Earthquakes Release Earthquakes Release EnergyEnergy

Seismic WavesSeismic Waves

►The vibrations caused by earthquakesThe vibrations caused by earthquakes

Focus and EpicenterFocus and Epicenter

►FocusFocus- the of an earthquake is the pint - the of an earthquake is the pint underground where the rocks first underground where the rocks first begin to move.begin to move.

►EpicenterEpicenter- the point of the Earth’s - the point of the Earth’s surface directly above the focus.surface directly above the focus. Scientists often name an earthquake after Scientists often name an earthquake after

the city that is closest to the epicenterthe city that is closest to the epicenter

Waves and EnergyWaves and Energy

Earthquakes produce three types of Earthquakes produce three types of waveswaves►Primary WavesPrimary Waves►Secondary WavesSecondary Waves►Surface WavesSurface Waves

Primary WavesPrimary Waves

►These are the fastest seismic wavesThese are the fastest seismic waves►Also called P wavesAlso called P waves►Average speed 5km/sec (3mi/sec)Average speed 5km/sec (3mi/sec)►Can pass through solids, liquids and Can pass through solids, liquids and

gasesgases

Secondary wavesSecondary waves

►Also called S- WavesAlso called S- Waves►Travel at about ½ the speed of P-Travel at about ½ the speed of P-

waveswaves►Start at the same time as P-wavesStart at the same time as P-waves►Can travel through solids (rocks) NOT Can travel through solids (rocks) NOT

through liquids or gasesthrough liquids or gases

Surface WavesSurface Waves►Seismic waves that move along Earth’s Seismic waves that move along Earth’s

surface, not through it’s interiorsurface, not through it’s interior►Make the ground roll or shake from side Make the ground roll or shake from side

to sideto side►Cause the largest ground movements and Cause the largest ground movements and

the most damage.the most damage.►The slowest type of seismic wavesThe slowest type of seismic waves

SeismographSeismograph►An instrument that constantly records An instrument that constantly records

ground movementsground movements

Read page 116 (B), using seismographs

Locating EarthquakesLocating Earthquakes► Scientist must have at least 3 seismic Scientist must have at least 3 seismic

stationsstations1)1) Scientist find the difference between the Scientist find the difference between the

arrival times of the P and S waves at each arrival times of the P and S waves at each of the three stationsof the three stations

2)2) The time difference is used to determine The time difference is used to determine the distance of the epicenter from each the distance of the epicenter from each station. The greater the difference in station. The greater the difference in time, the farther away the epicenter is.time, the farther away the epicenter is.

3)3) A circle is drawn around each station, with A circle is drawn around each station, with a radius corresponding to the epicenters a radius corresponding to the epicenters distance from that station. The point distance from that station. The point where the 3 circles meet is the epicenterwhere the 3 circles meet is the epicenter