Earthquakes Last time we considered faults. Today we discuss the consequence of a fault’s movement 1.Slinky 2.Beaker, Wet Sand, Weight 3.Ball Point Pen.

EarthquakesEarthquakes

Last time we considered faults. Today we discuss the consequence of a fault’s

movement

1. Slinky2. Beaker, Wet Sand, Weight3. Ball Point Pen 4. A seismogram

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

Rock subjected to elastic deformation snaps Rock subjected to elastic deformation snaps back and/or breaksback and/or breaks

Energy radiates in all directions from the Energy radiates in all directions from the break’s location, AKA source, the “break’s location, AKA source, the “focus”focus”

Energy moves like wavesEnergy moves like waves as moving rocks as moving rocks push on their neighborspush on their neighbors

Seismographs record the eventSeismographs record the event

Anatomy of EarthquakesAnatomy of Earthquakes

Earthquakes and faultsEarthquakes and faults Earthquakes are associated with faultsEarthquakes are associated with faults Motion along faults can be explained by plate Motion along faults can be explained by plate

tectonicstectonics

Causes of earthquakesCauses of earthquakes

Sudden release of accumulated strain Sudden release of accumulated strain energyenergy

Creation of fracture zones at faults by Creation of fracture zones at faults by rupturing rocksrupturing rocks

Creation of new faults by rupturing rocksCreation of new faults by rupturing rocks Shifting of rocks at preexisting faultsShifting of rocks at preexisting faults Deformed rock straightens back to Deformed rock straightens back to

original shape, but offset from matching original shape, but offset from matching beds on the other side of the fault.beds on the other side of the fault.

Elastic rebound 1Elastic rebound 1

Mechanism for EarthquakesMechanism for Earthquakes– Rocks on sides of fault are deformed by Rocks on sides of fault are deformed by

tectonic forces tectonic forces

– Rocks bend and store elastic energyRocks bend and store elastic energy

– Frictional resistance holding the rocks Frictional resistance holding the rocks

together is overcome by tectonic forces together is overcome by tectonic forces

Elastic rebound 2Elastic rebound 2

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.

) ) ) ) ) ) )( ( ( ( ( ( (

AftershocksAftershocksThe change in stress that follows The change in stress that follows a main shock creates smaller a main shock creates smaller earthquakes called earthquakes called aftershocksaftershocks

The aftershocksThe aftershocks““illuminate” the faultilluminate” the fault that ruptured in the mainthat ruptured in the main

shockshock RedRed dots show location of

aftershocks formed by 3earthquakes in Missouriand Tennessee in 1811/1812

Symbols: Reelfoot Fault is a Reverse Fault, teeth point to the upthrown side, so to the hanging wall. Cottonwood Grove a Transform Fault, note the arrows pointing in the directions of movement

Normal Fault Quake - NevadaReverse Fault Quake - Japan

Strike Slip Fault Quake - California

HW Down

HW UpConvergent

Divergent

Transform

San Andreas: An active San Andreas: An active earthquake zoneearthquake zone

San Andreas is the most studied strike-slip San Andreas is the most studied strike-slip (transform) fault system in the world(transform) fault system in the world

Displacement occurs along discrete segments 100 Displacement occurs along discrete segments 100 to 200 kilometers longto 200 kilometers long

Most segments slip every 100-200 years Most segments slip every 100-200 years producing large earthquakesproducing large earthquakes

However, some portions exhibit slow, However, some portions exhibit slow, gradual displacement known as gradual displacement known as fault creepfault creep. . The rocks have low strength minerals that The rocks have low strength minerals that cannot store strain, they just crumble and cannot store strain, they just crumble and smear continuously as the plates move.smear continuously as the plates move.

Fence offset by the 1906 Fence offset by the 1906 San Francisco earthquakeSan Francisco earthquake

San Andres Fault

Landscape Shifting, Wallace Landscape Shifting, Wallace CreekCreek

San Andres Fault

San Andreas Fault

Earthquake HazardsEarthquake Hazards

Four major hazards occur during Four major hazards occur during earthquakesearthquakes

One is well known: the collapse of One is well known: the collapse of buildings crushes peoplebuildings crushes people

Three more are less well knownThree more are less well known– FireFire– LiquifactionLiquifaction– TsunamiTsunami

Fires caused by 1906 San Francisco Earthquake

Gas mains break, fires shaken out of furnaces and fireplaces. Water mains break.Debris blocks streets. Fire Fighters cannot drive to the fire.

LiquefactionLiquefaction

Crystals from dredge muds are arranged like pick-up-sticks.Pressure waves from the earthquake force the crystals apart. Now your house is being supported by water.

Makes “quick clay”

Before we consider TsunamiBefore we consider Tsunami

We need some background in We need some background in seismologyseismology

SeismologySeismology

SeismometersSeismometers - instruments that - instruments that record seismic wavesrecord seismic waves

Formerly: Recorded the movement of Formerly: Recorded the movement of Earth in relation to a stationary mass Earth in relation to a stationary mass on a rotating drum or magnetic tapeon a rotating drum or magnetic tape

Today: use motion sensors similar to Today: use motion sensors similar to those in your smart phonethose in your smart phone

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.

Types of seismic wavesTypes of seismic waves

Surface wavesSurface waves

–Complex motion, great destructionComplex motion, great destruction

–High amplitudeHigh amplitude and low velocity and low velocity

–Longest periods (interval between Longest periods (interval between crests)crests)

–Termed long, or L wavesTermed long, or L waves

Two Types of Surface Two Types of Surface WavesWaves

Most of the destructionLarger amplitude than body 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

P and S waves

Locating the source of Locating the source of earthquakesearthquakes

FocusFocus - the place within Earth where - the place within Earth where earthquake waves originateearthquake waves originate

EpicenterEpicenter – location on the surface directly – location on the surface directly above the focusabove the focus

Epicenter is located using the difference in Epicenter is located using the difference in velocities of P and S wavesvelocities of P and S waves

Earthquake focus and Earthquake focus and epicenterepicenter

Note how much bigger the surface waves (aka L waves) are.

Body Waves

Delay between P and S arrivals gives distance to epicenter

P to S delay

Graph to find distance to Graph to find distance to epicenter epicenter

Average P wave speed is 3800 km in 7 minutes

Average S wave speed is 3800 km in 12 minutes

Locating the epicenter of an earthquakeLocating the epicenter of an earthquake

Three seismographs from different Three seismographs from different observatories needed to locate an epicenterobservatories needed to locate an epicenter

Each station determines the time interval Each station determines the time interval between the arrival of the first P wave and between the arrival of the first P wave and the first S wave at their locationthe first S wave at their location

A travel-time graph then determines each A travel-time graph then determines each station’s distance to the epicenterstation’s distance to the epicenter

Locating Earthquake Epicenter Locating Earthquake Epicenter

Locating the epicenter of an earthquakeLocating the epicenter of an earthquake

A circle with radius equal to distance to A circle with radius equal to distance to the epicenter is drawn around each the epicenter is drawn around each stationstation

The point where all three circles The point where all three circles intersect is the earthquake epicenterintersect is the earthquake epicenter

Epicenter located using three Epicenter located using three seismographsseismographs

Earthquake BeltsEarthquake Belts

95% of energy released by earthquakes 95% of energy released by earthquakes originates in narrow zones that wind originates in narrow zones that wind around the Eartharound the Earth

These zones mark of edges of These zones mark of edges of tectonic platestectonic plates

Locations of earthquakesLocations of earthquakesfrom 1980 to 1990from 1980 to 1990

Broad bands are subduction zone earthquakes, narrow are MOR

80% of seismic energy around Pacific Rim

Depths of EarthquakesDepths of Earthquakes

Earthquakes originate at depths ranging from 5 to Earthquakes originate at depths ranging from 5 to nearly 700 kilometersnearly 700 kilometers

Definite patterns existDefinite patterns exist – Shallow focus occur between mid-ocean ridgesShallow focus occur between mid-ocean ridges – Deep earthquakes occur in Pacific landward of oceanic Deep earthquakes occur in Pacific landward of oceanic

trenchestrenches– Central continent (intraplate) earthquakes are of Central continent (intraplate) earthquakes are of

various causes. Some causes still uncertain.various causes. Some causes still uncertain.

Devastating earthquakes occur less than 60 Devastating earthquakes occur less than 60 kilometers because cold rock is more elastic, and kilometers because cold rock is more elastic, and transmits waves better than warmer rocks belowtransmits waves better than warmer rocks below

Earthquake Depth and Plate Tectonic Earthquake Depth and Plate Tectonic SettingSetting

Subduction Zones discovered by Benioff

Weakest are the divergent zone earthquakes

Strongest Here

Strongest, with worst Tsunamis, at entrance to subduction zones

Earthquakes in Earthquakes in subduction zones subduction zones

Recent example, 9.0 Christmas 2004 Earthquake and Tsunami, Sumatra

Earthquakes at Divergent Earthquakes at Divergent Boundaries - IcelandBoundaries - Iceland

A new graben, down dropped hanging wall block - Normal Fault – divergent zone MOR

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 damage to buildings.damage to buildings.

MagnitudeMagnitude – estimates the amount of energy – estimates the amount of energy released by the earthquake.released 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 energytrue measure of earthquakes actual energy

Earthquake destructionEarthquake destructionAmount of structural damage depends Amount of structural damage depends

onon Intensity and duration of vibrationsIntensity and duration of vibrations Nature of the material upon which the Nature of the material upon which the

structure rests (hard rock good, soft structure rests (hard rock good, soft bad)bad)

Design of the structureDesign of the structure

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

Incorrectly called “tidal waves”Incorrectly called “tidal waves” Result from “push” of underwater fault Result from “push” of underwater fault

or undersea landslideor undersea landslideIn open ocean wave height is < 1 meterIn open ocean wave height is < 1 meter In shallow coast water wave can be > 30 In shallow coast water wave can be > 30

metersmeters (more than about 98 feet) (more than about 98 feet)Very destructiveVery destructive

Formation of a tsunamiFormation of a tsunami

SNAP

1.Water pushed up2. Wave as deep as water

4. In shallows wave rears up,and slows down.

3. Deep waterWave 1 meter above surface, very fast

The Hawaiian Islands vulnerable. Honolulu officials know exactly how long it takes a Tsunami to reach them from anywhere

Tsunami 1960, Tsunami 1960, Hilo HawaiiHilo Hawaii

Tsunami Model, Japan Tsunami Model, Japan EarthquakeEarthquake

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

Long Term PredictionsLong Term Predictions Strain Energy - accumulates Strain Energy - accumulates

uniformly - release irregularlyuniformly - release irregularly Some locked by friction “Seismic Some locked by friction “Seismic

gaps”gaps”– Prime candidates for major Prime candidates for major

earthquakeearthquake Some release energy Some release energy

continuously- creepcontinuously- creep– No major earthquakes thereNo major earthquakes there

Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE

Seismic Gap along HimalayasSeismic Gap along Himalayas

USGS web page on October 8 2005 magnitude 7.6 in Pakistan

Can earthquakes be Can earthquakes be predicted?predicted?

Short Term, Not very wellShort Term, Not very well Short-range predictionsShort-range predictions

Goal: provide warning location & magnitudeGoal: provide warning location & magnitude within a narrow time framewithin a narrow time frame Research on precursors due breaking prior Research on precursors due breaking prior

slip. slip. Breaking of rock lets the fault slipBreaking of rock lets the fault slip Breaks called fracture zones. The thickness Breaks called fracture zones. The thickness

of the fracture zone is proportional to the of the fracture zone is proportional to the length of the fault. length of the fault.

Breaking causes volume increase (dilation) Breaking causes volume increase (dilation) and and uplift in the rocks.uplift in the rocks.

Dilatancy causes many measurable changesDilatancy causes many measurable changes

5858

Dilatancy of Highly Stressed Dilatancy of Highly Stressed RocksRocks

Short-Term Earthquake Prediction

BOX OF ROCKS

Investigating Earth’s InteriorInvestigating Earth’s Interior

Earthquakes help us understand Earth’s Earthquakes help 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)

6060

Result: 3 Major Layers of Result: 3 Major Layers of EarthEarth

Shallow Components of EarthShallow Components of Earth

!

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 discontinuityCroatian seismologist Andrija Mohorovičić

Wave VelocitiesWave Velocities

Upper Mantle Fast

Asthenosphere Slow

Lower Mantle Fast

Mohorovičić discontinuity

Crust slow

http://pubs.usgs.gov/gip/interior/

Mineralogy of Earth’s LayersMineralogy of Earth’s Layers

This slide for graduate students.

The S-Wave Shadow ZoneThe S-Wave Shadow Zone

Since Shear (S) waves cannot travel through liquids, the liquid outer core casts a large shadow for S waves covering everything past 103 degrees away from the source.

The P-Wave Shadow ZoneThe P-Wave Shadow Zone

Behavior of waves through center reveals Earth’s Interior

P-waves pass ing 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

Discovery of the solid inner core

End of End of EarthquakesEarthquakes