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Natural Hazards
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
geo-
process
human
process
Hazardous
condition
or result
• The Earth’s 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)
Why is the human element so
critical?
•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
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)
• 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 …
Practical Exercise 1.
Impact of Environmental
Disasters
Earthquakes
and Tsunamis
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 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 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
San Francisco - Great Earthquake
• Magnitude 7.7 - 8.3
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
How do we mitigate the
hazard from earthquakes?
• Reinforce buildings
• Education
• Disaster plan
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
How does an earthquake form
a tsunami?
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
• 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!
Second largest recorded earthquake
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
A village
near the
coast of
Sumatra
lays in ruin
after the
tsunami.
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
• 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
How do we mitigate the
hazard from tsunamis?
• Seawall construction
– cause early wave breaking
– prevent wave run up into urban areas
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)
How do we mitigate the
hazard from tsunamis?
Practical Exercise 2.
Locating Earthquake Epicentres
Tropical Storms
Same Storm - Different Name
Tropical Cyclones
• Background: – Can be deadly!
• For example, in 1991 a large cyclone in Bangladesh killed >138,000 people in just two days!
• Most widespread
destructive weather hazard
– For example: Hurricane
Floyd (1999)
• only a moderate level
hurricane
• caused US$5.6 billion in
damage in the Bahamas
and North Carolina (USA)
and 57 fatalities
Tropical Cyclones
(Left) Three different cyclones spinning over
the western Pacific Ocean on August 7, 2006.
How do cyclones form?
The above figure shows how cyclones form. The green arrows show
where warm air is rising. The red arrows indicate where cool air is sinking.
Cyclone Categories Category Wind Speed
(mph)
Damage at
Landfall
Storm Surge
(feet)
1 74-95 Minimal 4-5
2 96-110 Moderate 6-8
3 111-130 Extensive 9-12
4 131-155 Extreme 13-18
5 > 155 Catastrophic 19+
The Saffir-Simpson Hurricane Scale
Hurricane Katrina
• Hurricane Katrina was the most costly and most deadly hurricane in the history of the USA. – Category 5
• At least 1,836 fatalities
• Damage estimated at US$ 81.2 billion
Hurricane Katrina
What damage is produced?
• Storm Surge
– water that is pushed toward the shore by the force of the cyclone winds.
What damage is produced?
• Wind – responsible for the
loss of power and utilities
– wind damage affects larger areas than surge
– flying debris
– tree loss
What damage is produced?
• Flying debris – debris
propelled at high speeds
How do we mitigate the
hazard from a cyclone?
• Monitoring
– early warning systems
• Infrastructure
– cyclone walls
– communal shelters
• Education and
planning
Natural Hazards Summary
Graph
showing the
number of
deaths per
year due to
natural hazard
events