# Chapter 7 Lesson 2 Earthquakes

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Chapter 7 Lesson 2 Earthquakes

What is an Earthquake? A sudden trembling of the ground caused by movement happening in the crust

Where Do Earthquakes Happen?
Most earthquakes happen at the edges of plates, where plates meet About 80 percent of all earthquakes happen along the edge of the Pacific Plate Along the edges, plates can collide, they may pull apart, or they may slide past each other These motions can cause the rocks to bend and stretch until they break As a result, faults form at or below the surface of the crust

Tectonic Plates

Earthquake Locations

What is a Fault? Fault- huge cracks in the Earth’s crust

Three Kinds of Faults 1. Normal Fault Produced at divergent boundaries
Rocks above the fault surface move down

Three Kinds of Faults 2. Reverse Fault Produced at convergent
boundaries Rocks above the fault move upward

Three Kinds of Faults 3. Strike Slip Fault Produced at transform
boundaries Rocks slide past each other in different directions

How Earthquakes Start Earthquakes start the moment rocks begin to scrape past each other along a fault.

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

How Earthquakes Start Seismic Waves
A vibration that spreads out away from a focus when an earthquake starts They soon reach the Earth’s surface at a point directly above the focus

How Earthquakes Start Epicenter
The point on the Earth’s surface directly above the focus of an earthquake People can first feel the ground shaking at the epicenter

How Earthquakes Start Epicenter This shaking is what causes most
earthquake damage Damage is usually greatest at the epicenter because it is so close to the focus As waves travel away from the focus they get weaker

Aftershocks After the first shaking, it may be quiet, and then there are aftershocks This is the shaking of the Earth’s crust after the initial shaking of an earthquake

Earthquake Waves There are different kinds of seismic waves which travel differently and at different speeds.

Earthquake Waves Primary Waves, or P Waves
Accordion-like motion shaking Rock material squeezes together and spreads apart repeatedly This motion produces seismic waves that move in the same direction that the rock is shaking They are the fastest seismic wave- the first to reach any faraway location

Earthquake Waves Secondary Waves, or S waves
Shaking like a ruler held off the edge of a desk and “twanged” This motion in rocks creates seismic waves that move in a perpendicular direction from the vibration These waves travel slower than primary waves They are second to arrive at any given faraway location.

Earthquake Waves Surface Waves
This shaking causes Earths surface to heave up and down like an ocean wave, or sway from side to side Tears apart structures built on the surface

Earthquake Waves Seismographs
Scientists study and identify waves using a seismograph A sensitive device that detects the shaking of the Earth’s crust during an earthquake

Seismograph

Seismogram

Tsunamis If the focus of the earthquake is beneath the sea floor, the seismic waves can travel through the ocean, producing huge ocean waves called tsunamis

See Mrs. Balducci’s Website….
…..For more information on earthquakes such as earthquake current events and earthquake simulations. Look under Chapter 7 Restless Earth LINKS