EARTHQUAKESEARTHQUAKES

MOTHER MOTHER NATURE’S NATURE’S

RUMBLINGSRUMBLINGS

What is that force?What is that force?

EarthquakeEarthquake – a vibration of the solid – a vibration of the solid earth produced by the very rapid earth produced by the very rapid release of energyrelease of energy

The energy that drives these quakes The energy that drives these quakes is derived from the earth’s interioris derived from the earth’s interior

The motions of the earth’s plates are The motions of the earth’s plates are frequently the cause of this rapid frequently the cause of this rapid energy releaseenergy release

Why bother in the first place?Why bother in the first place?

Why do we study Why do we study these earthquakes?these earthquakes?• Pure scientific Pure scientific

curiositycuriosity• Try and learn how to Try and learn how to

predict thempredict them• Gives us a view of Gives us a view of

the earth’s interiorthe earth’s interior• So we can try and So we can try and

avoid tragedies like avoid tragedies like the one shown herethe one shown here

In 1985, an earthquake measuring over 8.0 struck outside of Mexico City

An estimated 25,000 people were killed in this tragedy

Where in the world are they?Where in the world are they?

This map show world earthquake distribution

What familiar pattern do these earthquake locations line up with?

That’s right, the tectonic plates!

Where do they happen?Where do they happen?

FocusFocus – The point of – The point of origin of an Earthquakeorigin of an Earthquake

The earth’s lithosphere The earth’s lithosphere ruptures, and energy ruptures, and energy radiates out in all radiates out in all directionsdirections

Most earthquakes do Most earthquakes do not occur directly at not occur directly at the surfacethe surface

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

A diagram showing a fault with an earthquake focus occurring at depth with the epicenter directly above it

How deep?How deep? Although it’s possible, most Although it’s possible, most

earthquakes do not happen earthquakes do not happen right at the earth’s surfaceright at the earth’s surface

The depth that they occur, The depth that they occur, depends upon the plate depends upon the plate boundaryboundary

Convergent boundaries Convergent boundaries produce the largest quakes produce the largest quakes and also the deepest ones—and also the deepest ones—600 km depth is maximum600 km depth is maximum

Transform boundary quakes Transform boundary quakes can also be large but are can also be large but are generally shallow—less than generally shallow—less than 80 km below the surface80 km below the surface

Divergent boundaries Divergent boundaries produce weaker, shallower produce weaker, shallower quakes as wellquakes as well

This map and cross section of the Japanese Island Arc shows the range of earthquake depths found at convergent subduction zones

Waves on the WaterWaves on the Water Just as waves radiate out Just as waves radiate out

from a stone being from a stone being dropped in water, so will dropped in water, so will earthquake waves radiate earthquake waves radiate out in all directionsout in all directions

EQ waves will radiate out EQ waves will radiate out from the focus in 3-Dfrom the focus in 3-D

Energy dissipates very Energy dissipates very rapidly through solid rock, rapidly through solid rock, but sensitive instruments but sensitive instruments can pick up, that is “listen can pick up, that is “listen for” seismic events all for” seismic events all around the globearound the globe

Whose Whose faultfault is it? is it?

Most earthquakes occur Most earthquakes occur along faults or fault along faults or fault zoneszones

FaultFault – a fracture in the – a fracture in the earth’s crust along which earth’s crust along which there is movementthere is movement

Some are a few km in Some are a few km in length, others 1000s of length, others 1000s of km longkm long

Faults come in all varieties of angles, depths, and shapes

Earthquake ProcessesEarthquake Processes No one knew how earthquakes worked until No one knew how earthquakes worked until

early in the 20early in the 20thth century century H. F. Reid studied the 1906 San Fran quake H. F. Reid studied the 1906 San Fran quake

and came up with a theory called and came up with a theory called elastic elastic reboundrebound

Just like a stick under pressure, rocks will bend to a certain point and will then rupture and release stored energy with violent shaking

Understand the Physics Understand the Physics Elastic rebound can be Elastic rebound can be

explained once the idea explained once the idea of of stressstress and and strainstrain is is understoodunderstood

StressStress = pressure = pressure StrainStrain = deformation = deformation Rocks in the crust are Rocks in the crust are

under incredible under incredible amounts of pressure amounts of pressure and will slowly deformand will slowly deform

Once the rocks’ Once the rocks’ strength is exceeded strength is exceeded they will rupture causing they will rupture causing vibrationsvibrations

There are different kinds of stress as demonstrated by the above diagram

The Theory Behind the ShakeThe Theory Behind the Shake Elastic ReboundElastic Rebound – –

sudden released of sudden released of stored strain in stored strain in rocksrocks

The result of this The result of this rebound is sudden rebound is sudden movement along a movement along a faultfault

The rupture of this fault trace in California was formed due to elastic rebound

The strength of the rocks could only hold so much stress and then they gave way

What comes before and after?What comes before and after?

ForeshocksForeshocks – small earthquakes – small earthquakes preceding a major quake by days or preceding a major quake by days or even possibly yearseven possibly years

AftershocksAftershocks – the settling or – the settling or adjustment of crustal rocks after a adjustment of crustal rocks after a large earthquakelarge earthquake

SeismologySeismology The entire branch of The entire branch of

geology dedicated to geology dedicated to studying earthquakes studying earthquakes and earthquake waves and earthquake waves is called is called seismologyseismology

SeismographsSeismographs – the – the instruments used to instruments used to measure and record measure and record seismic wavesseismic waves

Almost 2000 years ago Almost 2000 years ago the Chinese made the the Chinese made the very first seismographvery first seismograph

The original seismograph was invented by the Chinese 2000 years ago

Metal spheres were delicately balanced in mouths of dragons, and the slightest vibrations would cause them to fall out

Modern SeismographModern Seismograph

Modern seismographs Modern seismographs incorporate the incorporate the principle of inertiaprinciple of inertia

InertiaInertia – an object at – an object at rest will stay at rest, & rest will stay at rest, & an object in motion will an object in motion will stay in motionstay in motion

Seismographs are Seismographs are placed at depth away placed at depth away from background noisefrom background noise

Always in a group of Always in a group of three to measure all three to measure all three dimensions of three dimensions of movement (3-D)movement (3-D)

A modern seismograph has a pen attached to a weight on a spring; the ground shakes the rotating drum while the pen remains stationary due to inertia and traces out the quake

Look at the DataLook at the Data

When seismographs When seismographs record earthquake record earthquake activity onto paper it is activity onto paper it is called a called a seismogramseismogram

Top: simple seismogram showing the arrival of different P & S waves

Bottom: a long track seismogram showing a major earthquake accompanied by both a foreshock and an aftershock

To Review:To Review: Earthquakes are the Earthquakes are the

result of a rapid result of a rapid release of stored release of stored energy from within the energy from within the crustcrust

The focus is where the The focus is where the EQ occurs and the EQ occurs and the epicenter is on the epicenter is on the surface directly above surface directly above the focusthe focus

EQ happen along faults EQ happen along faults which are long cracks which are long cracks in the earth’s crustin the earth’s crust

Earthquake energy Earthquake energy radiates out in all radiates out in all directions like ripples directions like ripples on a pondon a pond

H. F. Reid came up with H. F. Reid came up with theory of elastic theory of elastic rebound when studying rebound when studying the great 1906 quake of the great 1906 quake of San FranSan Fran

This theory is used to This theory is used to explain EQ even todayexplain EQ even today

Stress (pressure) causes Stress (pressure) causes strain (deformation)strain (deformation)

Seismograph is the Seismograph is the instrument used to instrument used to measure EQ and it measure EQ and it operates on the law of operates on the law of inertiainertia

Seismograms are Seismograms are recorded earthquakes recorded earthquakes on paperon paper