Earthquakes 2

1、EarthquakesWhat are Earthquakes?The shaking or trembling caused by the sudden release of energyUsually associated with faulting or breaking of rocksContinuing adjustment of position results in aftershocksWhat is the Elastic Rebound Theory?Explains how energy is stored in rocksRocks bend until the st

2、rength of the rock is exceededRupture occurs and the rocks quickly rebound to an undeformed shapeEnergy is released in waves that radiate outward from the faultThe Focus and Epicenter of an EarthquakeThe point within Earth where faulting begins is the focus, or hypocenterThe point directly above the

3、 focus on the surface is the epicenterSeismographs record earthquake eventsAt convergent boundaries, focal depth increases along a dipping seismic zone called a Benioff zoneWhere Do Earthquakes Occur and How Often?80% of all earthquakes occur in the circum-Pacific beltmost of these result from conve

4、rgent margin activity15% occur in the Mediterranean-Asiatic beltremaining 5% occur in the interiors of plates and on spreading ridge centersmore than 150,000 quakes strong enough to be felt are recorded each yearThe Economics and Societal Impacts of EQsDamage in Oakland, CA, 1989Building collapseFir

5、eTsunamiGround failureWhat are Seismic Waves?Response of material to the arrival of energy fronts released by ruptureTwo types:Body wavesP and SSurface wavesR and LBody Waves: P and S wavesBody wavesP or primary wavesfastest wavestravel through solids, liquids, or gasescompressional wave, material m

6、ovement is in the same direction as wave movementS or secondary wavesslower than P wavestravel through solids onlyshear waves - move material perpendicular to wave movementSurface Waves: R and L wavesSurface WavesTravel just below or along the grounds surfaceSlower than body waves; rolling and side-

7、to-side movementEspecially damaging to buildingsHow is an Earthquakes Epicenter Located? Seismic wave behaviorP waves arrive first, then S waves, then L and RAverage speeds for all these waves is knownAfter an earthquake, the difference in arrival times at a seismograph station can be used to calcul

8、ate the distance from the seismograph to the epicenter.How is an Earthquakes Epicenter Located?Time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- wav

9、esHow is an Earthquakes Epicenter Located?Three seismograph stations are needed to locate the epicenter of an earthquakeA circle where the radius equals the distance to the epicenter is drawnThe intersection of the circles locates the epicenter How are the Size and Strength of an Earthquake Measured

10、?Modified Mercalli Intensity Map1994 Northridge, CA earthquake, magnitude 6.7Intensitysubjective measure of the kind of damage done and peoples reactions to itisoseismal lines identify areas of equal intensityHow are the Size and Strength of an Earthquake Measured?MagnitudeRichter scale measures tot

11、al amount of energy released by an earthquake; independent of intensityAmplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scaleWhat are the Destructive Effects of Earthquakes?Ground Shakingamplitude, duration, and damage inc

12、reases in poorly consolidated rocksCan Earthquakes be Predicted?Earthquake Precursors changes in elevation or tilting of land surface, fluctuations in groundwater levels, magnetic field, electrical resistance of the groundseismic dilatancy modelseismic gapsCan Earthquakes be Predicted?Earthquake Prediction Programsinclude laboratory and field studies of rocks before, during, and after earthquakesmonitor activity along major faultsproduce risk assessmentsCan Earthquakes be Controlled?Graph showing the relationship between the amount of wa