Download Hazardous Environments resulting from crustal (tectonic) movement

Hazardous environments resulting
from tectonic (crustal) movements
earthquakes
• An earthquake is a sudden release of energy with
a series of vibrations or seismic (shock) waves
which originate from the focus – the point at
which the plates release their tension or
compression suddenly.
• The epicentre marks the point on the surface of
the earth immediately above the focus of the
earthquake.
• A large earthquake can be preceded by small
tremors known as foreshocks and followed by
numerous aftershocks.
Surface waves and body waves
• Seismic waves can travel along the surface of
the earth i.e. surface waves
• They may also travel through the body of the
earth i.e. body waves
• There are two types of body waves – P-waves
(primary or pressure waves) and S-waves
(transverse secondary waves), which are a
series of oscillations at right angles to the
direction of movement.
Cont…
• P-waves travel by compression and expansion,
passing through rocks, gases and liquids.
• S-waves travel with a side-to-side motion, and
can pass through solids but not liquids.
• Therefore the nature of rock and sediment
beneath the ground influences the pattern of
shocks and vibrations during an earthquake.
Liquefaction
• Unconsolidated sediments such as sand shake
in a less predictable way than solid rock.
• P-waves can turn solid sediments into fluids
like quicksand by disrupting sub-surface water
conditions.
• This is known as liquefaction or fluidisation
and can wreck foundations of large buildings.
Earthquakes and Plate boundaries
DESTRUCTIVE BOUNDARIES
• The subducting plate at a destructive boundary
creates very deep earthquakes (700 km below
ground).
• Plates move at a rate of 1.5 to 7.5 cm per year
but often get stuck, creating increasing pressure
and stress on rocks.
• The size of the earthquake depends upon the
thickness of the descending slab and the rate of
movement.
CONSTRUCTIVE BOUNDARIES
• Along mid-ocean ridges earthquakes are small
because the crust is very hot, and brittle faults
cannot extend more than a few kilometres.
Conservative margins
• Characterised by frequent small earth tremors
and occasional severe earthquakes as plates
try to slide past each other.
• The San Andreas fault is the most notorious of
several hundred known transform faults in
California.
The Richter Scale
• The Richter Scale measures earthquake
magnitude.
• It is logarithmic i.e. an earthquake of 5 is ten
times more powerful than one of 4, and 100
times more powerful than one of 3.
• It is numbered from 0 to 9
The Modified Mercalli Scale
• By contrast the Modified Mercalli Scale relates
ground movement to commonplace
observations around light bulbs and
bookcases. Its advantage is that it allows
ordinary eyewitnesses to assess the strength
of an earthquake.
• (see table 3.2 p264)
Factors affecting earthquake damage
• Strength and depth of earthquake, and
number of aftershocks. Shallow earthquakes
are potentially more dangerous than deep
ones.
• Population density
• Type of buildings
• Time of day
Cont….
• Distance from the epicentre
• Type of rocks and sediments
• Secondary hazards (mudslides, tsunami, fires,
contaminated water, disease, hunger and
hypothermia)
• Level of economic development
Resultant hazards from earthquakes
• Most earthquakes occur with little if any
advanced warning
• Damage to buildings, structures and transport
systems cause most problems
• Most deaths are caused by this damage
• Aftershocks may worsen damage
• Displacement may fracture gas pipes leading
to fires
Geomorphological hazards
• Landslides
• Liquefaction
• Tsunamis
Haiti earthquake January 2010
• Located in the Caribbean as an island shared
with the Dominican republic.
• Characterised by poverty, environmental
degradation, corruption and violence.
• An earthquake recording 7 on the Richter
scale just south of Port-au-Prince (the capital)
just 13 km deep occurred on January 12th.
Cont…
• Aftershocks as strong as 5.9 occurred just 9 km
below the surface and 56 km s.w. of the city.
• 300 000 people were killed, 250 000 injured and
1 000 000 made homeless.
• Most residents live in tin-roofed shacks perched
on unstable steep ravines.
• International aid poured in but distribution was
hampered because the capital had been
destroyed.
Cont…
• Since 2000, more than $4 billion has been given to
Haiti to rebuild communities and infrastructure
devastated by tropical storms, floods and landslides,
but mismanagement, a lack of co-ordination and
attempts by global institutions to use Haiti as an
economic test-bed are believed to have frustrated all
efforts.
• A foreign debt of $1.5 billion has weighed down the
economy.
• The earthquake took place where the Gonave
microplate is squeezed between the North American
Plate and the Caribbean Plate.
Earthquakes and human activity
Humans can trigger earthquakes in three ways:
• Through underground disposal of liquid
wastes
• By underground nuclear testing and
explosions
• By increasing crustal loading.
What should people do about
earthquakes?
• Do nothing and accept the hazard
• Adjust to living in a hazardous environment –
strengthen your home
• Leave the area
Earthquake preparedness
• Better forecasting and warning
• Improve building design, building location
• Establish emergency procedures
Predicting and monitoring earthquakes
Measurements of:
• Small-scale ground surface changes
• Small-scale uplift or subsidence
• Ground tilt
• Changes in rock stress
Cont…
• Micro-earthquake activity (clusters of small
quakes)
• Anomalies in the Earth’s magnetic field
• Changes in radon gas concentration
• Changes in electrical resistivity of rocks
Tsunamis
• The term ‘tsunami’ means ‘harbour wave’ in
Japanese.
• 90% of all tsunamis occur in the Pacific ocean
• They are generally caused by earthquakes but
can be caused by volcanoes (Krakatoa in
1883) and landslides (Alaska 1964).
The Boxing Day Tsunami 2004
• This was a global disaster, killing people from nearly 30
countries, many of whom were tourists.
• Between 180 000 and 280 000 people were killed.
• A massive earthquake (9 on the Richter scale) took
place off the coast of Indonesia, where the IndoAustralian plate is being subducted below the Eurasian
plate.
• Sumatra, an Indonesian island, suffered the most
deaths (70% of the populations of some villages
perished), and 31 000 people in Sri Lanka alone died.