2What are Earthquakes?The shaking or trembling of the earth/ground caused by the sudden release of energyUsually associated with faulting or breaking of rocksContinuing adjustment of position results in aftershocks.
3What is the Elastic Rebound Theory? Explains how energy is stored in rocksRocks bend until the strength of the rock is exceededRupture occurs and the rocks quickly rebound to an undeformed shapeEnergy is released in waves that radiate outward from the fault
8Where Do Earthquakes Occur and How Often? ~80% of all earthquakes occur in the circum-Pacific beltmost of these result from convergent margin activity~15% 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 year
9The Economics and Societal Impacts of EQs Damage in Oakland, CA, 1989Building collapseFireTsunamiGround failure
11What are Seismic Waves?Response of material to the arrival of energy fronts released by ruptureTwo types:Body wavesP and SSurface wavesR and L
12Body Waves: P and S waves P or primary wavesfastest wavestravel through solids, liquids, or gasescompressional wave, material movement is in the same direction as wave movementS or secondary wavesslower than P wavestravel through solids onlyshear waves - move material perpendicular to wave movement
13Surface Waves: R and L waves Travel just below or along the ground’s surfaceSlower than body waves; rolling and side-to-side movementEspecially damaging to buildings
16How is an Earthquake’s 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 calculate the distance from the seismograph to the epicenter.
17How is an Earthquake’s 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- waves
18How is an Earthquake’s 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 epicenterLocating an Earthquake
19How are the Size and Strength of an Earthquake Measured? Intensitysubjective measure of the kind of damage done and people’s reactions to itisoseismal lines identify areas of equal intensityModified Mercalli Intensity Map1994 Northridge, CA earthquake, magnitude 6.7
20How are the Size and Strength of an Earthquake Measured? MagnitudeRichter scale measures total 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 scale
21What are the Destructive Effects of Earthquakes? Ground Shakingamplitude, duration, and damage increases in poorly consolidated rocks
22Can Earthquakes be Predicted? Earthquake Precursorschanges in elevation or tilting of land surface, fluctuations in groundwater levels, magnetic field, electrical resistance of the groundseismic dilatancy modelseismic gaps
23Can Earthquakes be Predicted? Earthquake Prediction Programsinclude laboratory and field studies of rocks before, during, and after earthquakesmonitor activity along major faultsproduce risk assessments
24Can Earthquakes be Controlled? Graph showing the relationship between the amount of waste injected into wells per month and the average number of Denver earthquakes per monthSome have suggested that pumping fluids into seismic gaps will cause small earthquakes while preventing large ones
27Earth’s Layered Structure Moho is the boundary of the crust and the mantle. Lehmann discontinuity is boundary between the inner and outer core
28Layers Defined by Composition CrustOceanic CrustBasaltic CompositionDensity 3g/ccYounger (180MY younger than the continental crust)Continental CrustMantleBelow crust to a depth of 2900 kilometersComposition of the uppermost mantle is the igneous rock peridotite (changes at greater depths).
29Layers Defined by Composition CoreBelow mantleSphere with a radius of 3486 kilometersComposed of an iron-nickel alloyAverage density of nearly 11 g/cm3
30Layers Defined by Physical Properties LithosphereCrust and uppermost mantle (about 100 km thick)Cool, rigid, solidAsthenosphereBeneath the lithosphereUpper mantleTo a depth of about 660kmSoft, weak layer that is easily deformed
31Layers Defined by Physical Properties Lower MantlekmMore rigid than the asthenosphereRocks are very hot and capable of gradual flowOuter CoreLiquid layer 2270 km thickConvective flow of metallic iron within generates Earth’s magnetic fieldInner CoreSolid sphere with a radius of 1216 km
32More … What to do when there’s an earthquake? Forces of Nature : Earthquakes