Section 1: How and Where EarthquakesHappen
Key Ideas
•Describe elastic rebound.
•Compare body waves and surface waves.
•Explain how the structure of Earth’s interior affectsseismic waves.
•Explain why earthquakes generally occur at plateboundaries.
Why Earthquakes Happen
•Earthquake - a movement or trembling ofthe ground that is caused by a suddenrelease of energy when rocks along a faultmove
•Earthquakes occur when rocks understress suddenly shift along a fault.
•A fault is a break in a body of rock alongwhich one block moves relative to another.
•The rocks along both sides of a fault arecommonly pressed together tightly.Although the rocks may be under stress,friction prevents them from moving pasteach other. In this state, a fault is said to belocked.
Why Earthquakes Happen, continued
•Parts of a fault remain locked until the stressbecomes so great that the rocks suddenlyslip past each other. This slippage causes thetrembling and vibrations of an earthquake.
Elastic Rebound
•When the rock fractures, it separates at theweakest point and snaps back, or rebounds,to its original shape, which causes anearthquake.
•Elastic rebound - the sudden return ofelastically deformed rock to its undeformedshape
Why Earthquakes Happen, continued
Anatomy of an Earthquake
•Focus - the location within Earth along a fault atwhich the first motion of an earthquake occurs
•Epicenter - the point on Earth’s surface abovean earthquake’s starting point, or focus
•Although the focus depths of earthquakes vary,about 90% of continental earthquakes have shallowfoci.
•Earthquakes that have deep foci usually occur insubduction zones.
•Earthquakes that cause the most damage usuallyhave shallow foci.
Why Earthquakes Happen, continued
The diagram below shows the parts of an earthquake.
Why Earthquakes Happen, continued
Seismic Waves
•As rocks along a fault slip into new positions, the rocksrelease energy in the form of vibrations called seismicwaves.
•Seismic waves travel outward in all directions from thefocus through the surrounding rock.
•Body wave - a seismic wave that travels through thebody of a medium.
•Surface wave - a seismic wave that travels along thesurface of a medium and that has a stronger effect nearthe surface of the medium than it has in the interior
Why Earthquakes Happen, continued
•Each type of wave travels at a different speed andcauses different movements in Earth’s crust.
Body Waves
•P waves and S waves are two types of body waves.
•P wave - a primary wave, or compression wave; aseismic wave that causes particles of rock to move in aback-and-forth direction parallel to the direction in whichthe wave is traveling.
Why Earthquakes Happen, continued
Body Waves, continued
•P waves are the fastest seismic waves and can travelthrough solids, liquids, and gases.
•The more rigid the material is, the faster the P wavetravels through it.
•S wave - a secondary wave, or shear wave; a seismicwave that causes particles of rock to move in a side-to-side direction perpendicular to the direction in which thewave is traveling
•S waves are the second-fastest seismic waves and canonly travel through solids.
Why Earthquakes Happen, continued
Surface Waves
•Surface waves form from motion along a shallow fault orfrom the conversion of energy when P waves or S wavesreach Earth’s surface.
•Although surface waves are the slowest-moving seismicwaves, they can cause the greatest damage during anearthquake.
•Love waves are surface waves that cause rock to moveside-to-side and perpendicular to the direction in whichthe waves are traveling.
•Rayleigh waves are surface waves that cause theground to move with an elliptical, rolling motion.
Seismic Waves and Earth’s Interior
•By studying the speed and direction of seismicwaves, scientists can learn more about themakeup and structure of Earth’s interior.
Earth’s Internal Layers
•In 1909, Andrija Mohorovičić discovered that thespeed of seismic waves increases abruptly atabout 30 km beneath the surface of continents,where the crust and mantle meet.
•By studying seismic waves, scientists havediscovered Earth’s three compositional layers(the crust, the mantle, and the core) and Earth’sfive mechanical layers (the lithosphere, theasthenosphere, the mesosphere, the outer core,and the inner core).
Shadow Zones
•Shadow zone - an area on Earth’s surfacewhere no direct seismic waves from a particularearthquake can be detected.
•Shadow zones exist because the materials thatmake up Earth’s interior are not uniform inrigidity.
•When seismic waves travel through materials ofdifferent rigidity, they change in both speed anddirection.
•S waves do not reach the S wave shadow zonebecause cannot pass through the liquid outercore.
•P waves do not reach the P wave shadow zonebecause of the way the P waves bend and theytravel through Earth’s interior.
Seismic Waves and Earth’s Interior
Seismic Waves and Earth’s Interior,continued
The diagram below shows how seismic waves interact withEarth’s interior.
Earthquakes and Plate Tectonics
•Earthquakes are the result of stresses in Earth’slithosphere.
•Most earthquakes occur at or near tectonic plateboundaries, where stress on the rock is greatest.
•The three main types of tectonic settings are
–convergent oceanic environments
–divergent oceanic environments
–continental environments
Earthquakes and Plate Tectonics
Convergent Oceanic Environments
•At convergent plate boundaries,plates move toward each other andcollide.
•The denser plate moves down, orsubducts, into the asthenosphereunder the other plate, causingearthquakes.
•Convergent oceanic boundariescan occur between two oceanicplates or between one oceanicplate and one continental plate.
Divergent Oceanic Environments
•At the divergent plate boundaries that makeup the mid-ocean ridges, plates are movingaway from each other.
•Earthquakes occur along mid-ocean ridgesbecause oceanic lithosphere is pulling awayfrom both sides of the ridge.
Continental Environments
•Earthquakes also occur at locations wheretwo continental plates converge, diverge, ormove horizontally in opposite directions.
•As the continental plates interact, the rocksurrounding the boundary experiencesstress, which causes earthquakes.
Earthquakes and Plate Tectonics
The diagram below shows the different tectonic boundarieswhere earthquakes occur.
Earthquakes and Plate Tectonics,continued
Fault Zones
•Fault zone - a region of numerous,closely spaced faults.
•Fault zones form at plate boundariesbecause of the intense stress thatresults when plates separate,collide, subduct, or slide past eachother.
•When enough stress builds up,movement occurs along one ormore of the individual faults in thefault zone and sometimes causesmajor earthquakes.
The Japanese city of Kobe is rocked by the Great HanshinEarthquake which destroys most of its buildings
Fault Zones, continued
•Not all earthquakes result frommovement along plate boundaries.
•In 1811 and 1812 the most widelyfelt series of earthquakes in UnitedStates history occurred in themiddle of the continent near NewMadrid, Missouri.
•In the late 1970s scientistsdiscovered an ancient fault zonedeep within the crust of theMississippi River region.
Earthquakes Away from Plate Boundaries
