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