75TH ANNUAL SEISMIC GAMES

What is seismology?

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

 •Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs.

How are SEISMIC WAVES detected?

DETECTION OF SEISMIC WAVES

•Seismic waves are detected using an instrument known as a seismograph.

• John Milne was the English seismologist and geologist who invented the first modern seismograph in 1880.

DETECTION OF SEISMIC WAVES

•Most seismographs have a mass or an object attached which can move relative to the frame of the instrument.

•When seismic waves arrive at a seismograph station, the frame (which is fixed to the solid earth) moves with the earth but the mass attached does not because of it's inertia.

Types of Seismic waves

Body waves

•Travels through the interior of the earth

•Arrive before surface waves

•Higher frequency than surface waves

Body waves: Primary

•Fastest kind of seismic wave and is the first to arrive at a seismic station

Can move through solid, liquid and gas

•Pushes and pulls the rock it moves through

•Particles move in the same direction as the wave

Body waves: secondary

•Slower than a primary wave

•Can only move through solid rock

•S waves move rock particles up and down, or side-to-side (perpendicular to the direction that the wave is travelling in).

Surface waves•Travels only through the crust

•Lower frequency than body waves

•Almost entirely responsible for the damage and destruction associated with earthquakes

Surface waves: love•Named after A.E.H. Love, a British mathematician

•Fastest surface waves

•Moves the ground from side-to-side

•Produces entirely horizontal motion

Surface waves: Rayleigh•Named after John William Strutt, Lord Rayleigh

•Rolls along the ground

•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 an earthquake is due to the Rayleigh wave

summary

Primary Waves Secondary Waves

Love Waves Rayleigh Waves

•P waves•Fastest wave•Passes through liquid, solid and gas•Moves tiny rock particles back and forth in line with the direction the wave is traveling

•S waves or shear waves•Slower than P waves•Can only pass through solid•Displaces rock particles outwards, pushing them perpendicular to the path of the waves

•Fastest surface waves •Moves the ground from side-to-side•Produces entirely horizontal motion

•Rolls along the ground•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 an earthquake is due to the Rayleigh wave

Body Waves Surface Waves

Locating the epicenter using seismic waves

Locating the epicenter•Epicenter - A point on the Earth’s surface that is directly above the hypocenter or focus, the point where an earthquake originates

•Hypocenter/focus – the position where the strain energy stored in the rock is fi rst released, marking the point where the fault begins to rupture

Locating the epicenter

•To locate the epicenter of an earthquake, seismologists use at least 3 seismographs.

•Using the seismographs, the arrival times of the p-waves and s-waves are determined.

Diagram 1. In the example P-wave is at 0 minutes and S-wave is at 5 minutes

Locating the epicenter

•After fi nding out the arrival time, the time diff erence needs to be computed (S-wave arrival time minus P-wave arrival time equals time diff erence).

5 minutes – 0 minutes = 5 minute time interval, based on the example.

Locating the epicenter

•Next step is to use the Earthquake Time Travel Graph. Place a sheet of paper along the Time Travel axis of the graph, then mark the time diff erence computed earlier.

Diagram 2

Locating the epicenter

•Slide the paper across the graph until the bottom mark lines up with the p-wave line and the top mark lines up with the s-wave line.

Diagram 3

Locating the epicenter

•Read where the paper is along the Epicenter distance axis. This will give you the distance of the seismograph from the epicenter of the earthquake.

Diagram 4. Based on the example the epicenter distance is 3,400 km.

sources

•http://www.geo.mtu.edu/UPSeis/waves.html •http://regentsprep.org/regents/earthsci/eqwaves.htm•http://en.wikipedia.org/wiki/Hypocenter•http://en.wikipedia.org/wiki/Epicenter•http://inventors.about.com/od/sstartinventions/a/seismograph.htm•http://science.howstuffworks.com/environmental/earth/geophysics/question142.htm•http://en.wikipedia.org/wiki/Seismometer