Seismic Waves

Chapter 15.4

Types of Seismic Waves• Earthquakes occur when rocks

beneath the Earth’s surface move• When pressure in the rock builds

up because of forces found deep inside the Earth the rock breaks or changes shape, which releases energy in the form of waves

• These waves are seismic waves, which are the waves produced by earthquakes– Seismic waves include P waves, S

waves, and surface waves– These waves ripple out in all

directions from the point where the earthquake occurred and can travel from one side of the Earth to the other

P Waves

• P waves, also known as primary waves, are longitudinal seismic waves

• They are called primary waves because they travel faster than other seismic waves and therefore arrive at distant points before other seismic waves

• P waves are made up of refractions and compressions of the rock inside Earth

• P waves compress and expand the ground as they move through it

S Waves

• S waves (secondary waves) are transverse seismic waves with crests and troughs

• S waves shake the ground up and down and side to side as they move through it

• S waves CANNOT travel through liquid– Since part of the Earth’s core is made of liquid S waves

do not travel directly through the Earth like P waves– Therefore, S waves CANNOT be detected on the side of

the Earth opposite of the Earthquake

Surface Waves• When P waves and S waves reach the Earth’s

surface they can create surface waves• A surface wave is a combination of longitudinal

and transverse waves that travels along the surface of a medium

• Surface waves move more slowly than P and S waves

• Surface waves cause more severe ground movements than P and S waves because they combine up and down and side to side motions that can make the ground roll like ocean waves

Surface Waves that Occur Underwater• Earthquakes that occur

underwater can produce huge surface waves on the ocean called Tsunamis– Tsunamis come in all sizes from

2 cm to 30 meters tall– Tsunamis can travel thousands

of kilometers– In the deep oceans waves that

cause tsunamis may begin at 1 meter high, but as they near land the tsunamis slow down in shallow waters to allow the waves in the back to catch up with the waves in the front and pile on

Detecting Seismic waves• To detect and measure earthquake waves scientists use

seismographs• A seismograph records the ground movements caused by

seismic waves as they move through the earth– The frame of the seismograph is attached to the ground, so the

frame shakes when seismic waves arrive– Seismographs use to have pens attached to the frame that

would wiggle back and forth on a roll of paper as the ground shook, but now electronic seismographs are used

• P waves arrive at the seismograph first followed by slower moving S waves– Scientists can tell how far away the earthquake was by

measuring the time of arrival between P and S waves– Scientists can tell where an earthquake occurred by using three

seismographs at three different locations

Seismographs • Seismographs can be used to find

valuable resources such as water and oil

• Geologists will set off explosives at the Earth’s surface

• Seismic waves from the explosions reflect from structures underground

• Seismographs are then used to locate the underground resources

Research Report:• You and a Partner will write an article for a boating

magazine about tsunamis• Include: Details about what causes tsunamis, why

they are dangerous, and explain what is being done to reduce damage from tsunamis

• Also include pictures• You may use Macs and your book for research• You will have today and tomorrow to finish– When you are done share it as a Google doc with

kajarikl@gmail.com