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EESA06H3 (234)
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Chapter 3

Chapter 3 notes

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University of Toronto Scarborough
Environmental Science
Nick Eyles

Chapter 3 Earthquakes What causes earthquakes? • An earthquake is a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath Earth’s surface. • Great forces acting deep in the earth may put a stress on the rock, which may bend or change in shape (strain) – the rock can only deform so far before it breaks, waves of energy are released and send out through the Earth • Seismic waves = waves of energy produced by an earthquake. It is the seismic waves that cause the ground to tremble and shake during an earthquake • Fault = the break between the 2 rock masses as they move past each other • Classic explanation of why earthquakes take place is called the elastic rebound theory = sudden release of progressively stored elastic strain energy in rocks, causing movement along a fault. Eventually the stored energy in the rock exceeds the breaking strength, rock breaks causing and earthquake • Brittle behaviour of breaking rock is only in rock near Earth’s surface. Rocks at depth are subjective to increase temperature and pressure = less brittle. Why do earthquakes cause so much damage? • The point within the earth where seismic waves first originate is called the focus (or hypocentre) of the earthquakes – this is the centre of the earthquake, the point of initial breakage and movement on a fault • The point on the Earth’s surface directly above the focus is the epicenter • 2 types of seismic waves are generated during earthquakes: o body waves – are seismic waves that ravel through the Earth’s interior, spreading outward from the focus in all directions o surface waves – seismic waves that travel on Earth’s surface away from the epicenter (like water moving when throwing pebble) FIG 3.5 page 73 – diagram that explains the waves Body Waves • 2 types of body waves: o P wave – a compressional or longitudinal wave in which rock vibrates back and forth parallel to the direction of wave propagation Very fast wave, 4-7kn per second First to arrive at a recoding station following an earthquake o S wave – secondary wave, slower transverse wave that ravels through near-surface rocks at 2-5km per second Characterized by shearing motion (i.e. shaken rope), the rock vibrates perpendicular to the direction of wave propagation • Both waves pass easily through solid rock. P wave can also pass through fluid (gas or liquid) but S wave can’t Surface Waves • Slowest waves set off by earthquakes, but cause more property damage than body waves because they produce more ground movement and takes longer to pass • 2 types: o Love waves – similar to S waves that have no vertical displacement. The ground moves side to side in a horizontal plate that is perpendicular to the direction of the wave travelling or propagating Do not travel through fluid Due to horizontal movement, love waves tend to knock buildings off their foundation and destroy highway bridge support www.notesolution.com o Rayleigh waves – behave like rolling ocean waves, it causes the ground to move in a elliptical path opposite to the direction the wave passes Incredibly destructive to buildings because they produce more ground movement and takes longer to pass How do we know where earthquakes occur? • Instrument we use to measure seismic waves is a seismometer o The principle of the seismometer is to keep a heavy suspended mass as motionless as possible – suspending it by springs or hanging it as a pendulum from the frame of the instrument o When the ground moves, the frame of the instrument moves with it, heavy mass suspended act as a point of reference in determining ground motion o Measure axis, X, Y and Z (3Dimensions) • A seismogram is a recording device that produces a permanent record of Earth motion detected by a seismometer, in digital format • Because of different types of waves that travel at different speeds, they arrive in seismograph stations in a definite order: o First P waves o S-waves o Finally surface waves • By analyzing seismograms, can learn about location, and size Determining the Location of an Earthquake • P and S waves start from the focus of an earthquake at essentially the same time. As they travel away from the quake, the 2 kinds of body waves gradually separate because they travel in different speeds • The farther the seismic waves travel (from focus to seismograph station) the longer the time interval between the arrival of P and S waves and more they are separated on the seismograms o This time interval can be used to determine the distance form the seismograph station to a quake o The increase in P-S interval is regular with increasing distance fro several thousands km and can be graphed in a travel-time curve = plots seismic wave arrival time against distance • Analyses of seismogram can also indicate at what depth beneath the surface the quake occurred, most quakes occur near the surface o The maximum depth of focus – the distance between focus and epicenter for quakes is about 670km o Quakes can be classified into 3 group according to their depth of focus: Shallow focus (0-70km deep) = most common 85% of total quake energy release Intermediate focus (70-350km deep) = 12% Deep focus (350-670km deep) = 3% Measuring the size of an earthquake • Size of earthquake can be measured in 2 ways: o Intensity – find out how much and what kind of damage the quake has causes. Measure of an earthquake’s effect on people and buildings Intensity is expressed in roman numerals from I to XII on the modified Mercalli scale (high number = greater damage) Limitations in using intensity as a measure of earthquake strength: • Damage lessens with distance intensities for same earthquake • Damage to buildings and other structures depend on type of geological material it was built on • Damage estimates are also subjective www.notesolution.com Advantage – doesn’t involve instruments o Magnitude – calculate the amount of energy released by the quake. Usually measure the height (amplitude) of one of the wiggles on a seismogram. The larger the quake, the more the ground vibrates, the larger the wiggle Report on
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