Natural Hazards

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Natural Hazards
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Introduction to Natural Hazards

• What is a Geohazard?
• Earth processes (involving the lithosphere,
  hydrosphere atmosphere) that, upon interaction
  with human activity, cause loss of life and
  property
• It is important to understand the human element
• without it, there would be no hazard
• because of it, the science of geohazards becomes
  more important every year
• mitigation reduction/prevention

Hazardousconditionor result
geo-process
humanprocess
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Why is the human element so critical?

• The Earths population is increasing
• more people living in hazard-prone areas
• populations are becoming hyper-concentrated
• consumption of resources
• examples
• today there are 6 billion people on Earth ( 50
  live in cities)
• by 2025, there will be 8 billion people ( 66
  in cities)
• of these cities, 40 are coastal
• prone to severe storm and tsunami damage
• and a large majority lie in areas subject to
  other geohazards (for example volcanoes and
  earthquakes)

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• CANNOT stop the geologic processes
• CANNOT stop the population growth/expansion

• Therefore, we must try to reduce (mitigate) the
  hazards through
• scientific study
• population education
• changes in engineering/building practices
• management plans and hazard response scenarios

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Types of Natural Hazards

• Volcanoes, floods, earthquakes, tornadoes,
  tsunamis, etc.
• can act adversely on human processes
• can occur
• without warning (e.g. earthquakes)
• with warnings (precursors) (e.g. satellite
  monitoring of cyclone tracks, or the presence of
  ground deformation at a volcano before an
  eruption)

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• To help mitigate the hazard we need to know
• Frequency vs. Magnitude
• F how often a given event occurs in a certain
  region
• M how powerful (amount of energy released) an
  event is
• for example, high M hazards happen with low F,
  but are much more destructive
• Scope
• S area affected by a given hazard
• local landslides, floods, earthquakes, fire
• regional tsunamis, volcanoes, larger
  earthquakes, cyclones
• global large volcanoes, global warming,
  meteorite impacts

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Practical Exercise 1.

• Impact of Environmental Disasters

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Earthquakes and Tsunamis
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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 the actual location
of the energy released inside the Earths
crust. The epicentre is the point on the Earths
surface directly above the focus.
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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 of earthquakes
around the globe
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Earthquake Magnitude
Magnitude Earthquake Effects Approx. number each year
lt 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
gt 8.0 Great earthquake. Can be totally destructive near the epicentre. 1 every 5-10 years
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San Francisco - Great Earthquake

• Magnitude 7.7 - 8.3

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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 (gt 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 gt M7.0 and these account for 90 of the
  energy released 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 Tsunamis

• 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
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How does an earthquake form a tsunami?
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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

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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 of
  energy
• Equivalent to
• 475,000,000 kg of TNT
• 23,000 Hiroshima atomic bombs!

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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
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A village near the coast of Sumatra lays in ruin
after the tsunami.
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How do we mitigate the hazard from tsunamis?

• Monitoring
• process is very technology-intensive
• high costs for many poorer countries
• often no technology available to monitor local
  tsunamis
• for example,
• Papua New Guinea has no monitoring stations
• reliant on the Pacific Tsunami Warning Center
• tsunami in 1998 was not detected

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How do we mitigate the hazard from tsunamis?

• Building restrictions in hazard prone areas
• In Hawaii, Hilo harbor and downtown was destroyed
  by the tsunamis of 1946 and 1960
• The town is now rebuilt on higher ground and the
  devastated area is a park

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How do we mitigate the hazard from tsunamis?

• Seawall construction
• cause early wave breaking
• prevent wave run up into urban areas

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How do we mitigate the hazard from 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)
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Practical Exercise 2.

• Locating Earthquake Epicentres

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Tropical Storms
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Same Storm - Different Name
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Tropical Cyclones

• Background
• Can be deadly!
• For example, in 1991 a large cyclone in
  Bangladesh killed gt138,000 people in just two
  days!

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Tropical Cyclones

• Most widespread destructive weather hazard
• For example Hurricane Floyd (1999)
• only a moderate level hurricane
• caused US5.6 billion in damage in the Bahamas
  and North Carolina (USA) and 57 fatalities

(Left) Three different cyclones spinning over the
western Pacific Ocean on August 7, 2006.
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How do cyclones form?
The above figure shows how cyclones form. The
green arrows show where warm air is rising. The
red arrows indicate where cool air is sinking.
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Cyclone Categories
Category Wind Speed (mph) Damage at Landfall Storm 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 gt 155 Catastrophic 19
The Saffir-Simpson Hurricane Scale
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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

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Hurricane Katrina
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What damage is produced?

• Storm Surge
• water that is pushed toward the shore by the
  force of the cyclone winds.

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What damage is produced?

• Wind
• responsible for the loss of power and utilities
• wind damage affects larger areas than surge
• flying debris
• tree loss

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What damage is produced?

• Flying debris
• debris propelled at high speeds

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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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Natural Hazards Summary
Graph showing the number of deaths per year due
to natural hazard events