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.

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

San Francisco - Great EarthquakeMagnitude 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

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

How does an earthquake form a tsunami?

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

Same Storm - Different Name

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

What damage is produced?

Flying debris debris

propelled at high speeds

How do we mitigate the hazard from a cyclone?

Monitoringearly warning systems

Infrastructurecyclone wallscommunal shelters

Education and planning