what are earthquakes, Lecture notes of Earth Sciences

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What are Earthquakes?

• The shaking or trembling caused by the sudden

release of energy

• Usually associated with faulting or breaking of rocks

• Continuing adjustment of position results in

aftershocks

What is the Elastic Rebound Theory?

• (^) Explains how energy is stored in rocks - (^) Rocks bend until the strength of the rock is exceeded - (^) Rupture occurs and the rocks quickly rebound to an undeformed shape - (^) Energy is released in waves that radiate outward from the fault

Seismographs record

earthquake events

At convergent boundaries, focal depth increases along a dipping seismic zone called a Benioff zone

Where Do Earthquakes Occur and How Often?

~80% of all earthquakes occur in the circum-Pacific belt

• (^) most of these result from convergent margin activity
• (^) ~15% occur in the Mediterranean-Asiatic belt
• (^) remaining 5% occur in the interiors of plates and on spreading ridge centers
• (^) more than 150,000 quakes strong enough to be felt are recorded each year

Seismic WaveSeismic Wave

• Seismic wavesSeismic waves are the waves of energyare the waves of energy

caused by the sudden breaking of rockcaused by the sudden breaking of rock

within the earth or an explosion. They arewithin the earth or an explosion. They are

the energy that travels through the earththe energy that travels through the earth

and is recorded on seismographs.and is recorded on seismographs.

• There are several different kinds of seismicThere are several different kinds of seismic

waves, and they all move in differentwaves, and they all move in different

ways. The two main types of waves areways. The two main types of waves are

body waves body waves andand surface wavessurface waves ..

What are Seismic Waves?

• (^) Response of material to the

arrival of energy fronts released

by rupture

• (^) Two types:
  • (^) Body waves
    • (^) P and S
  • (^) Surface waves
    • (^) R and L

Body WavesBody Waves

• P Waves (compression wave)P Waves (compression wave)
• The first kind of body wave is theThe first kind of body wave is the P waveP wave oror primaryprimary

wavewave. This is the fastest kind of seismic wave. The P wave. This is the fastest kind of seismic wave. The P wave

can move through solid rock and fluids, like water or thecan move through solid rock and fluids, like water or the

liquid layers of the earth. It pushes and pulls the rock itliquid layers of the earth. It pushes and pulls the rock it

moves through just like sound waves push and pull the air.moves through just like sound waves push and pull the air.

Body Waves

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vp 

  vs   3 s p v v Bulk modulus  =  P / (  V/ V)  Shear modulus or „rigidity“  = (  F/A) / (  L/L)  Young´s or „stretch“ modulus E = (F/A)/ (  L/L) and Poisson ratio  = (  W/W) / (  L/L)  Deformation of material samples for determining elastic moduli P. Bormann, NMSOP

Stop and ThinkStop and Think

• Have you ever heard a big clap ofHave you ever heard a big clap of thunder and heard the windows rattlethunder and heard the windows rattle at the same time?at the same time?
• The windows rattle because the soundThe windows rattle because the sound waves were pushing and pulling on thewaves were pushing and pulling on the window glass much like P waves pushwindow glass much like P waves push and pull on rock. Sometimes animalsand pull on rock. Sometimes animals can hear the P waves of ancan hear the P waves of an earthquake. Usually we only feel theearthquake. Usually we only feel the bump and rattle of these waves.bump and rattle of these waves.

Surface Waves: R and L waves

• (^) Surface Waves
  • (^) Travel just below or along the ground’s surface
  • (^) Slower than body waves; rolling and side-to-side movement
  • (^) Especially damaging to buildings

Surface WavesSurface Waves

• Love WavesLove Waves

• The first kind of surface wave is called aThe first kind of surface wave is called a Love waveLove wave ,,

named after A.E.H. Love, a British mathematician whonamed after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind ofworked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moveswave in 1911. It's the fastest surface wave and moves the ground from side-to-side.the ground from side-to-side.

Surface WavesSurface Waves

• Rayleigh WavesRayleigh Waves
• The other kind of surface wave is theThe other kind of surface wave is the^ Rayleigh waveRayleigh wave , named for John, named for John William Strutt, Lord Rayleigh, who mathematically predicted theWilliam Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls along theexistence of this kind of wave in 1885. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because itground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the samerolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from andirection that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much largerearthquake is due to the Rayleigh wave, which can be much larger than the other waves.than the other waves.

Locating the Source of an Earthquake

• Need distance of earthquake from^ three^ stations to

pinpoint location of earthquake:

• Computer calculation
• Visualize circles drawn around each station for appropriate

distance from station, and intersection of circles at

earthquake’s location

• Method is most reliable when earthquake is near surface

How is an Earthquake’s Epicenter Located?

Seismic wave behavior

• (^) P waves arrive first, then S waves, then L and R
• (^) Average speeds for all these waves is known
• (^) After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.