Chapter 2 \u2013 Earthquakes - Chapter 2 Earthquakes Earthquakes Result from rupture of rocks along a fault Energy from an earthquake is released in the

Chapter 2 u2013 Earthquakes - Chapter 2 Earthquakes...

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Chapter 2 – Earthquakes Earthquakes -Result from rupture of rocks along a fault -Energy from an earthquake is released in the form of seismic waves -They are mapped according to the epicenter; the focus is located directly below the epicentre oMeasured by Seismograph and compared by Magnitude Earthquakes Magnitude -Expressed as a number to one decimal place (developed by Richter in 1935)-Richter Scale– measured of the strength of a wave at a distance of 100 km from the epicenter oSince then, more accurate method have been developed and the Richter Scale is no longer in use Moment Magnitude Scale (M)Scale determined by:-Area raptured along a fault -Amount of movement along the fault -Elasticity of the crust a the focus oSimilar to Richter Scale – a logarithmic scale oExample: an M7 earthquake represent 10x the amount of ground motion as an M6oCompare the M5 to an M8 – how many more time would M8 be compare to M5? = 1000 (take the difference and power it to 10) Magnitude & Frequency of Earthquakes -Except for very large earthquakes, magnitude on the Moment Magnitude scale is similar to Richter scale -The strongest earthquake to ever occur is M9.5 in Chile in 1960-In Canada it is M8.1 in B.C. 1949 – only few M9+ earthquakes each century
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-Earthquake Intensity -Modified Mercalii Intensity Scaleis a qualitativescale based on damage to structures and the affect on people (12 categories) oSeismic scale – scale of the magnitude of the energy released by the rupture Earthquake Process -Motion at plate boundaries is not usually smooth or constant -Friction along plate boundaries exerts a force (stress) on the rocks, exerting train or deformation oWhen the stress exceeds the strength of the rocks, there is a sudden movement along a fault -Movement starts at the focus and propagate in all directions (known as seismic waves) – thus faults are considered seismic sources -Identifying fault is necessary to evaluate the risk of an earthquake in a given area -Not all faults reach the earth’s surface oBlind faults are located below the surface Fault Types -2 basic types of geologic faults distinguished by the direction of the displacement of rock or sediment -Strike-slip faults - displacement are horizontal (San Andreas Fault) -Dip-slip faults – displacement are vertical Dip-Slip Faults -3 types: reverse faults, trust faults and normal faults Reverse Fault – hanging-wall has moved up relative to the footwall inclined at an angle steeper than 45 degreesThrust Fault – these are similar to reverse fault except the angle is 45 or lessNormal Fault – hanging-wall has moved down relative to the footwall -Comprise of 2 walls on an incline defined by miners oFootwall (where miners placed their feet)oHanging-wall where miners placed their lanterns) Fault Activity Active – movement during the past 11,600 years Potentially Active – movement during the past 2.6 million years Inactive– no movement during the past 2.6 million years Tectonic Creep -The slow movement of rock or sediment along a fracture caused by stress (fault creep-Can damage roads and buildings foundations (movement of few cm per decade)
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