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Geography of Hazards Final Exam Review.doc

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Geography 2152F/G
Mark Moscicki

FINAL EXAM NOTES GEOGRAPHY OF HAZARDSLecture Seven 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 byan estimate of the area ruptured along a fault the amount of movement along the faultthe elasticity of the crust at the focus Similar 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 1960 In canada it is M81 in BC 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 in 12 categoriesMaps are produced showing the differences in Modified Mercalli intensities over broad areas Earthquake ProcessesEarthquakes are most common at or near plate boundariesMotion at plate boundaries is not usually smooth or constantFriction along plate boundaries exerts force stress on the rocks exerting strain or deformationWhen the stress exceeds the strength of the rocks there is a sudden movement along a fault The 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 area Not all faults reach the Earths surface Blind faults are located below the surfaceEarthquake Distribution Fault TypesThere are two basic types of geologic faults distinguished by the direction of the displacement of rocks or sedimentStrikeslip faults displacements are horizontal Dipslip faults displacements are vertical StrikeSlip FaultsThe San Andreas Fault is the best example of this typeDipSlip FaultsThere are three types reverse faults thrust faults and normal faults They are comprised of two walls on an incline defined by minersfootwall where the miners placed their feetHanging wall where miners placed their lanterns more likely to cause a tsunamiReverse FaultThe hanging wall has moved up relative to the footwall incline at an angle steeper than 45 degrees Thrust FaultThese are similar to reverse faults except the angle is 45 degrees or lessNormal FaultThe hangingwall has moved downward relative to the footwallFault ActivityIn terms of activity faults can fall into one of three categoriesActive movement during the past 11600 yearsPotentially Active movement during the past 26 million yearsInactive no movement during the past 26 million years Tectonic CreepDefinition The slow movement of rock or sediment along a fracture It 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 be expected Seismic 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 Primary or compressional waves They move fast with a push pull motion and can travel through solids or liquidsS waves Secondary or shear wavesThey move more slowlyin an up and down motion and can only travel through solids Surface WavesDefinition Seismic waves that form when P and S waves reach Earths surface and move along itThese waves move more slowly than body wavesThey are responsible for damage near the epicenter Two types of surface wavesLove wavescause horizontal shaking Rayleigh waves rolling waves that travel in an elliptical motion Earthquake ShakingFactors that determine the shaking people experience during an earthquakemagnitudedistance to the epicenter focal depth direction of rupture local soil and rock conditions engineering and construction practices of local buildings Seismographs record the arrival of waves to a recording stationBecause P waves travel faster than S waves they appear first on a seismogramEarthquake shaking with decreases with distance from the epicentre Distance to the Epicentre
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