Lecture 09 earthquakes p(6)
- 1. What is an earthquake?What is an earthquake? An earthquake is the vibration of EarthAn earthquake is the vibration of Earth produced by the rapid release of energyproduced by the rapid release of energy Energy radiates in all directions from itsEnergy radiates in all directions from its source, thesource, the focusfocus Energy moves like wavesEnergy moves like waves Seismographs record the eventSeismographs record the event Slinky, Rubber Band SEISMOGRAM Beaker, Wet Sand, Weight Cardboard Fault models Chewing Gum Wood meter stick or plastic ruler pencil
- 2. Anatomy of EarthquakesAnatomy of Earthquakes Earthquakes are associated with faultsEarthquakes are associated with faults
- 3. ) ) ) ) ) ))((( ( ( ( ( Earthquakes are causedEarthquakes are caused by sudden release ofby sudden release of accumulated strain energyaccumulated strain energy along Faultsalong Faults Rocks onRocks on sides of faultsides of fault are deformedare deformed by tectonicby tectonic forcesforces Rocks bendRocks bend and storeand store elastic energyelastic energy FrictionalFrictional resistanceresistance holding theholding the rocks togetherrocks together is overcomeis overcome by tectonicby tectonic Hands Demo
- 4. Earthquake mechanismEarthquake mechanism – Slip starts at the weakest point (the focus)Slip starts at the weakest point (the focus) – Earthquakes occur as the deformed rockEarthquakes occur as the deformed rock “springs back” to its original shape (“springs back” to its original shape (elasticelastic reboundrebound)) – The motion moves neighboring rocksThe motion moves neighboring rocks – And so on.And so on. – DEMO – elastic rebound w/ rulerDEMO – elastic rebound w/ ruler
- 5. RelationshiRelationshi p Betweenp Between Stress andStress and StrainStrain Strain can be a change in shape (a deformation) due to an applied stress Demo: Rubber Band
- 6. RelationshipRelationship BetweenBetween Stress andStress and Strain at lowStrain at low Temps andTemps and Pressure orPressure or Sudden StressSudden Stress Demo: Pencil
- 7. RelationshipRelationship BetweenBetween Stress andStress and Strain underStrain under High TempsHigh Temps or Pressureor Pressure Demo: gum
- 8. Strike and DipStrike and Dip Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface Strike is long line, dip is short line Note the angle of dip given 45o
- 10. Horizontal Movement AlongHorizontal Movement Along Strike-Slip FaultStrike-Slip Fault
- 11. Normal Fault Quake - NevadaReverse Fault Quake - Japan Strike Slip Fault Quake - California DEMO – Types of faults
- 12. Fence offset by the 1906Fence offset by the 1906 San Francisco earthquakeSan Francisco earthquake San Andreas is the most studied transform faultSan Andreas is the most studied transform fault system in the worldsystem in the world discrete segments 100 to 200 kilometers longdiscrete segments 100 to 200 kilometers long slip every 100-200 years producingslip every 100-200 years producing large earthquakeslarge earthquakes Some portions exhibit slow, gradual displacementSome portions exhibit slow, gradual displacement known as fault creepknown as fault creep
- 14. Fires caused by 1906 San Francisco Earthquake Gas mains break, fires shaken out of furnaces. Water mains break, cannot fight fires. Debris in streets, Fire department cannot reach fires.
- 15. Landscape Shifting, Wallace CreekLandscape Shifting, Wallace Creek San Andreas Fault, a Transform Margin
- 17. SeismographSeismograph Data-data aktual getaran tanah dariData-data aktual getaran tanah dari seismograph dikenal sebagai sebuahseismograph dikenal sebagai sebuah seismogram, dapat menyediakanseismogram, dapat menyediakan informasi tentang gempa secara alami.informasi tentang gempa secara alami. Data-data seismograph terdiri dari:Data-data seismograph terdiri dari: Percepatan terhadap waktuPercepatan terhadap waktu Kecepatan terhadap waktuKecepatan terhadap waktu Perpindahan terhadap waktuPerpindahan terhadap waktu
- 18. SeismographSeismograph Sebuah seismograph adalah sebuah instrumen yangSebuah seismograph adalah sebuah instrumen yang mencatat, sebagai fungsi waktu, geteran permukaanmencatat, sebagai fungsi waktu, geteran permukaan bumi akibat timbulnya gelombang-gelombang seismicbumi akibat timbulnya gelombang-gelombang seismic oleh gempabumi.oleh gempabumi. Data-data aktual getaran tanah dari seismograph dikenalData-data aktual getaran tanah dari seismograph dikenal sebagai sebuah seismogram, dapat menyediakansebagai sebuah seismogram, dapat menyediakan informasi tentang gempa secara alami.informasi tentang gempa secara alami.
- 19. SeismographSeismograph Data-data seismograph terdiri dari:Data-data seismograph terdiri dari: Percepatan terhadap waktuPercepatan terhadap waktu Kecepatan terhadap waktuKecepatan terhadap waktu Perpindahan terhadap waktuPerpindahan terhadap waktu
- 20. SeismologySeismology SeismometersSeismometers - instruments that- instruments that record seismic wavesrecord seismic waves Records the movement ofRecords the movement of Earth in relation to a stationaryEarth in relation to a stationary mass on a rotating drum ormass on a rotating drum or magnetic tapemagnetic tape
- 21. A seismograph designed toA seismograph designed to record vertical ground motionrecord vertical ground motion The heavy mass doesn’t move much The drum moves
- 22. Lateral Movement DetectorLateral Movement Detector In reality, copper wire coils move around magnets, generating current which is recorded.
- 23. Seismic Waves 1: Surface wavesSeismic Waves 1: Surface waves –Complex motion, great destructionComplex motion, great destruction –High amplitudeHigh amplitude and low velocityand low velocity –Longest periods (interval between crests)Longest periods (interval between crests) –Termed long, or L wavesTermed long, or L waves
- 24. Types of seismic waves (continued)Types of seismic waves (continued) Body wavesBody waves – Travel through Earth’s interiorTravel through Earth’s interior – Two types based on mode of travelTwo types based on mode of travel – Primary (P) wavesPrimary (P) waves Push-pull motionPush-pull motion Travel thru solids, liquids & gasesTravel thru solids, liquids & gases – Secondary (S) wavesSecondary (S) waves Moves at right angles to theirMoves at right angles to their direction of traveldirection of travel Travels only through solidsTravels only through solids
- 25. Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L P and S waves Demo: P and S waves
- 26. Earthquake focus andEarthquake focus and epicenterepicenter
- 27. Note how much bigger the surface waves are
- 28. Graph to find distance toGraph to find distance to epicenterepicenter
- 29. Locating Earthquake EpicenterLocating Earthquake Epicenter
- 30. Epicenter located using threeEpicenter located using three seismographsseismographs
- 31. 95% of energy released by earthquakes originates95% of energy released by earthquakes originates in narrow zones that wind around the Earthin narrow zones that wind around the Earth These zones mark of edges of tectonic platesThese zones mark of edges of tectonic plates Broad are subduction zone earthquakes, narrow are MOR. Lead to recognition of plates
- 32. Earthquake Depth and Plate TectonicEarthquake Depth and Plate Tectonic SettingSetting Subduction Zones discovered by Benioff
- 33. Earthquake in subductionEarthquake in subduction zoneszones
- 34. Earthquakes at DivergentEarthquakes at Divergent Boundaries - IcelandBoundaries - Iceland Crust pulling apart – normal faults
- 35. Measuring the size ofMeasuring the size of earthquakesearthquakes Two measurements describe the size of anTwo measurements describe the size of an earthquakeearthquake IntensityIntensity – a measure of earthquake shaking– a measure of earthquake shaking at a given location based on amount ofat a given location based on amount of damagedamage MagnitudeMagnitude – estimates the amount of energy– estimates the amount of energy released by the earthquakereleased by the earthquake
- 36. Intensity scalesIntensity scales Modified Mercalli Intensity ScaleModified Mercalli Intensity Scale waswas developed using California buildings as itsdeveloped using California buildings as its standardstandard Drawback is that destruction may not beDrawback is that destruction may not be true measure of earthquakes actual severitytrue measure of earthquakes actual severity
- 37. Magnitude scalesMagnitude scales Richter magnitudeRichter magnitude - concept introduced by- concept introduced by Charles Richter in 1935Charles Richter in 1935 Richter scaleRichter scale –Based on amplitude of largest seismicBased on amplitude of largest seismic wave recordedwave recorded –LOGLOG1010 SCALESCALE Each unit of Richter magnitudeEach unit of Richter magnitude corresponds to 10X increase in wavecorresponds to 10X increase in wave amplitude and 32X increase in Energyamplitude and 32X increase in Energy
- 38. Magnitude scalesMagnitude scales Moment magnitudeMoment magnitude was developed becausewas developed because Richter magnitude does not closely estimateRichter magnitude does not closely estimate the size of very large earthquakesthe size of very large earthquakes –Derived from the amount of displacementDerived from the amount of displacement that occurs along a fault and the area ofthat occurs along a fault and the area of the fault that slipsthe fault that slips
- 39. TsunamisTsunamis, or seismic sea waves, or seismic sea waves Destructive waves called “tidal waves”Destructive waves called “tidal waves” Result from “push” of underwater faultResult from “push” of underwater fault or undersea landslideor undersea landslide In open ocean height is > 1 meterIn open ocean height is > 1 meter In shallow coast water wave can be > 30In shallow coast water wave can be > 30 metersmeters Very destructiveVery destructive
- 40. Formation of a tsunamiFormation of a tsunami Tsunamis are actually huge, extending from the fault on the sea floor up to the surface, but they don’t stick up more than a meter or so in the deep ocean. However, when they reach shallow water they must rear up and slow down. Discussion: Kinetic vs. potential energy
- 41. Honolulu officials know exactly how long it takes a Tsunami to reach them from anywhere
- 42. Tsunami 1960,Tsunami 1960, Hilo HawaiiHilo Hawaii
- 44. Earthquake predictionEarthquake prediction Long-range forecastsLong-range forecasts Calculates probability of a certainCalculates probability of a certain magnitude earthquake occurring over amagnitude earthquake occurring over a given time periodgiven time period Short-range predictionsShort-range predictions Ongoing research, presently not muchOngoing research, presently not much successsuccess
- 45. Long Term Predictions Seismic Gaps
- 46. Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE
- 47. Seismic Gap along HimalayasSeismic Gap along Himalayas 2005
- 48. 4848 Dilatancy of Highly Stressed RocksDilatancy of Highly Stressed Rocks Short-Term Earthquake Prediction
- 49. Investigating Earth’s InteriorInvestigating Earth’s Interior Seismology helps us understand Earth’sSeismology helps us understand Earth’s Interior Structure. We use:Interior Structure. We use: Speed changes in different materialsSpeed changes in different materials due changes rigidity, density, elasticitydue changes rigidity, density, elasticity Reflections from layers with different propertiesReflections from layers with different properties Attenuation of Shear Waves in fluidsAttenuation of Shear Waves in fluids Direction changes (Refraction)Direction changes (Refraction)
- 50. 5050 Investigating Earth’s InteriorInvestigating Earth’s Interior
- 51. Surface Components magnifiedSurface Components magnified !
- 52. Seismic-wave velocities are faster in the upper mantleSeismic-wave velocities are faster in the upper mantle Waves that travel via mantle arrive sooner at far destinations Velocity increases w depth, waves bend back to surface. Mohorovičić discontinuity
- 53. Wave VelocitiesWave Velocities Upper Mantle Fast Asthenosphere Slow Lower Mantle Fast
- 54. The S-Wave Shadow ZoneThe S-Wave Shadow Zone Since Shear (S) waves cannot travel through liquids, the liquid outer core casts a larger shadow for S waves covering everything past 103 degrees away from the source. http://en.wikipedia.org/wiki/Richard_Dixon_Oldham
- 55. The P-Wave Shadow ZoneThe P-Wave Shadow Zone Behavior of waves through center reveal Earth’s Interior P-waves through the liquid outer core bend, leaving a low intensity shadow zone 103 to 143 degrees away from the source, here shown as the north pole HOWEVER, P-waves traveling straight through the center continue, and because speeds in the solid inner core are faster, they arrive sooner than expected if the core was all liquid. Inge Lehmann http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html