5-2: Earthquakes and Seismic Waves

6th Grade Earth Science

5-2: Earthquakes and Seismic Waves

What is an Earthquake?

• An earthquake is ground shaking from the sudden movement of rock along a fault.

5-2: Earthquakes and Seismic Waves

Why are there Earthquakes?

• Earthquakes happen because of the movement of plates and the stress it causes, which builds up in the rock till it breaks.

5-2: Earthquakes and Seismic Waves

Epicenter and Focus

5-2: Earthquakes and Seismic Waves

Epicenter and Focus

• An earthquakes focus is the point under the earth where the fault begins to break.

• The epicenter is the spot on the earth’s surface directly above the focus.

5-2: Earthquakes and Seismic Waves

Epicenter and Focus

• An earthquakes focus is the point under the earth where the fault begins to break.

• The epicenter is the spot on the earth’s surface directly above the focus.

5-2: Earthquakes and Seismic Waves

Seismic Waves

• Seismic waves carry the energy released in an earthquake, far from the epicenter.

• There are three types of seismic waves, P-Waves, S-Waves, and surface waves.

5-2: Earthquakes and Seismic Waves

• P-Waves (primary waves) compress and expand the ground like an accordion – they are the first to arrive. They are able to go through liquid.

P-Waves

5-2: Earthquakes and Seismic Waves

• S-Waves (secondary waves) are a side to side or up and down motion and they arrive after the P-Waves. They are NOT able to go through liquid.

S-Waves

5-2: Earthquakes and Seismic Waves

• Surface waves form when the P and S waves reach earth’s surface.

• They go the slowest and sometimes look like ocean waves going across the land.

• They cause the most damage.

Surface Waves

5-2: Earthquakes and Seismic Waves

• Surface waves form when the P and S waves reach earth’s surface.

• They go the slowest and sometimes look like ocean waves going across the land.

• They cause the most damage.

Surface Waves

5-2: Earthquakes and Seismic Waves

• Scientists measure earthquakes in three ways. 1. Mercalli Scale. This measures the intensity or the

amount of damage at one particular location.

2. Richter Scale. This measures the size of the quake, or how much of the rock broke. Works best on small quakes that are close by.

3. Moment Magnitude Scale. This measures the amount of energy released by a quake. Works on all quakes and is the method used today when reporting earthquakes … even though the still say Richter Scale!!!

Measuring Earthquakes

5-2: Earthquakes and Seismic Waves

Mercalli Scale

5-2: Earthquakes and Seismic Waves

Richter Scale

• Goes from 0 to about 9.

5-2: Earthquakes and Seismic Waves

Moment Magnitude Scale

• Goes from 0 to about 10.

5-2: Earthquakes and Seismic Waves

• Magnitude scales range from 0 to about 9 or 10

• An increase of 1 means the ground motion goes up by 10 times and the energy goes up by 30 times.

Magnitude Scales

x (5.0)

xxxxxxxxxx (6.0)

xxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx (7.0)

5-2: Earthquakes and Seismic Waves

• One seismogram can be used to determine there was an earthquake and how far away it was.

• Three seismograms can be used to locate the epicenter

Locating the Epicenter

5-2: Earthquakes and Seismic Waves

• A circle is drawn around the location of each seismogram.

• The circle size is the distance to the quake.

• Where the three intersect is where the epicenter was.

5-2: Earthquakes and Seismic Waves

• Since P-Waves go faster than S-Waves, the difference in time between their arrivals tells you the distance from the quake.

How do I know the distance to the quake?

5-2: Earthquakes and Seismic Waves

• A table is used to convert the difference in time to a distance from the epicenter.

5-2: Earthquakes and Seismic Waves Summary

1. When stress builds up in rock, the rock breaks causing an earthquake.

2. Three types of waves carry the energy, P, S, and Surface waves.

3. Three scales are used to measure quakes.

4. Seismograms record ground motion and are used to locate quakes.