Chapter 3; Global hazard patterns


Chapter 3; Global hazard patterns

The distribution of geophysical hazards
The 3 main geophysocal hazards are earthquakes, volcanoes and tsunamis.

The main earthquake zones are clustered along plate boundaries. The most powerful earthquakes are assosciated with destructive or conservative plate boundaries.

Plate tectonics
Hotspots from within the asthenosphere generate thermal convection currents which cause magma to rise towards the Earth's surface. This continuous process forms new crust along the line of constructive boundaries, where the plates are diverging.
At the same time, older crust is being destroyed at destructive boundaries, where plates converge. The type of activity here depends on whether both plates are continental, bot plates are oceanic or an oceanic plate is being subducted or gragged down beneath a ligher continental plate.
At conservative boundaries, two plates slide past each other and there is no creation  or destruction of crust.
The type of movement and the degree of activity at the plate margins almost totally controls the distribution, frequency and magnitude if earthquakes and volcanic eruptions. 

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Chapter 3; Global hazard patterns

So, there are 3 boundaries:
Conservative boundary: A boundary between plates where the movement of the plates is parallel to the palte margin and the plates slide past each other.
Constructive boundary: A boundary between plates where the plates are diverging or moving apart.
Destructive boundary: A boundary between plates where the plates are converging (moving together). Ocean plate being subducted beneath a ligher continental plate.

Earthquakes at destructive plate boundaries
Where 2 continental plates are colliding to form fold mountains, shallow, highly damaging earthquakes occur. These present a hazard risk over a wide area in countries such as India and Iran.

Earthquakes at constructive plate boundaries
Constructive boundaries are associated with large numbers of shallow, low magnitude earthquakes as magma rises. Most are submare and so pose little hazard to people

Earthquakes at conservative plate boundaries
At this boundary frequent shallow earthquakes, sometimes with high magnitude, are produced; for example along the San Andreas fault system of the western USA 

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Chapter 3; Global hazard patterns

Volcanoes at destructive plate boundaries
Some 80% of the worlds active volcanoes occur along destructive boundaries. When oceanic plates are subducted beneath continental plates, explosive volcanoes such as Mt St Helens are formed. The 'ring of fire' around the Pacific has many such volcanoes. 

The distribution of hydro-meteorological hazards

Drought has a spread out pattern - over one-third of the world's lamd sirface has some level of drought exposure. This includes 70% of the world's people and agricultural value, which means that drought has an effect on global security. 

Flooding is a frequent hazard and is evident in some 33% of the world's area, which is inhabited by over 80% of its population. Regional-scale, high-magnitude floods are frequent events in India/Bangladesh and China. 

Storms include tropical cyclones, mid-latitude storms and tornadoes. Tropical cyclones are violent storms between 200 and 700 km in diameter. Tropical cyclones or hurricanes will only occur over warm ocean (over 26C) of at least 70m depth at least 5N or S of equator.

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Chapter 3; Global hazard patterns

Disaster hotspots

Identifying and defining hazard hotspots

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Chapter 3; Global hazard patterns

Managing a hazard hotspots
The identification of multiple hazard zones has major implications for development and investment planning, and for disaster preparedness and loss prevention.
However, many hazard-prone areas have long lists of priorties more immediate thank risk manangement, such as poverty reduction or fighting HIV/AIDS, and may be unable to afford the technology to cope with with multiple hazards.
2 key case studies, California and Philippines, illustrate the features of multiple hazard zones. 

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Chapter 3; Global hazard patterns. Philippines.

Disaster hotspots: the Philippines

The Philippines, an island arc in southeast Asia, consists over 7,000 islands. It lies in a belt of tropical cyclones (typhoons), and astrided an active plate boundary. The dense oceanic Philippines plate is being subducted beneath the Eurasian plate. The country experiences a tropical monsoon climate and is subject to heavy rainfall. Flooding can lead to landslides because of deforestation of many hillsides. 
The Philippines is a lower-middle-income country which is developing fast. With a rapidly increasing young population, average population densities for the whole country are high at 240 people km2, with up to 2,000 people per km2 in the megacity of Manila. Many of these people are very poor and live on the coast, making them vulnerable to locally generated tsunamis and typhoon-generated storm surges. On averagem about ten typhoons occur each season, especially in Luzon.
In response, the government has established several organisations to carry out forecasting, warning, hazard risk assessment, disaster training and education. These include the National Disaster Co-ordinating Council; Philippine Atmospheric, Geophysical and Astronomical Services; and the Philippine Insitute of  Volcanology and Seismology. Land-use planning and building regulation, and structural programmes of defences help people survive the huge range of hazards facing them.

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Chapter 3; Global hazard patterns. California.

Disaster hotspots: the California Coats

The state of California contains nearly 40 million people and has an economy the size of a high-income country. However, it suffers from a vast range of hazards, including huge risks from geophysical hazards (especially earthquakes) as well as a range of atmospheric hazards such as fog, drough and associated wildfires, and major impacts from the El Nino cycle. The hazardous zone is concentrated along the San Andreas fault, which runs parallel to the coast.

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