Natural Hazards What is an Earthquake Ground movement

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. Figure showing the distribution of earthquakes around the globe �This commonly occurs due to movement along tectonic plate boundaries

Earthquake 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 > M 7. 0 and these account for 90% of the energy released and 80% of all the fatalities �

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

Tsunami Above: Countries most effected by the tsunami Animations of the Boxing Day tsunami showing how the tsunami radiated from the entire length of the 1, 600 km (994 miles) rupture.

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 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

Pressure Systems 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.

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

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