waves to explore Earth’s interior layers.

How to measure an earthquake

People Power

Directly or indirectly, tectonic plate motion causes most of Earth’s earthquakes. But human activity can also trigger small, shallow earthquakes. For example, water filling a reservoir or injecting water into underground rocks can cause small earthquakes. Big explosions also register on seismometers. Seismologists use waves generated from these human activities the same way they study waves from natural earthquakes.

Sometimes, seismic waves come from unexpected sources. On January 8, 2011 Seattle Seahawks football player Marshawn Lynch, nicknamed Beast Mode, made an incredible touchdown run. The crowd of over 60,000 spectators went wild. Seismometers nearby registered their cheering and stomping as the same intensity as an M2 earthquake! The crowd-created “earthquake” was called Beast Quake in honor of Lynch.

Studying Beast Quake and other shaky events has helped seismologists develop an earthquake early warning system. This enables them to detect an earthquake just seconds ahead, and send out an alert by text and email. Seconds may not sound like much time. But it’s enough to give people time to duck into doorways or under desks for safety.

Written by Laura McCamy

Edited by Beth Geiger, MS Geology

Illustrated by Renee Barthelemy