Download - Lecture 09 earthquakes p(6)
What is an earthquake?What is an earthquake? An earthquake is the vibration of Earth An 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 its Energy radiates in all directions from its
source, the source, the focusfocus Energy moves like wavesEnergy moves like waves Seismographs record the eventSeismographs record the event
Slinky, Rubber Band SEISMOGRAMBeaker, Wet Sand, WeightCardboard Fault modelsChewing GumWood meter stick or plastic rulerpencil
Anatomy of EarthquakesAnatomy of Earthquakes
Earthquakes are associated with faultsEarthquakes are associated with faults
) ) ) ) ) ) )( ( ( ( ( ( (
Earthquakes are caused Earthquakes are caused by sudden release of by sudden release of accumulated strain accumulated strain energy along Faultsenergy along Faults
Rocks on Rocks on sides of fault sides of fault are are deformed by deformed by tectonic tectonic forces forces
Rocks bend Rocks bend and store and store elastic elastic energyenergy Frictional Frictional resistance resistance holding the holding the rocks rocks together is together is overcome by overcome by tectonic tectonic forcesforces
Hands Demo
Earthquake mechanism Earthquake mechanism
– Slip starts at the weakest point (the focus) Slip starts at the weakest point (the focus) – Earthquakes occur as the deformed rock Earthquakes occur as the deformed rock
“springs back” to its original shape (“springs back” to its original shape (elastic elastic 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
RelationshRelationship ip Between Between Stress and Stress and StrainStrain
Strain can be a change in shape (a deformation) due to an applied stress
Demo: Rubber Band
Relationship Relationship Between Between Stress and Stress and Strain at low Strain at low Temps and Temps and Pressure or Pressure or Sudden Sudden StressStress
Demo: Pencil
Relationship Relationship Between Between Stress and Stress and Strain under Strain under High Temps High Temps or Pressure or Pressure
Demo: gum
Strike and DipStrike and Dip
Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface
Strike is long line, dip is short lineNote the angle of dip given 45o
Vertical Vertical Movement Movement along Dip-along Dip-Slip FaultsSlip Faults
Divergent
Convergent
Horizontal Movement Along Horizontal Movement Along Strike-Slip FaultStrike-Slip Fault
Normal Fault Quake - NevadaReverse Fault Quake - Japan
Strike Slip Fault Quake - California
DEMO – Types of faults
Fence offset by the 1906 Fence offset by the 1906 San Francisco earthquakeSan Francisco earthquake
San Andreas is the most studied transform fault San 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 producing slip every 100-200 years producing large earthquakes large earthquakes
Some portions exhibit slow, gradual displacement Some portions exhibit slow, gradual displacement known as fault creepknown as fault creep
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.
Landscape Shifting, Wallace Landscape Shifting, Wallace CreekCreek
San Andreas Fault, a Transform Margin
LiquefactionLiquefaction
Demo: Liquifaction
SeismographSeismograph
Data-data aktual getaran tanah dari Data-data aktual getaran tanah dari seismograph dikenal sebagai sebuah seismograph dikenal sebagai sebuah seismogram, dapat menyediakan seismogram, 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
SeismographSeismograph
Sebuah seismograph adalah sebuah instrumen Sebuah seismograph adalah sebuah instrumen yang mencatat, sebagai fungsi waktu, geteran yang mencatat, sebagai fungsi waktu, geteran permukaan bumi akibat timbulnya gelombang-permukaan bumi akibat timbulnya gelombang-gelombang seismic oleh gempabumi. gelombang seismic oleh gempabumi.
Data-data aktual getaran tanah dari seismograph Data-data aktual getaran tanah dari seismograph dikenal sebagai sebuah seismogram, dapat dikenal sebagai sebuah seismogram, dapat menyediakan informasi tentang gempa secara menyediakan informasi tentang gempa secara alami.alami.
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
SeismologySeismology
SeismometersSeismometers - instruments that - instruments that record seismic wavesrecord seismic waves
Records the movement of Records the movement of Earth in relation to a stationary Earth in relation to a stationary mass on a rotating drum or mass on a rotating drum or magnetic tapemagnetic tape
A seismograph designed to A seismograph designed to record vertical ground record vertical ground
motionmotionThe heavy mass doesn’t move much
The drum moves
Lateral Movement DetectorLateral Movement Detector
In reality, copper wire coils move around magnets, generating current which is recorded.
Seismic Waves 1: Surface wavesSeismic Waves 1: Surface waves–Complex motion, great destructionComplex motion, great destruction –High amplitudeHigh amplitude and low velocity and low velocity –Longest periods (interval between crests)Longest periods (interval between crests) –Termed long, or L wavesTermed long, or L waves
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 their Moves at right angles to their
direction of traveldirection of travel Travels only through solidsTravels only through solids
Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L
P and S waves
Demo: P and S waves
Earthquake focus and Earthquake focus and epicenterepicenter
Note how much bigger the surface waves are
Graph to find distance to Graph to find distance to epicenter epicenter
Locating Earthquake Epicenter Locating Earthquake Epicenter
Epicenter located using three Epicenter located using three seismographsseismographs
95% of energy released by earthquakes originates 95% of energy released by earthquakes originates in narrow zones that wind around the Earth in 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
Earthquake Depth and Plate Tectonic Earthquake Depth and Plate Tectonic SettingSetting
Subduction Zones discovered by Benioff
Earthquake in subduction Earthquake in subduction zones zones
Earthquakes at Divergent Earthquakes at Divergent Boundaries - IcelandBoundaries - Iceland
Crust pulling apart – normal faults
Measuring the size of Measuring the size of earthquakesearthquakes
Two measurements describe the size of an Two 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 of at 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
Intensity scalesIntensity scales
Modified Mercalli Intensity ScaleModified Mercalli Intensity Scale was was developed using California buildings as its developed using California buildings as its standardstandard
Drawback is that destruction may not be Drawback is that destruction may not be true measure of earthquakes actual severitytrue measure of earthquakes actual severity
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 seismic Based on amplitude of largest seismic
wave recordedwave recorded–LOGLOG1010 SCALE SCALE Each unit of Richter magnitude Each unit of Richter magnitude
corresponds to 10X increase in wave corresponds to 10X increase in wave amplitude and 32X increase in Energyamplitude and 32X increase in Energy
Magnitude scalesMagnitude scales
Moment magnitudeMoment magnitude was developed because was developed because Richter magnitude does not closely estimate Richter magnitude does not closely estimate the size of very large earthquakesthe size of very large earthquakes
–Derived from the amount of displacement Derived from the amount of displacement that occurs along a fault and the area of that occurs along a fault and the area of the fault that slipsthe fault that slips
TsunamisTsunamis, or seismic sea waves, or seismic sea waves
Destructive waves called “tidal waves”Destructive waves called “tidal waves” Result from “push” of underwater fault Result from “push” of underwater fault
or undersea landslideor undersea landslideIn open ocean height is > 1 meterIn open ocean height is > 1 meter In shallow coast water wave can be > 30 In shallow coast water wave can be > 30
metersmeters Very destructiveVery destructive
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
Honolulu officials know exactly how long it takes a Tsunami to reach them from anywhere
Tsunami 1960, Tsunami 1960, Hilo HawaiiHilo Hawaii
Tsunami Tsunami Model, Model, Alaska Alaska QuakeQuake
Earthquake predictionEarthquake prediction Long-range forecasts Long-range forecasts
Calculates probability of a certain Calculates probability of a certain magnitude earthquake occurring over a magnitude earthquake occurring over a given time periodgiven time period
Short-range predictionsShort-range predictions Ongoing research, presently not much Ongoing research, presently not much
successsuccess
Long Term Predictions
Seismic Gaps
Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE
Seismic Gap along HimalayasSeismic Gap along Himalayas
2005
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Dilatancy of Highly Stressed Dilatancy of Highly Stressed RocksRocks
Short-Term Earthquake Prediction
Investigating Earth’s InteriorInvestigating Earth’s Interior Seismology helps us understand Earth’s Seismology helps us understand Earth’s
Interior Structure. We use:Interior Structure. We use: Speed changes in different materials Speed changes in different materials
due changes rigidity, density, elasticity due changes rigidity, density, elasticity Reflections from layers with different Reflections from layers with different
propertiesproperties Attenuation of Shear Waves in fluidsAttenuation of Shear Waves in fluids Direction changes (Refraction)Direction changes (Refraction)
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Investigating Earth’s InteriorInvestigating Earth’s Interior
Surface Components magnifiedSurface Components magnified
!
Seismic-wave velocities are faster in the upper Seismic-wave velocities are faster in the upper mantlemantle
Waves that travel via mantle arrive sooner at far destinations
Velocity increases w depth, waves bend back to surface.
Mohorovičić discontinuity
Wave VelocitiesWave Velocities
Upper Mantle Fast
Asthenosphere Slow
Lower Mantle Fast
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
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