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EOSC114 - Earthquakes 2.docx

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University of British Columbia
Earth and Ocean Sciences
EOSC 114

EOSC114 - Earthquakes - Part 2  Earthquakes result from the rupture of rocks along a fault Fault: a fracture in the earth’s crust Seismic waves: the form in which energy is released when an earthquake occurs  Focus/Hypocentre: the point at which the earthquake originates from (initial rupture), where seismic waves radiate from  Earthquakes are compared by the amount of energy released, their magnitude, and by their intensity Magnitude: can be measured by moment magnitude, the Richter scale, etc. Intensity: the effects of ground motion on people and structures Epicentre: the point on the surface of the earth directly above the fault rupture. Richter Scale: expressed as a single decimal number (ex. 6.8), and quantifies the magnitude of earthquakes as the logarithm to the base 10 o News reports still use the Richter scale, however, seismologists now use moment magnitude Moment magnitude: determined from an estimate of the area that ruptured along a fault plane during the quake, the amount of movement or slippage along the fault, and the rigidity of the rocks near the focus. (ex. An increase from one whole number to the next represents a 10-fold increase in the amount of shaking and a 30-fold increase in the amount of energy released  Moment magnitude~=Richter scale, except for with large quakes Magnitude and Frequency of Earthquakes Worldwide Descriptor Magnitude Avg. Annual # of Events Great 8 and higher 1 Major 7-7.9 17 Strong 6-6.9 134 Moderate 5-5.9 1319 Light 4-4.9 13,000 (estimated) Minor 3-3.9 130,000 (estimated) Very Minor 2-2.9 1,300,000 (estimated - approx. 150/hour) Modified Mercalli Intensity Scale: measures the degree to which an earthquake affects people, property, and the ground.  ShakeMaps show both perceived shaking and potential damage (valuable to emergency response teams who must locate and rescue people) Modified Mercalli Intensity Scale (abridged) Intensity Effects I Felt by very few ppl II Felt by only a few ppl at rest, esp on upper floors. Delicate suspended objects may swing. III Felt noticeably indoors, esp on upper floors, no recognized as earthquake. Vibration feels like truck passing. IV Day= felt by many indoors, few outdoors. Night=some ppl awakened. Walls make cracking noise. Stationary cars rock noticeably. Sensation= heavy truck striking a building. V Felt by nearly everyone, many awakened. Some dishes and windows broken, unstable objects overturned. Disturbed tall objects sometimes noticed. VI Felt by all; many ppl frightened and run outside. Some heavy furniture moved. Some fallen plaster/damaged chimneys. Slight damage. VII Almost everyone runs outdoors. Damage: well built=negligible, moderate=slight, poor=considerable. Noticed by ppl driving cars. VIII Damage: well-built=slight, moderate=considerable, poor=great. Tall walls and columns collapse. Heavy furniture overturned. Changes in well water. Disturbs ppl driving cars. IX Damage: considerable in all makes of buildings. Some buildings shifted off of foundations. Ground cracks conspicuous. Underground pipes are broken. X Some well-built wooden structures destroyed. Most structures w/foundations destroye
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