Earthquakes.
What Are Earthquakes?
• Earthquakes are a
result of a sudden
release of energy
caused by the rupture
of geological faults
deep beneath the
earths crust. They
cause shaking and
displacement of the
ground.
Here are some examples of the differing magnitudes
and levels of damage an earthquake can cause.
Tectonic Processes
• The Earths crust is made up of
many huge segments known
as plates.
• Where two plates meet and
press against each other, or
sink beneath each other,
earthquakes often occur.
• New Zealand lies along one
such point. It straddles the
boundary between the pacific
plate and the Australian
plate.
• Existing fractures in the crust
are called ‘faults’ and it is
along these faults that
earthquakes most often occur.
Tectonic Processes cont’
• The earth's crust consists of plates that float on the molten rock of the mantle.
• Plates move on convection currents caused by heat rising from the centre of the
earth.
• The hot magma rises and spreads on the surface, creating new crust.
• The crust spreads out forming a new plate until it meets another plate.
• One of the plates will be pushed down into the interior of the earth and reabsorbed
into the mantle.
• Plates can be compressed to push up mountains when they collide or move sideways
or past each other along transform faults.
Subduction
• Where two plate boundaries meet, often, one is
forced under the other in the strain. Pressure
building along these boundaries can release in
the form of Earthquakes.
• In NZ, the pacific plate is subducting under the
Australian plate, causing earthquakes.
• The areas where earthquakes occur are often in
the overriding plate
Earthquakes Processes
• The place where a fault ruptures is called the focus or
origin, and the point directly above on the earth’s
surface is called the epicentre. The pulse of energy
released by a fault movement radiates outwards as
seismic waves, or shock waves. Large, shallow
earthquakes sometimes cause a surface rupture, as
shown here.
Earthquakes Processes cont’
• Earthquakes send out Seismic Waves
which can be felt on the surface of the
earth.
• There are two main types of seismic
waves: Body Waves and Surface Waves
• Body Waves: These travel beneath the
surface at depths of up to 700km.
Earthquakes Processes cont’
• There are two types of body waves.
• Primary waves: sound waves which reach the
instruments first
• Secondary waves: Cause the earth to shake
horizontally and vertically. (Cannot travel
through water).
• Surface Waves: These travel out from the
epicentre and along the surface of the earth.
Surface waves can be slower then body waves,
but often cause more damage.
How are Earthquakes Measured?
Earthquakes are measured using two scales; The Richter scale which
measures the intensity and the Mercalli scale which measures the
magnitude
Recording Earth’s movements
• The machine used to
record seismic waves is
a ‘Seismograph’.
• This machine constantly
records the motion of the
ground
• From this data we can
determine where an
earthquake might
measure on the Richter Here is an example of seismic recordings,
and Mercalli scales illustrating a large earthquake.
Where do Earthquakes occur in NZ?
This image of NZ shows the active branches This image shows the areas where Earthquakes
of the Alpine fault. The evidence that a fault is occur in NZ and the depth (per km) at which
active is that it repeatedly ruptures the surface. they happen.
Alpine Fault
• In the central South Island, the
colliding Australian and Pacific
Plates are both thick
continental crust, so one plate
cannot sink under the other.
• Instead, the crust of the Pacific
Plate is being forced upward,
creating the Southern Alps.
• The boundary between the
plates is the Alpine Fault.
• Earthquakes along the Alpine
Fault have often been very
large – the last occurred in
about 1717 AD.
• East of the Alpine Fault,
earthquakes also occur on
numerous smaller faults that
criss-cross the region.
The effect of earthquakes on people
• Major Earthquakes effect peoples lives in numerous
ways.
• Other than the damage to buildings, people can:
• Lose loved ones
• Have their crops or homes destroyed
• Lose income
• Die due to lack of infrastructure to provide provisions
and relief efforts into the country
Effects on people cont’
• Water and power supplies may be cut forcing people to
find their own fresh water. If sewage pipes have been
broken, water may be contaminated.
• Transport links may be cut, leaving many stranded.
• Schools and workplaces may be closed causing further
disruption.
• Coastal farmlands may be flooded by subsequent
Tsunamis.
• In many cases countries communities will come to a
standstill and have to band together with teamwork to
rebuild damaged buildings and lives.
Economic effects of Earthquakes
• The economic effects of major earthquakes can be
devastating for the people involved.
• Communities may lose land for agriculture, due to
ruptures in the earth or landslides.
• Damage to buildings and land for agriculture may mean
that communities lose income from exports.
• Rebuilding means new facilities, which may prove to be
a positive economic effect as new designs mean better
structures able to better withstand the forces of an
earthquake.
Reducing the effect of Earthquakes
• Although there is no way to prevent
earthquakes, humans are doing everything they
can to prepare and help minimize the impact.
• These include; constructing buildings that can
move with the shaking of the ground.
• Preparing provisions in the home (survival kits)
to cope with the loss of a number of essential
services.
Reducing the effect of Earthquakes
• Insurance against damage so people are able to
replace buildings and belongings quickly.
• The government in New Zealand has introduced
height controls and building specifications in
areas venerable to earthquakes.
• Education of the dangers posed by earthquakes
and urging all citizens to be prepared.
Case Study: Napier 1931
• One of the most famous NZ earthquakes
occurred in Napier on 3rd February 1931.
• It was a large scale ‘quake magnitude 7.1
on the Richter scale.
• 256 people died in the Earthquake. Many
of these deaths were after the earthquake
as buildings collapsed.
Napier earthquake Continued
• The effects on the land
were significant.
• Landslides caused much
damage in the hilly
Napier suburbs.
• The Ahuriri lagoon was
uplifted more than 2.7
meters, draining the
lagoon and tidal flats
• Many buildings were
destroyed by the ‘quake
Napier earthquake Continued
• After the earthquake,
a fire broke out in the
city and six inner city
blocks were burned.
• The inner city area of
Napier was reduced
to a pile of debris.
• There were 647 after
shocks, which put a
huge strain on the
task of rebuilding.