4.2Earthquakes & Seismic Waves

• earthquakes - movements or shaking of the ground when rock (plates) move suddenly and release energy.

• aftershock – a release in energy after an earthquake

Where do most earthquakes happen?On plate boundaries

What causes earthquakes?• Movement of the tectonic plates

• When the plates break at a fault located at a boundary

• An earthquake releases an enormous amount of stored energy

• The energy travels in the form of seismic waves

• Seismic waves – vibrations that are similar to sound waves. They travel through Earth carrying energy released by an earthquake.

• Seismic waves vary in speed and direction depending on the material it goes through

• focus – the point inside the Earth where an earthquake begins

• epicenter – the point on the Earth’s surface directly above the earthquake’s starting point

• seismic waves – waves of energy that travel through and on the Earth

• 2 types of seismic body waves:1. P-waves (primary waves) – the fastest seismic waves that

travel through solids, liquids, and gases• (the P-waves move like a slinky)

2. S-waves (secondary waves) – the second fastest seismic waves that travel through solids• (the S-waves move like a snake)

• 1 type of seismic surface waves :3. L-waves (land & last waves) – the slowest and most

destructive seismic waves that travel only on the surface/land• (the L-waves move in big slow circles)

• Seismograph/Seismometer – an instrument at or near the surface of the Earth that records seismic waves (machine)

• seismogram – is a tracing of earthquake motion created by a seismograph (paper)

• The Modified Mercalli scale measures the amount of shaking from an earthquake based on people’s observations (no machine used).

• The Richter scale measures the magnitude (size) of an earthquake.• magnitude – a number based on the earthquakes

size (strength) – the seismic waves are measured by a seismograph

• The moment magnitude scale rates the total energy an earthquake releases using seismographs.

How do you find an epicenter?1. You need to find the S-P time difference and that

is the distance from the seismograph to the epicenter.

• P-wave time minus the S-wave time

2. Draw a circle around the seismograph location (radius)

3. Do this three times because you need three seismograph stations to find an epicenter.

4. Where all three circles intersect is where the epicenter is located.

To find an epicenter you

need three seismograph

stations. Where the three radi

intersect is where the epicenter is

located.