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Final Exam Notes

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Western University
Geography 2152F/G
Leichelle Little

EarthquakesThey result from the rupture of rocks along a faultEnergy from an earthquake is released in the form of seismic wavesThey are mapped according to the epicenter the focus is located directly below the epicenterThey are measured by seismographs and compared by magnitude Earthquake MagnitudeThe magnitude of an earthquake is expressed as a number to one decimal placeThis type of measurement was first developed by Richter in 1935The Richter Scale was a measure of the strength of a wave at a distance of 100 km from the epicenterSince then more accurate methods have been developed and the Richter Scale is no longer in useThe Moment Magnitude ScaleToday earthquakes are measured using the Moment Magnitude scale MThe scale is determined by o The area ruptured along a fault o The amount of movement along the fault o The elasticity of the crust at the focusSimilar to the Richter Scale it is a logarithmic scaleExample An M7 earthquake represents 10 times the amount of ground motion as an M6 earthquakeMagnitude and Frequency of EarthquakesExcept for very large earthquakes the magnitude on the Moment Magnitude Scale is similar to the Richter ScaleThe strongest earthquake to ever occur is M95 in Chile in 1960In Canada it is M81 in BC in 1949There are only a few M9 earthquakes each centuryEarthquake IntensityThe Modified Mercalli Intensity Scale is a qualitative scale based on damage to structures and the affect on peopleIt is based on 12 categoriesMaps are produced showing the differences in Modified Mercalli Intensities over broad areaEarthquake ProcessesEarthquakes are most common at or near plate boundariesMotion at plate boundaries is not usually smooth or constantFriction along plate boundaries exerts a force stress on the rocks exerting strain or deformationWhen the stress exceeds the strength of the rocks there is a sudden movement along a faultThe movement or rupture starts at the focus and propagates in all directions called seismic wavesThus faults are considered seismic sourcesIdentifying faults is necessary to evaluate the risk of an earthquake in a given areaNot all faults reach the Earths surfaceBlind faults are located below the surfaceFault TypesThere are two basic types of geologic faults distinguished by the direction of the displacement of rock or sedimentStrikeslip faults o Displacements are horizontal o The San Andreas Fault is the best example of this typeDipslip faults o Displacements are vertical o Dipslip faults are the faults to worry aboutDipSlip FaultsThere are 3 types o Reverse faults thrust faults and normal faultsThey are comprised of 2 walls on an include defined by miners o Foot wall where miners placed their feet o Hangingwall where miners placed their lanternsReverse Fault The hangingwall has moved up relative to the footwall inclined at an angle steeper than 45 degreesThrust Fault These are similar to reverse faults except the angle is 45 degrees or lessNormal Fault The hangingwall has moved downward relative to the footwallFault ActivityIn terms of activity faults can fall into one of three categories o Activemovement during the past 11600 years o Potentially Activemovement during the past 26 million years o Inactiveno movement during the past 26 million yearsTectonic CreepDefinition The snow movement of rock or sediment along a fracture caused by stressIt is also referred to as fault creepThis can damage roads and building foundations ie movement of a few cm per decadeAlong these faults periodic sudden displacements producing earthquakes can also occurSeismic WavesSome seismic waves generated by fault rupture travel within the body of the Earth and others travel along the surfaceBody waves These include P waves and S wavesP Waves o They are also called primary or compressional waves o They move fast with a pushpull motion and can travel through solids or liquidsS Waves o They are also called secondary or shear waves o They move more slowly in an upanddown motion and can only travel through solidsSurface WavesDefinition Seismic waves that form when P and S waves reach Earths surface and then move along itThese waves move more slowly than body wavesThey are responsible for damage near the epicenterEarthquake ShakingFactors that determine the shaking people experience during an earthquake listed in order of importance o Magnitude o Distance to the epicenter o Focal depth o Direction of rupture o Local soil and rock types o Local engineering and construction practicesSeismographs record the arrival of waves to a recording stationBecause P waves travel faster than S waves they appear first on a seismogramEarthquake shaking decreases with distance from the epicenterDistance to the EpicentreThe difference between the arrival times of the first P and S waves at different locations determine the distance to the epicenterThe distance to the epicenter is calculated at 3 different seismic stationsA circle with radius equal to that distance is drawn around the stationLocating an EarthquakeThe epicenter is located where the circles intersect this process is called triangulationFocal DepthSeismic waves lose some of their energy before they reach the surfaceThe greater the focal depth the less intense the shaking at the surfaceThis loss of energy is referred to as attenuation
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