1. Chapter 7 Lesson 2 - Earthquakes

2. How Do Earthquakes Happen?
An earthquake is defined as a sudden trembling in the ground caused by something happening in the crust.
Most happen at the edges of plates, or plate boundaries. Very few happen in the center of plates.
About 80% happen along the edges of the Pacific plate.
Caused by plates colliding, sliding, or pulling apart. The rocks bend and stretch.

3. When plates move… Great forces are exerted on the rocks.
First, the rocks bend and stretch.
Once they reach their limit, they break.
Faults form at or below the surface of the crust.

4. Faults When rocks in the crust reach their breaking limit and crack.
San Andres Fault

5. Earthquakes in NJ
Why do most earthquakes occur in the mid-northern section of the state?

6. Normal Fault Happens at divergent boundaries Where plates pull apart
Rocks above the fault surface move down.
Ex: Sierra Nevada in CA

7. Reverse Fault Happens at convergent boundaries
Where plates push together
Rocks above the fault move upward
Ex: Himalayas in India

8. Strike-slip Fault Happens at transform boundaries
Plates slide past each other without moving up or down.
Where rocks slide past each other in different directions.
Ex: San Andreas Fault in CA

9. How Do Earthquakes Make Waves?
An earthquake starts the moment rocks begin to scrape past each other along a fault.
It may be a new fault that forms at that moment or an old fault that has already been formed.

10. Focus
The point where the earthquake starts, where rocks begin to slide past each other
It is usually below the surface
The sudden motion causes vibrations to spread out from the focus
These vibrations travel through the crust in the form of waves

11. Seismic Waves The vibrations travel through the crust.
They reach Earth’s surface at a point directly above the focus called the epicenter.

12. Epicenter
Definition: The point on Earth’s surface directly above the focus of an earthquake
People can first feel the ground shaking.
This shaking is what causes most earthquake damage because it is so close to the focus.
As waves travel away from the focus they get weaker.

13. Aftershocks
After the first shaking, it may be quiet, and then aftershocks occur.
Aftershocks continue the damage of an earthquake.
Additional shaking and damage after the first shaking and relative quiet.

14. Earthquake Waves
Vibrations that are given off by an earthquake and travel through the crust are called seismic waves.
A seismic wave starts with shaking caused by rocks scraping against each other.
This shaking results in several kinds of seismic waves.
The waves travel at different speeds.
Three kinds:
Primary Waves – P waves
Secondary Waves – S waves
Surface Waves

15. Primary Waves – P waves
Accordion-like motion where rock squeezes together and spreads apart repeatedly.
Produces seismic waves that move in the same direction that the rock is shaking.
The fastest seismic wave
The first to reach any faraway location

16. Secondary Waves – S Waves
Shaking like a ruler held off the edge of a desk and “twanged”
Produces seismic waves that move in a different direction from the vibration
Travel slower than primary waves
They are the second to arrive at any given faraway location.

17. Surface Waves
Causes Earth’s surface to heave up and down like an ocean wave, or sway from side to side.
Tears apart structures built on the surface as the land stretches and compresses.

18. When a 7.0-magnitude earthquake rocked the south island of New Zealand one train track was permanently altered in what looks more like a Photoshopped image or a cartoon gag than an actual photograph.

19. Seismograph Definition: A sensitive device
that detects the shaking of the
Earth’s crust during an earthquake.
Used by scientists to study earthquake waves
Shows patterns in the waves.
Can identify the P waves, S waves, and surface waves from studying the readings of the seismograph.

20. Seismograph

21. Ocean Waves
When an earthquake’s focus is beneath the ocean floor, the seismic waves can travel through the ocean.
They can produce huge ocean waves called tsunamis

22. Tsunami Formation http://www.youtube.com/watch?v=aHljDIDf6js
When tsunamis reach a shoreline, the top of the wave travels faster than the bottom and they can reach heights of more than 50 ft.
They destroy everything in their path.
Can reach speeds up to 560 mph.

23. Richter and Mercalli Scales
Pg. D25 in textbook

24. Latest Earthquakes Latest Earthquakes in the World - Past 7 days
Earthquakes around the world