Natural Hazards What is an Earthquake? Ground movement caused by the sudden release of seismic...
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Transcript of Natural Hazards What is an Earthquake? Ground movement caused by the sudden release of seismic...
What is an Earthquake? Ground movement caused by the sudden
release of seismic energy due to tectonic forces.
The focus of an earthquake is the actual location of the energy released inside the Earth’s crust.
The epicentre is the point on the Earth’s surface directly above the focus.
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 of earthquakes around the globe
Earthquake MagnitudeMagnitude 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 and other structures
500
6.1-6.9 May cause a lot of damage in very populated areas
100
7.0-7.9 Major earthquake. Serious damage.
20
> 8.0 Great earthquake. Can be totally destructive near the epicentre.
1 every 5-10 years
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 past century)
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
energy released and 80% of all the fatalities
Earthquakes and Tsunami’sAn earthquake under the ocean has the
potential to form a tsunami.The earthquake must vertically displace
overlying water (extensional or compressional faults - not transform)
Extension CompressionTransform
2004 South Asian Boxing Day event
Biggest earthquake in 40 years!
Magnitude 9.2150 km off the
west of Northern Sumatra
Generated a disastrous tsunami in 12 countries
Tsunami
Animations of the Boxing Day tsunami showing how the tsunami radiated from the entire length of the 1,600 km (994 miles) rupture.
Above: Countries most effected by the tsunami
How do we mitigate the hazard from tsunamis?Monitoring
process is very technology-intensive
high costs for many poorer countries
Building restrictions in hazard prone areas
Seawall construction
Cyclone CategoriesCategory Wind Speed
(mph)Damage at
LandfallStorm Surge
(feet)
1 74-95 Minimal 4-5
2 96-110 Moderate 6-8
3 111-130 Extensive 9-12
4 131-155 Extreme 13-18
5 > 155 Catastrophic 19+
The Saffir-Simpson Hurricane Scale
A low-pressure system is a large body of circulating air with low pressure at its center and higher pressure outside of the system.
It has warm, moist air and generally brings stormy weather with strong winds.
Pressure Systems
Hurricane KatrinaHurricane Katrina
was the most costly and most deadly hurricane in the history of the USA.Category 5
At least 1,836 fatalitiesDamage estimated at
US$ 81.2 billion
What damage is produced? Wind
responsible for the loss of power and utilities
wind damage affects larger areas than surge
flying debris tree loss