1. Earthquakes
A sudden movement of the Earth’s crust that causes vibrations of surrounding rock called seismic waves.
Intro to Earthquakes

2. Where Do Earthquakes usually occur?
Along tectonic plate boundaries (usually convergent or transform boundaries)
Along Faults (large cracks in the Earth’s crust)
Around volcanoes due to the movement of magma below the Earth’s surface.
USGS Earthquake map

3. Earthquakes and Plate Tectonics
Earthquakes typically occur along two tectonic plate boundaries.
Tectonic Plate Motion causes plates to move, but some rock will get “hung up” and plates cannot slide.
When stress becomes too high, the rock slips and an earthquake occurs.

4. Elastic Rebound

5. Anatomy of an Earthquakes
The area under the Earth’s surface where and earthquake occurs is known as the focus.
The area on the surface directly above the focus is known as the epicenter.

6. Seismic Waves
P-Waves – Also know as compression waves, these move forwards and backwards.
S-Waves – Also known as secondary waves (or shear waves).
Surface Waves – When P & S waves come together at Earth’ surface, causes violent shaking.
Earthquake wave rap

7. Seismic Waves
Earthquake Damage Bay Bridge San Fransisco

8. Review Questions: Describe the 3 places Earthquakes occur.
Explain how an Earthquake happens.
What is the difference between the focus and epicenter of an earthquake?
List the 3 types of seismic waves from earthquake. Explain how each wave moves through the Earth’s crust.

9. Locating an Earthquake
Because P & S waves travel at different speeds, the difference in arrival times can be used to identify the epicenter of an earthquake.
A seismogram, or seismic wave record, can determine the difference between the two waves based on the amplitude (or wave height).
S – Waves Arrive
P – Waves Arrive

10. Earthquake Triangulation
The difference in arrival times of both P and S Waves at a location can be used to determine how far away an earthquake’s epicenter is.
Three locations must be used to determine how far away the event is. This is called “triangulation”.

11. Earthquake Triangulation
Finding Earthquakes - Youtube

12. Seismic Waves do not Travel Through all Types of Material
P – Waves are the only seismic wave that can travel through liquids. This means that P – waves can travel through the Earth’s molten core.
S – Waves cannot travel through liquids. Therefore, the side of the Earth opposite an earthquake epicenter will be in an S-Wave shadow zone.

13. Seismic Waves in the Earth’s Interior