EARTHQUAKES
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Rocks move along faults…
A fault is a fracture or break in theEarth’s lithosphere where blocks ofrock move past each other.
Along some parts of the fault, therock on either side may slide alongslowly and constantly.
Along other parts of the fault, therocks may stick or lock together.
Rocks bend as stress is put on them.
As the stress increases, the rocksfinally break free.
Stress in Lithosphere =Earthquake
A sudden release of stress in thelithosphere causes an earthquake.
An earthquake is a shaking of theground caused by suddenmovement of large blocks of rockalong the fault.
Most faults are located along plateboundaries, so most earthquakesoccur in these areas.
About 80% of the earthquakes occur inthe belt around the edges of the PacificOcean.
What is the best known fault inthis belt in the United States?
San Andreas Fault in California.
It forms part of the boundarybetween the North American Plateand the Pacific Plate.
Unlike many other faults, parts ofthe San Andreas Fault can be seenon the surface of the ground.
Strength of an earthquake
The strength of an earthquakedepends in part on…
How much stress builds up beforethe rocks move
The distance the rocks movealong the fault
3 Types of Faults
Normal Faults
Reverse or Thrust Faults
Strike-Slip Faults
The blocks of rock along different types offaults move in different directions,depending on the kinds of stress they areunder.
More than one type of fault may bepresent along the same plate boundary.
Normal Faults
Earthquakes along normal faults arecommon near boundaries where tectonicplates are moving apart. (Divergent Boundaries)
Pull
Pull
Directionrocks move
Example: GreatRift Valley ofAfrica
The Great Rift Valley
Reverse or Thrust Faults
Stress that presses rock together causesreverse faults. (Convergent Boundaries)
The Himalaya Mountains, which rise in the areawhere the Indian Plate is pushing into the EurasianPlate, have may earthquakes along reverse faults.
Directionrocks move
Push
Strike-Slip Faults
These faults can occur where platesscrape past each other. (Transform Boundaries)
The San Andreas Fault is a strike-slip fault.
Earthquakes ReleaseEnergy
Earthquake energy travels outward in alldirections – up, down, and to the sides.
The energy travels as seismic waves,which are vibrations caused byearthquakes.
All earthquakes start beneath Earth’ssurface.
The focus of an earthquake is the pointunderground where the rock first beginsto move.
Seismic waves travel outward from theearthquakes focus.
The epicenter is the point on the Earth’ssurface directly above the focus.
Earthquakes produce 3types of seismic waves…
Primary Waves
Secondary Waves
Surface Waves
Each type moves through materialdifferently.
Scientists learn about Earth’s layersby studying the paths and speedsof seismic waves traveling throughEarth.
Primary Waves, P Waves
These are the fastest waves that are thefirst to reach any particular location afteran earthquake occurs.
Primary waves can travel through solids,liquids, and gases.
Primary waves = P waves
Secondary Waves, S Waves
Secondary waves travel through Earth’sinterior at the same time as primary waves,but at about half the speed.
Therefore, they are the second waves toarrive at any particular location after anearthquake.
Secondary waves can travel through rock,but unlike primary waves they cannot travelthrough liquids or gases.
Secondary waves = S waves
Surface Waves
Surface waves move along Earth’ssurface, not through its interior.
They make the ground roll up anddown or shake from side to side.
Surface waves cause the largestground movements and the mostdamage.
They travel more slowly than theother types of seismic waves.
Seismic waves can bemeasured…
There are seismic stations all over theworld where ground movements aremeasured.
A seismograph is an instrument thatconstantly records ground movement.
The recording produced by aseismograph is called a seismogram.
By studying seismograms, scientists candetermine the locations and strengths ofearthquakes.
Seismograph and Seismogram
The height of the wiggles on theseismogram indicates the amount ofground movement produced by seismicwaves at the seismograph’s location.
Interesting Facts…
Most injuries and deaths due toearthquakes are not directlycaused by the movement of theground.
They are caused by collapsingbuildings and other structures andby fires.
After an earthquake, fires may startdue to broken natural-gas lines,broken electrical power lines, oroverturned stoves.
How do we measure anearthquakes magnitude?
The Richter Scale : An earthquake’smagnitude is based on how fast theground moves at the seismic station.
The Modified Mercalli Intensity Scale: Theintensity scale consists of a series ofcertain key responses such as peopleawakening, movement of furniture,damage to chimneys, and finally - totaldestruction.
Review Questions…
What is a fault?
How does an earthquake occur in thelithosphere?
What are the 3 types of faults? How arethey different?
What are seismic waves?
What are the 3 types of seismic waves?How are they the same and how are theydifferent?
How are seismic waves measured?
How do we measure an earthquakesmagnitude?