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Chapter 3

Planet Earth - Chapter 3 Textbook Notes

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Environmental Science
Nick Eyles

Chapter 3EarthquakesHistorical EarthquakesCharlesvoixKamouraska QuebecFebruary 1663Some of the earliest to be documented in North AmericaNames the CharlesvoixKamouraska Seismic Zone because of repeated earthquakesNo deaths in 1663 but was very violentcaused landslidesEastern CanadaNovember 1929Seafloor beneath Atlantic Ocean movedEarthquake triggered submarine landslideallowed scientists to record speed of movement of current for first timeTsunami resulted from quake in Newfounland28 people diedLargest earthquake in eastern Canada hit Saguenay region of Quebec in 1988San Francisco CaliforniaApril 18 1906Great earthquakeCalifornia slid abruptly past the rest of the stateVisible scar 450 km where Earth was torn openOriginated on San Andreas fault near San FranciscoGround shook for one minutedamage broken gas mainsfire3000 people died and 400 mill spent90 of destruction caused by firesAGAIN San Francisco 1989Loma Prieta earthquake on the San Andreas fault near Santa CruzGround was NOT torn but buildings collapsedSection of Bay Bridge collapsedFires by broken gas mains again hard to extinguish since water mains were broken tooSevere damage in small towns NEAR THE CENTRE of the quakeDeath toll63 Damage6 billionAlaska 1964Earthquake for 3 minutesForce was twice as strong as 1906 San Francisco but loss of life and property was low because of SMALL populationOnly 15 died due to shaking and damage cost 300 millionGreatest loss of life due to tsunami generated by earthquakealmost 100 drownedNorthridge Earthquake in San Fernando Valley Los Angeles California40 secondsBuildingschools damaged16 killedAlaska 2002Greatest earthquake ever recorded in the interior of AlaskaRuptured the ground surfaceTriggered thousands of landslidesNo deaths and minimal damage since it occured in a remote areaPipeline did not break due to good engineering designDecember 26 2004Earthquake93 magnitudend2 largest recorded since 1900Deformed Indian Ocean floorSo severe that it slightly changed the shape of the planet reducing the length of day by 3 microsecondsThe quake epicentre was located to the east of the Sunda Trench where the India plate is subducted below the Burma plate and was caused by thrust faulting along the boundary between the two platesThis type of megathrust earthquake is also possible along the western coast of Canada and US Pacific Northwest where the Juan de Fuca plate is being subducted beneath the North American plateWhat Causes EarthquakesAn earthquake is a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath Earths surfaceGreat forces acting deep in the Earth may put a stress on the rock which may bend or change in shape strainStrainbeding of the rockchange in shapeBending stick analogyRock can deform only so far before it breaksSeismic Waves the waves of energy that are released when rock breaks or in other words the waves of energy produced by an earthquakeEslastic Rebound TheoryClassic explanation of why earthquakes take placeSudden release of progressively stored elastic strain energy in rocks causing movement along a faultDeepseated internal forces tectonic forces act on a mass of rock over many decadesFirst rock bends or stretched but not stretchesMore energy is built upeventually rock breaks causing an earthquakeEnergy is expended by moving the rock into new positions and by creating seismic wavesTwo masses of rock move past one another along a faultDiagram p 772 Models for Fault BehaviourSome say faults are strong to begin with other says its weak so a small amount of stress is enough to trigger movement along the fault and subsequent earthquake Ductile Behaviour vs Brittle BehaviourBreakage of rocks is usually near the Earths surface since theyre brittle and the rocks that are deeper are more flexible ductile they dont breakAlthough most earthquakes are associated witrh movement on faults some are not Some are associated with buried thrust faults and even volcanic eruptionsMost earthquakes in eastern North America are not associated with surface displacementReasons for deep earthquakesAll deep earthquakes are found on cold subducting plates sliding down into the mantleSince then theres high temperature and pressure at depth rocks in plates SHOULD behave plastically rather than breakingSo whats the causeMaybe mineral transformation within the downgoing rock as pressure collapses one mineral into a denser formWhy Do Earthquakes Cause So Much DamageFocus hypocentrePoint within the Earth where seismic waves first originateThis is the point of initial breakage and movement on a faultRupture begins at the focus and then spreads along the fault planeEpicentrePoint on the Earths surface directly above the focus is the epicentre
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