Earthquakes and Seismic WavesQuestionsWhat is an earthquake?Causes of earthquakesSlide 5Slide 6How energy is applied to rocksHow does energy build up in rocks?Elastic-Rebound TheorySlide 10PowerPoint PresentationThe amount of energy a rock can store before it ruptures is dependent on many variablesParts of a fault-related earthquakeSlide 14Slide 15Slide 16Slide 17Slide 18Earthquakes may be preceded by smaller FORESHOCKS, or there may be AFTERSHOCKSEarthquake factoidsRelationship of earthquake occurrence and focal depth to Plate BoundariesNot All earthquakes are associated with Modern Plate boundariesThe released energy travels as Seismic WavesTypes of Seismic WavesP-WavesS-WavesSurface waves: originate when body waves travel to the epicenter, where they cause the earth’s surface to vibrateSurface WavesSeismographs – Stations that Record the passage of seismic wavesSlide 30Seismogram - the record of passing seismic wavesExample of a seismogramLocating the epicenterSlide 34Slide 35Slide 36Measuring earthquakesMercalli ScaleMercalli Intensity maps 1886 Charleston, SC 1811 New Madrid, MORichter ScaleSlide 41Relationship of Richter Magnitude with Released energySlide 43Slide 44Earthquakes and Earthquakes and Seismic WavesSeismic WavesLecture 22Lecture 22Questions•How does plate tectonics relate to earthquakes?•What are the types of seismic waves and how are they different?•What do the Mercalli Scale and Richter Scale Measure?•What are the hazards from earthquakes?What is an earthquake?•Shaking and trembling of the ground initiated by a sudden release of energy•The Energy radiates as Seismic Waves that travel within earth and onto the surface•Large seismic waves result in observable ground motion, but small energy events can be detected by instruments•This energy release can be caused by a variety of factors:Volcanic eruptionsCauses of earthquakesLandslidesCauses of earthquakesCave collapse ExplosionsThe 2001 World Trade Center collapse produced detectable seismic waves!Causes of earthquakes•The Main cause of earthquakes is from rock movement in earth’s crust•Most of these earthquakes occur along a Fault, or breakage in the rocks•100,000’s – 1,000,000 earthquakes occur each year, but, the vast majority (95 % +) are too small to be felt by humansHow energy is applied to rocksHow does energy build up in rocks?•Plate tectonics - the constant shifting of earth’s crusts allow rocks to deform or behave elastically•Rocks elastically deform until the strength of the rocks is exceeded, and then they break•The breakage (and rebound) of rocks transmits energyElastic-Rebound Theory•Strain (energy) builds up in rocks•Rocks deform•Strain exceeds rock strength > Rocks break•Rocks “snap back” to original, undeformed position–The Rapid return to original shape (Rebound), releases energyStress is appliedStrain builds upcausing deformationRocks break and rebound to former Undeformed shapeDisplacement, and movement Along Fault occursElastic-Rebound TheoryDisplacement is the Amount of movement along fault planeThe amount of energy a rock can store before it ruptures is dependent on many variables•The type of rock •Depth of rocks•The nature of the stress–Slow and gradual build up of stressversus–Rapid build up of stress–Shear v. Tension v. Compression–Therefore, earthquakes are difficult to predict in terms of size and effects!Parts of a fault-related earthquakeEpicenterFocusParts of a fault-related earthquake•FOCUS – the point within the earth where the rupture occurs•EPICENTER – point on the earth’s surface directly above Focus•FAULT PLANE – where rock movement occurs•Faults can expand at velocities up to 7200 mph!Earthquakes may be preceded by smaller FORESHOCKS, or there may be AFTERSHOCKSEarthquake factoids•No earthquakes occur below 450 miles from the surface. Why?•90% of all earthquakes occur within 60 miles (~100 km) of the surface (Shallow FOCUS)•Almost ALL catastro p h ic earthquakes occur within 40 (~60 km) miles of the surface•Shallow Focus earthquakes are associated with Divergent or Transform Plate boundaries•Shallow, Intermediate or Deep Focus earthquakes can occur at Convergent BoundariesRelationship of earthquake occurrence and focal depth to Plate BoundariesNot All earthquakes are associated with Modern Plate boundariesThe released energy travels as Seismic WavesSeismic waves radiate outward from focusSeismograph stations can detect these wavesTypes of Seismic Waves•Body Waves – Travel through earthP-Wave (Primary Waves)Fastest wave (4 miles/second)S-Wave (Secondary wave)Slower than p-waves (2 miles/second•Surface Waves – travel on surface; larger than body waves, but travel slower (1.5 miles/second)–Rayleigh–Love wavesP-WavesTravel by compressionCan travel through Solids, Liquids and AirOccur as a jolt whenthey intersect the surfaceS-WavesTravel only through solidsShear motionOccur as a jerkingMotion when they reach Earth’s surfaceSurface waves: originate when body waves travel to the epicenter, where they cause the earth’s surface to vibrateSurface WavesRayleigh Waves Love WavesSurface Waves –cause most of the damagesAlthough larger waves than Body waves, Surface waves dissipate more rapidlySeismographs – Stations thatRecord the passage of seismic wavesSeismogram - the record of passing seismic wavesExample of a seismogramThe P-S time interval depends how far theSeismograph station is from epicenterLocating the epicenterThe P-S arrival time interval increases with increasing distance from the epicenter(The P and S wave travel at different velocities)Requires at least 3 seismograph stations to triangulate epicenter3 minutes8 minutes11 minutesP WaveS waveMeasuring earthquakes•2 Systems are used•Mercalli Scale–Measures Intensity of shaking by assessing the damages–Scale from I to XII•Richter Scale–Measures Magnitude of energy released–Scale from 1 - 10Mercalli ScaleMercalli Intensity maps 1886 Charleston, SC 1811 New Madrid, MORichter Scale•Measures the magnitude of energy•It is a Log scale system•Each 1 whole unit represents a 10-fold increase in Magnitude, but a 30-fold increase in the amount of Energy released for each whole unit increase!•Magnitude, then, is based upon the amplitude of the seismic waveMagnitude = Log (X/T) + (S-P interval)X = Maximum Amplitude (height) of waveT = the Duration of 1 oscillationRelationship of Richter