EARTHQUAKES

Did you know…Globally, there are over 50o ooo earthquakes each year

100 000 can be felt

100 cause damage to Earth’s surface

Introduction video

What is an earthquake?Trembling or shaking of

Earth’s crust created by a release of energy stored in the rocks beneath the surface

Rocks between two plates bend until the strain is too great, and then the rock breaks, releasing seismic waves that produce shaking.

Where?Focus: actual location in the earth where movement originates

Epicentre: point on the surface directly above the earthquake's focus VIDEO

• The seismic waves produced by an earthquake radiate out from its focus.

• These waves cause the earth to vibrate in response to the pushing and pulling forces that are applied to them.

• Two category of waves (each with subsets):

– Body waves: Primary(P), Secondary(S)

– Surface waves: Love(L), Rayleigh (R)

• Body waves arrive first since they are the fastest. Body waves are broken down into two types:

•Primary (P) waves, which cause the rock particles through which they pass to shake back and forth (compression effect) and can result in a noise similar to a sonic boom.

•Secondary (S) waves make the particles vibrate both vertically and horizontally (shearing effect).

• Note that Body waves travel mostly downwards into the interior of the earth and are rarely felt by humans. They disappear as soon as they hit a gas (atmosphere).You can hear Body waves, but it is the next set of waves that actually move outwards along the ground – causing damage.

SEISMIC WAVES: SEISMIC WAVES: Body WavesBody Waves

Surface waves arrive last and as their name implies they travel along the surface of the earth. It is these waves that cause damage. They begin with:

•Love/Long (L) waves that move the ground from side to side, again, in a shearing effect. These are followed by:

• Rayleigh (R) waves which are similar to ocean waves. These cause surface materials to move in a vertical circle just as a floating object would move as a sea wave passes under it. These waves are responsible for most of the damage to buildings.

• The greatest shaking occurs near the centre of large earthquakes. It is here that all the waves originate.

SEISMIC WAVES: SEISMIC WAVES: Surface WavesSurface Waves

Measuring and Predicting

• A seismograph is used to detect and record seismic waves

• There are more than 10 000 seismograph stations worldwide

• Video: How seismographswork

The violence of an earthquake can be measured using two different scales:

The Richter scale - devised in 1932 by Charles RichterThe Richter scale actually uses a numerical value. It is important to note that this scale is logarithmic in nature meaning that the numbers measure in factors of 10. In other words, an earthquake measuring a 4 on the richter scale is 10x as damaging as an earthquake that measured 3 on the richter scale.

The Mercalli scale - devised in 1905 by a gentlemen named Mercalli. The Mercalli scale is a visual scale based on the destruction that occurs.

MEASURING THE DAMAGEMEASURING THE DAMAGE

Earthquake Hazards-Ground shaking causes buildings to collapse

-Liquefaction occurs when loose soil temporarily turns from a solid to a liquid during an earthquake- resembles quicksand and will not support buildings

Earthquake Hazards

- Tsunami- a series of large destructive sea waves caused by an earthquake under the ocean floor

Earthquake Hazards

- Aftershocks- a series of smaller earthquakes that occur after the main tremor. Usually smaller than initial quake but can cause considerable damage

-Fire- due to broken gas and water mains and fallen electrical wires

Earthquake Hazards

Earthquakes in Canada?VIDEO There are 3-4 earthquakes in Canada each

DAYMost can only be detected by seismographsOnly one person has died in Canada as a

result of an earthquake in Montreal in 1732Read the case study about Vancouver,

answer questions # 1,2,5