Chapter 7 Lesson 2 - Earthquakes

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.

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.

Faults• When rocks in the crust reach their breaking

limit and crack.

San Andres Fault

Earthquakes in NJWhy do most earthquakes occur in the mid-northern section of the state?

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

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

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

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.

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

Seismic Waves• The vibrations travel through the crust. • They reach Earth’s surface at a point directly

above the focus called the epicenter.

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.

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.

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

Primary Waves – P waves• http

://www.youtube.com/watch?v=2rYjlVPU9U4&feature=player_embedded

• 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

Secondary Waves – S Waves• http://

www.youtube.com/watch?v=en4HptC0mQ4&feature=player_embedded

• 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.

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.

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.

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.

Seismograph

• http://www.youtube.com/watch?v=83GOKn7kWXM&feature=player_embedded

• http://www.youtube.com/watch?feature=player_embedded&v=DX5VXGmdnAg

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

Tsunami Formationhttp://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.

Richter and Mercalli Scales

• Pg. D25 in textbook

• http://www.iknowthat.com/mhscience/Earthquakes/earthquake_movie.html

Latest Earthquakes

• Latest Earthquakes in the World - Past 7 days• http://earthquake.usgs.gov/earthquakes/rece

nteqsww/

• Earthquakes around the world • http://www.guardian.co.uk/news/datablog/in

teractive/2011/feb/22/earthquake-map-world