Types of Seismic Waves An earthquake is the shaking that results when rocks move inside Earth. An earthquake is caused by stress along a fault. Stress increases until the rocks break and release the stored energy. Earthquakes happen all the time, every day. Most are too weak to be felt, and some can cause major damage.
Google Earth has the ability to show - in real time - earthquakes from the last 7 days. http://earthquake.usgs.gov/learn/kml.php
Types of Seismic Waves The place where rocks break and cause an earthquake is called the focus. The point directly above the focus, on the surface, is known as the epicenter. Most earthquakes begin in the lithosphere within 100 kilometers of Earth’s surface.
Types of Seismic Waves Earthquakes cause waves, called seismic waves, to travel through Earth. Seismic waves carry the energy released by the rocks. There are three kinds of seismic waves: P waves, S waves, and surface waves.
Types of Seismic Waves P waves (or Primary Waves) move rocks back and forth, like a wave passing through a spring toy when you push the coils. This is known as a compression or longitudinal wave. P waves are the fastest waves and arrive first to a seismic station. P waves are able to travel through all forms of matter.
Types of Seismic Waves S waves (or Secondary Waves) move rocks up and down, like a wave in a spring toy when you flick it. This is known as a transverse wave. S waves move slower than P waves and therefore will arrive second at a seismic station. S waves cannot travel through liquids.
Types of Seismic Waves Surface waves are the mechanical waves that physically move Earth’s surface. They can be a combination of both Love waves and Rayleigh waves. Surface waves travel the slowest and cause the most structural damage of all seismic waves.
Measuring Earthquakes The Richter scale is based on the size of the seismic waves. A stronger earthquake makes bigger seismic waves. An instrument called a seismograph measures the size of seismic waves. Each number on the Richter scale is 10- times greater than the previous number. For example a 6.0 is ten times more powerful than a 5.0. A 7.0 is one hundred times greater than a 5.0!
Measuring Earthquakes The moment magnitude scale is based on the amount of energy an earthquake releases. The amount of energy is based on many things, including the size of the seismic waves. This is usually the number that news stories quote in their reports. Every number increase in magnitude is usually a 32x increase in energy release. Therefore 32 times more energy is released in a 5.0 earthquake than a 4.0. And about 1000 times more energy than a 3.0.
Locating the Epicenter The epicenter is the point on the surface that lies directly above an earthquake’s focus. Scientists use P waves and S waves to find an earthquakes epicenter. P waves travel faster than S waves. So P waves arrive at a seismograph sooner than S waves. The longer it takes S waves to reach the seismograph after P waves have arrived, the farther away the epicenter is.
Locating the Epicenter Imagine a race track on which two cars are racing. One car’s top speed is 80 mph. The other car’s top speed is 60 mph. The longer they race, the farther behind the second car will be. In 15 minutes, the 2nd car would be 5 miles behind the first. In 30 minutes, the 2nd car would be 10 miles behind, and in one hour it’d be 20 miles behind.
1.5 minutes = 900 km 3 minutes = 1800 km 5 minutes = 3300 km
Locating the Epicenter To find the exact location of the epicenter, you need seismographs in three different locations. You can draw a circle around each station to show how far the epicenter is from that seismograph. The point where all three circles cross is the epicenter. This process is known as triangulation.