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 NJ Why 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 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.

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/recenteqsww/ Earthquakes around the world http://www.guardian.co.uk/news/datablog/interactive/2011/feb/22/earthquake-map-world