EarthquakesEarthquakes occur when rock masses in the Earth’s crust move suddenly along ruptures called faultsfaults. The rock movement may occur vertically, horizontally or in combination.

Earthquakes and Volcanoes

FocusFocus – The location on a fault where the greatest amount of movement is. It is far underground.

EpicenterEpicenter- the point on the surface directly above the focus

EnergyEnergy travels outward from the focus in waveswaves.

The intensityintensity of an earthquake can be expressed on the Richter ScaleRichter Scale.

Earthquake Waves

Types of Earthquake Types of Earthquake WavesWaves

An “S WaveS Wave” (“shear” or “secondary”) causes material to move up and down as it goes through.

A “P Wave (or “Primary” or “Pressure” wave) consists of compressions which move straight through a material.

p & s waves animations

P wavesP waves can travel through solids, liquids and gases.S wavesS waves can travel only through solids.

Earthquake waves are detected on a SeismometerSeismometer.

How a Seismograph Works.swf

Seismometers produce a seismographseismograph

PP waves travel fasterfaster than S waves. The farther awayfarther away from the epicenter, the greater the the greater the difference in arrival timedifference in arrival time of P and S waves.

This graph lets a person calculate the distance distance from the from the epicenterepicenter, knowing the time interval time interval between between arrival of P arrival of P and S and S waves.waves.

An earthquake occurred on June 23, 1946 and these were recorded times of P and S wave arrivals at different stations.

For example, the P/S time interval in Saskatoon was 0 h, 2 min and 37 seconds.

0 2 37

2 minutes and 37 seconds would correspond to about 1400 1400 kmkm.

A circle with a 1400 km radius could be drawn on a map with Saskatoon in the middle. The epicenter would be somewhere on this circle.

1400

km

Saskatoon

The procedure could be repeated for all the other stations. The point where all 5 circles overlapcircles overlap would be the epicenterepicenter of the earthquake.

Eg. All 3 circles interceptintercept at this point, so this would be the location of the epicentreepicentre

In a different example, circlescircles were drawn

around 3 seismic stations.

High Risk Zones for Earthquakes:1. They are located near a plate boundary.2. There is a history of earthquakes in the region.3. There are numerous active faults.The West Coast of North America has had many earthquakes.

More Information on Earthquakes from National Geographic

Volcanoes

A volcano is formed when magmamagma makes its way to the surfacesurface.

Three types of VolcanoesThree types of Volcanoes

The lava is quite liquidliquid and flows down the sides. These volcanoes are very largevery large but not very steep. An example is Hawaiian volcanoes like Kilauea

form mainly from explosive eruptions of lava. Blown violently into the air, the erupting lava breaks apart into fragments called cinders. The fallen cinders form a cone around the volcano's central vent.

such as Mount St. Helens, build from both explosive and slow flowing eruptions. Layers of tephra alternating with layers of viscous lava flows create steep-sided, often symmetrical cones that we think of as the classic volcano shape.

Effects of Effects of VolcanoesVolcanoes

Not all volcanoes have all these effects. EarthquakeEarthquakess always occur when a volcano erupts

More Information on Volcanoes from National Geographic