Seismic Events

Causes of EarthquakesDistribution of EarthquakesMeasuring EarthquakesThe Effects of Earthquakes

WHAT IS AN EARTHQUAKE?

•Sudden release of energy

•Creates seismic waves (similar to waves through water)

•Waves spread out spherically

•Greatest damage occurs at the epicentre

P & S waves

• Two types of wave explained.

• Circle of power demonstration.

• P & S travel through the interior of the earth – out from the epicentre.

P-waves

• P waves (primary waves) are compressional waves.

• In solids, these waves generally travel almost twice as fast as S waves.

• P waves can travel through any type of material. • In air, these pressure waves take the form of

sound waves.• They travel at are 330 m/s in air, 1450 m/s in

water and about 5000 m/s in granite. (m/s = meters per second.)

PRIMARY WAVE ‘p-wave’

– Longitudinal– Sudden jolt felt as the quake hits.

S waves

• S waves (secondary waves) are transverse which means that the ground is displaced perpendicularly (sideways) to the direction of movement.

• The ground moves alternately to one side and then the other. (or up or down… depending on the direction of the wave.)

• S waves can travel only through solids, as fluids (liquids and gases)

• Their speed is about 60% of that of P waves in a given material.

• S waves arrive second in a seismic station because of their slower speed.

• They are sometimes called shear waves.

SECONDARY WAVE ‘s-wave’

– Transverse (shake at right angles to the direction of the wave.)– Sustained shaking felt as quake hits

L wave• The particle motion of a Love wave forms a horizontal line

perpendicular to the direction of movement (i.e. are transverse waves). • Since Love waves travel on the Earth's surface, the strength (or

amplitude) of the waves decrease exponentially with the depth of an earthquake.

• Surface waves therefore decay more slowly with distance than do body waves, which travel in three dimensions.

• Large earthquakes may generate Love waves that travel around the Earth several times before dissipating.

• Since they decay so slowly, Love waves are the most destructive outside the immediate area of the focus or epicentre of an earthquake. They are what most people feel directly during an earthquake.

LOVE WAVE. L-wave

Lets get an overview.

Read the book pages 24 - 27

Worksheet– Sheet 5 & 6

Distribution of Earthquakes

• What is the distribution of Earthquakes?

• Read the section in the book then use following statements to make a short paragraph explaining the distribution of Earthquakes.

– The Pacific Ring of Fire– 70% of Earthquake and Volcanic activity is found around

it’s edges– Volcanoes and Earthquakes are frequent.– Represents the destructive plate boundaries around the

edges of the plate.

THE RICHTER SCALE

•Invented by Charles Francis Richter in 1935

•Measure of an earthquake’s magnitude

•Scale between 0 and 10(theoretically)

•Logarithmic scale

How does a seismograph work?

Measurement – using a seismograph http://www.wwnorton.com/college/geo/egeo/flash/8_3.swf

Magnitude: A measure of the size of an earthquake.Frequency: Is the number of occurrences of a repeating event per unit time.

• Worksheet 1.10

Effects of an Earthquake

• National Geographic 4 min clip – the effects of earthquakes. Link

• The text book explains several effects– Liqufaction– Landslides/avalanches– Effects on built environment– Tsunamis

Liquefaction Explained

http://www.youtube.com/watch?v=PwvvYxSZ7PI

http://www.youtube.com/watch?v=sV2WeR670ls

What happened in San Francisco?

Year 9 Worksheet 1.20. 1.21.

Aftershocks

A tremor (or one of a series of tremors) occurring after the main

shock of an earthquake : Video

Mass Movement when masses of rock, earth, or debris move down a slope.

• Mass movement is movement of masses of bodies of soil, bed rock, rock debris, soil, or mud.

• They may be very small or very large, and can move at slow to very high speeds.

• Usually occur along steep-sided hills and mountains because of the pull of gravity.

• An avalanche is a sudden flow of a large mass of snow or ice down a slope, sometimes at speeds exceeding 160 km/hr.

• Mass movement and avalanches can be started when earthquakes destabilise rock and start a slide.

• http://www.youtube.com/watch?v=JrV4uCVwmfk

Tsunami -

• What is a Tsunami?• Tsunami -a large wave, or series of

waves, caused when an earthquake causes massive undersea crust movements and/or collapses which displace the water above.

• Earthquakes deform the ocean floor, pushing the overlying water up into a tsunami wave.

Video of Boxing Day Tsunami

• Tsunami – Caught on Camera

Scale showing the size of the tsunami waves that hit Indonesia.

Case study – Boxing Day Tsunami.

• Make a 5 point PowerPoint to get the key facts about the Tsunami across. Use case study on page 29 to help.– Why it occurred.– Where it started.– What it effected.– Positive side effects.

Plenary

Earthquakes don’t kill people, buildings do.

Starter

Earthquakes don’t kill people… BUILDINGS DO!!!

Port-au-Prince, Haiti

BUILDINGS• Side to side forces that cause collapse

• Damage caused by lateral forces

• Building tilted due to first story building collapse

HOW TO BUILDSMALL BUILDINGS

• Shear walls

• Concentrate the damage

• Cross Bracing• Reinforced stone wall

• Cross bracing • Construction of Pearl River Tower, China

LARGER BUILDINGS

• Base isolation

• Left: regular building Right: base isolated building

• Ways of improving resistance to earthquakes

SKY SCRAPERS• Tuned Mass Damper

• The tuned mass damper at the top of the Taipei 101

HAITI

• A poor neighbourhood, Port-au-Prince

CHILE

• Damaged building, Concepcion