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SEISMIC WAVESSEISMIC WAVES

What is Seismology?What is Seismology?

• Seismology is the study of earthquakes and seismic waves that move through and around the earth

• A seismologist is a scientist who studies earthquakes and seismic waves

What Are Seismic What Are Seismic Waves?Waves?

•Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth.

•They are the energy that travels through the earth and is recorded on seismographs.

•Seismic comes form the Greek word meaning to “SHAKE”

Types of Seismic WavesTypes of Seismic WavesEarthquakes produce 3 basic Earthquakes produce 3 basic

kinds of wave kinds of wave motions:motions:

•P Waves

•S Waves

•L Waves

Primary WavesPrimary Wavesaka compressional wavesaka compressional waves

• back and forth wave motion

• this motion alternates squeezing and stretching the rock through which it passes

• can travel through any material- solid rock, magma, ocean water, and air

• travel about twice as fast as S waves.

S Wave-Secondary Waves S Wave-Secondary Waves aka Shear Waveaka Shear Wave

• side to side waves

• cause particles to move at right angles to the direction the waves are traveling

• can travel through solids but not through liquids or gases

L Waves-Surface WavesL Waves-Surface Waves• When P and S

waves reach the surface, they set up a third wave called L waves or surface waves

• These waves move like “ripples” on a pond and travel more slowly than P or S waves

P Waves(Primary orCompressional

)

S Waves (Secondary or Shear)

L Waves (Surface)

Back & Forth motion

Side to Side motion

“Ripples” on a pond

travel through: solid rock, magma, ocean water, and air

travel thru: Solids but not liquids or gases.

Surface Waves

Fastest, first to reach seismograph

Twice as Slow as P waves, reach second

Slowest, last to reach seismograph

How Do Seismologists How Do Seismologists Measure earthquake waves?Measure earthquake waves?

• Seismographs are instruments that detect and record earthquake waves

• Earthquakes are measured according Earthquakes are measured according to the to the RICHTER SCALE. This is based . This is based on the size of the largest seismic on the size of the largest seismic wave produced by the earthquake. wave produced by the earthquake.

• The damage done by an earthquake The damage done by an earthquake is measured according to the is measured according to the MERCALI SCALE.

                                                                                                                        

                                             

Modified Mercalli Scale

Intensity

Verbal Description

Magnitude

Witness Observations

I Instrumental 1 to 2 Detected only by seismographs

II Feeble 2 to 3 Noticed only by sensitive people

III Slight 3 to 4 Resembling vibrations caused by heavy traffic

IV Moderate 4 Felt by people walking; rocking of free standing objects

VRather Strong

4 to 5 Sleepers awakened and bells ring

VI Strong 5 to 6 Trees sway, some damage from overturning and falling object

VII Very Strong 6 General alarm, cracking of walls

VIII Destructive 6 to 7 Chimneys fall and there is some damage to buildings

IX Ruinous 7 Ground begins to crack, houses begin to collapse and pipes break

X Disasterous 7 to 8 Ground badly cracked and many buildings are destroyed.There are some landslides

XIVery

Disasterous 8

Few buildings remain standing; bridges and railways destroyed;water, gas, electricity and telephones out of action.

XII Catastrophic8 or

greater Total destruction; objects are thrown into the air,much heaving,shaking and distortion of the ground

What Do Seismologists What Do Seismologists Learn From Seismic Waves?Learn From Seismic Waves?• Seismologists use seismographs to record

the amount of time it takes seismic waves to travel through different layers of the Earth.

• As the waves travel through different densities, seismologists can deduce the type of material the waves are travelling through.

• The results can provide a snapshot of the Earth’s internal structure and help us to locate and understand fault planes and the stresses and strains acting on them.

Results of Seismic WavesResults of Seismic Waves

TRIVIA QUESTIONSTRIVIA QUESTIONS

• What is the most earthquake prone state in the US?

ALASKA- it’s one of the most seismically active regions of the world

• Why does California have so many EQ’s and NY does not?

CA is on a plate boundary• What states have the least amount of EQ’s?Florida and N. Dakota• Where and when did the largest EQ occur in

the 20th Century?1960 Chilean EQ: Magnitude 9.6 and broke a

fault over 1,000 miles long!