Tectonic hazards - brief overview

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Unit 4 - Tectonic activity and Hazards
1. What are tectonic hazards and what causes them?
A tectonic event is a physical occurrence resulting from the movement or deformation of the Earth's crust.
They are predominantly earthquakes or volcanic eruptions and their associated activities. Such events can
become tectonic hazards when they have the potential to cause loss of life and damage to property.
Not all events are hazardous. Many earthquakes in active areas are low in magnitude and may occur deep
beneath the surface (low focus). People living in the area may not even feel the vibrations from the quake. In
April 2009, there were over 150 earthquakes in the area around Los Angeles but virtually all were below 3.0
on the Richter scale and thus they presented no danger to the local people.
A hazard becomes a tectonic disaster when an event materialises and causes extensive destruction and a
number of fatalities. For example, the eruption of Mt Pinatubo in the Philippines (1991) killed about 300
people, over 80,000 homes were destroyed or damaged and 800km2 of agricultural land was buried
beneath an extremely thick layer of debris and ash.
The hazard event profile is a diagram that attempts to represent the characteristics:
Speed of onset
Areal extent
Spatial predictability
These are used to highlight the effects of different types of hazards and making comparisons between those
two events however different or similar they may be the comparisons drawn are extremely interesting.
The causes of tectonic hazards:
The tectonic plates of the Earth's crust move relative to one another so there is a slow build up of stress
within the rock forms below the visible surface. When pressure is released, parts of the Earth's surface may
experience an intense period of intense shaking in action, which typically only lasts a few seconds but there
have been exceptions where those periods continue for a number of horrifying minutes. This is the main
cause of earthquakes. Earthquakes are classified according to depth of their focus, which affects the amount
of surface damage in result of the particular event. Three broad categories are usually recognised:
Deep focus: 300-700km deep
Intermediate focus: 70-300km deep
Shallow focus: 0-70km deep
Shallow focus earthquakes usually cause the greatest degree of damage and account for approximately 75%
of all earthquake types reported globally.
A volcano is a landform that develops around the weakness in the Earth's crust. At this point, molten rock
(magma) is forced out and extruded. The nature of the volcanic explosion, i.e. viscosity (thickness and
stickiness of magma), the amount of dissolved gases and how easily they can escape, is determined by the
tectonic context. Volcanic eruptions can be classified by the `volcanic explosivity index' (VEI):
Occurrences in last
VEI Ejecta volume Classification Description Frequency Examples
10,000 years*

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³ Hawaiian effusive constant Kilauea, Piton de la Fournaise many
1 > 10,000 m³ Hawaiian/Strombolian gentle daily Stromboli, Nyiragongo (2002) many
Galeras (1993), Mount
2 > 1,000,000 m³ Strombolian/Vulcanian explosive weekly 3477*
Sinabung (2010)
> 10,000,000 Nevado del Ruiz (1985),
3 Vulcanian/Peléan severe few months 868
m³ Soufrière Hills (1995)
Mount Pelée (1902),
4 > 0.1 km³ Peléan/Plinian cataclysmic 1 yr 421
Eyjafjallajökull (2010)
Mount Vesuvius (79 CE), Mount
5 > 1 km³ Plinian paroxysmal 10 yrs 166
St.…read more

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Global distribution of earthquakes and volcanoes is shown here.…read more

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Tsunamis (`harbour waves' ­ Japanese) are waves caused by the rapid movement or deformation of the
seabed. Submarine earthquakes, volcanoes, landslide or slumps can initiate this motion. They are classified as
secondary hazards since they are events cause by a tectonic occurrence.
The distribution of earthquakes and volcanoes has a distinct geography. There are two important areas of
activity: the oceanic fracture zone (OFZ) and the continental fracture zone (CFZ).
1.…read more

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Formed by major mantle plume of rising that may cause a
The Isle of Arran ­ west
eruptions from vents or magma, volcanic hot dooming up of the
coast of Scotland,
fissures. Accumulation of spot Reunion in Indian surface as they form.
intrusion of large granite
deposition = layers Ocean, 700-900m Only exposed after
formed. above sea level. weathering and erosion
of the less resistant
overlying country rock.…read more

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Destructive Western coast South
(convergent) boundary America ­ Nazca plate
volcano: subduction subducted beneath
zone causes build up of continental South
pressure beneath the American Plate.
plate boundary with
Mount St. Helens 1980
magma rising to the
surface causing
eruptions. Contrasting Fold mountains, Island
cone types. Eruptions Arcs, cinder cones,
less frequent and much stratovocanoes, volcanic
more explosive, rising domes or calderas. Lava
magma has a much = andesitic. Eruptions ­
greater thickness of strombolian, vulcanian,
crust to pass through plinian, or pelean.…read more

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Microfracturing is also common as the effects of earthquakes on landscapes ­ also known as microseismicity.
Explained as the process of small-scale fractures on very small areas releasing stress under high-strain
conditions. Sufficient when microfractures link up to a large slip surface of significance seismic event or
earthquake can occur. After large event ­ microfracturing is thus much lower because stress has been
released. Can predict rock failures in mines, and applications are being made for the portions of the faults
within brittle geological conditions.…read more

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Volcano ­ e.g. explosive blast, lava flows, ash flows and ash falls, mud flows (lahars), and the release
of poisonous gases
Some areas show concentrations of tectonic and other hazards. These are known as multiple hazard zones.…read more

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Social Human costs, e.g. primary, secondary and tertiary casualties. Hazards can also cause
misery and suffering as well as poor health, e.g. loss of electricity/ services/
Economic Costs of repairing damage. Indirect costs/secondary impacts, such as loss of
earnings, loss of tourism income
Tectonic hazards can have beneficial impacts on people. These include geothermal sources of energy, rich
fertile soils and minerals such as gold. There is also potential income streams associated with tourism foreign
exchange, e.g. visitors to Iceland.…read more

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Range of responses:
The response to natural hazards can occur at a range of scales, from the individual within the local community
to regional, national and international level. Various `players' may be involved in hazard response.
National National policies, civil protection and defence with a thorough check of the information
Government available to the public
Local Government Operational policies at local level, e.g. rescue, welfare, medical, transport, supply of
emergency aid in the given situation
Scientists/academi Researching, understanding causes, producing hazard-risk maps.…read more


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