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Natural Hazards

Natural Hazards. 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.

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Natural Hazards

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  1. Natural Hazards

  2. 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.

  3. 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

  4. Earthquake Magnitude

  5. San Francisco - Great Earthquake • Magnitude 7.7 - 8.3

  6. 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

  7. Earthquakes and Tsunami’s • An 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 Compression Transform

  8. How does an earthquake form a tsunami?

  9. 2004 South Asian Boxing Day event • Biggest earthquake in 40 years! • Magnitude 9.2 • 150 km off the west of Northern Sumatra • Generated a disastrous tsunami in 12 countries

  10. 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

  11. 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

  12. Same Storm - Different Name

  13. Cyclone Categories The Saffir-Simpson Hurricane Scale

  14. Pressure Systems A low-pressure systemis 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.

  15. Hurricane Katrina • Hurricane Katrina was the most costly and most deadly hurricane in the history of the USA. • Category 5 • At least 1,836 fatalities • Damage estimated at US$ 81.2 billion

  16. What damage is produced? • Wind • responsible for the loss of power and utilities • wind damage affects larger areas than surge • flying debris • tree loss

  17. What damage is produced? • Flying debris • debris propelled at high speeds

  18. How do we mitigate the hazard from a cyclone? • Monitoring • early warning systems • Infrastructure • cyclone walls • communal shelters • Education and planning

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