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

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