earthquakes and tsunamis - environmental engineering - lecture slides pdf

8/15/2019 Earthquakes and Tsunamis - Environmental Engineering - Lecture Slides PDF

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What is an Earthquake?

• Ground movement caused by the sudden release of

seismic energy due to tectonic forces.

The focus of an earthquake is theactual location of the energy

released inside the Earth’s crust.

The epicentre is the point on the

Earth’s surface directly above the

focus.

D o c s i t y . c o m


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Why do earthquakes occur?• Seismic energy is usually caused by the brittle

failure (fracturing) of rocks under stress.

• This commonly occurs due to movement along

tectonic plate boundaries

Figure showing the

distribution ofearthquakes

around the globe



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Earthquake Magnitude

Magnitude Earthquake Effects Approx. number

each year

< 2.5 Usually not felt, but recorded 900,000

2.5-5.4 Often felt, only minor damage 30,000

5.5-6.0 Slight damage to buildings andother structures

500

6.1-6.9May cause a lot of damage in very

populated areas 100

7.0-7.9 Major earthquake. Seriousdamage.

20

> 8.0 Great earthquake. Can be totallydestructive near the epicentre.

1 every 5-10 years



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Earthquake Hazards

• These are important hazards to understand: – the natural hazard that on average kills the highest

number of people per year (> 1 million during the pastcentury)

– commonly strikes without warning

– no time for evacuation

– not a predictable trend to earthquake numbers,magnitude or location

• 1000's of large earthquakes every year

• ~ 20 are > M7.0 and these account for 90% of the energyreleased and 80% of all the fatalities



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How do we mitigate the hazard from

earthquakes?

• Reinforce buildings

• Education

• Disaster plan



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Earthquakes and Tsunami’s

• An earthquake under the ocean has the

potential to form a tsunami.

• The earthquake must vertically displaceoverlying water (extensional or

compressional faults - not transform)

Extension Compression

TransformD o c s i t y . c o m


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How does an earthquake form a

tsunami?



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2004 South Asian

Boxing Day event

• Biggest earthquake in40 years!

• Magnitude 9.2

• 150 km off the westof Northern Sumatra

• Generated adisastrous tsunami in12 countries



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Second largest recorded earthquake• The earthquake occurred at a convergent tectonic

plate boundary (subduction zone)

• An estimated 1,600 km (994 miles) of faultline slipped about 15 m (50 ft)!

• The earthquake released 20 x 1017 Joules ofenergy

– Equivalent to:• 475,000,000 kg of TNT

• 23,000 Hiroshima atomic bombs!



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tsunamis?

• Monitoring – process is very technology-

intensive• high costs for many poorer countries

– often no technology available tomonitor local tsunamis

• for example,

• Papua New Guinea has no monitoringstations

– reliant on the Pacific TsunamiWarning Center

• tsunami in 1998 was not detected



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tsunamis?

• Building restrictions in

hazard prone areas – In Hawaii, Hilo harbor anddowntown was destroyed bythe tsunamis of 1946 and

1960 – The town is now rebuilt on

higher ground and thedevastated area is a park



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tsunamis?

• Seawall construction – cause early wave breaking

– prevent wave run up into urban areas



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tsunamis?

• Education – warning systems

– evacuation plans – general understanding of

the hazards involved

Punishment

From God

45%

Natural event

35%

Bomb 20%

Population reaction:Papua New Guinea (1998)


earthquakes and tsunamis - environmental engineering - lecture slides pdf

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