Earthquakes in New Zealand

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Earthquakes in New Zealand
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Global Distribution of Earthquakes
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What is an Earthquake? An earthquake is a
sudden slipping or displacement of part of the
earths crust.  When it happens, energy is
released and travels outwards in waves. Why do
they happen in New Zealand? New Zealand is
slowly being deformed by the interaction of two
large pieces of crust or tectonic plates. This
causes stresses in the rock that makes up the
crust, and from time to time it fractures,
releasing energy which we may feel as
earthquakes. Some of these fractures are many
kilometres long we often refer to the visible
ones as fault lines. The Alpine fault, the
Wellington fault and the Wairarapa fault are
famous features of our landscape. But most
earthquakes occur on unnamed, unknown faults
beneath the earth's surface.
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New Zealand sits on the boundary between two
tectonic plates.
Indo-Australian Plate
Pacific Plate
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The surface of the earth is made up of a crust
which varies in depth from 5 to 70 km. The word
shallow is used to describe earthquakes that
occur in the earth's crust.
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Canterburys Unstable World
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Earthquakes which have caused damage in New
Zealand
CANTERBURY (2010)
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(07/09/10)
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Focus the point of energy release.
Epicentre the point on the earths surface
directly above the focus.
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A seismograph or seismometer is an instrument
that measures motions in the ground, including
those of seismic waves generated by earthquakes.
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This image represents one day's recording of the
seismometer located in McQueens Valley on Banks
Peninsula, Canterbury.
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Earthquakes generate several kinds of seismic
waves including P, for "Primary" and S, for
"Secondary" waves.
P wave
S wave
The P waves move in a compressional motion
similar to the motion of a slinky, while the S
waves move in a shear motion perpendicular to the
direction the wave is travelling.
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Richter Scale
Each whole number increase corresponds to an
increase of approximately 30 times the amount of
energy released. Each increase of 0.2 corresponds
to a doubling of the energy released.
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How the magnitude of an earthquake is
calculated. This data also enables Seismologists
to determine the location of the epicentre and
the depth of the focus of the earthquake
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• Epicentre
• 10 kms S.E. of Darfield
• 40 kms W. of Christchurch.
• Magnitude 7.1
• Focal Depth 10 kms

Canterbury Earthquake 04/09/10 4.35am
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This scale is more useful than the Richter Scale
when describing the impact of an earthquake.
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Canterbury Earthquake 04/09/10 4.35am - Modified
Mercalli Scale Felt Intensities
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Aftershocks
An aftershock is a smaller earthquake that occurs
after a previous large earthquake in the same
area (the main shock). If an aftershock is larger
than the main shock, the aftershock is
re-designated as the main shock and the original
main shock is re-designated as a foreshock.
Aftershocks are smaller earthquakes formed as
the crust around the displaced fault plane
adjusts to the effects of the main shock. Most
aftershocks are located over the full area of
fault rupture and either occur along the fault
plane itself or along other faults within the
volume affected by the strain associated with the
main shock.
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Canterbury Earthquake Animation
Thousands of aftershocks have struck the
Canterbury region since the huge 7.1 magnitude
earthquake at 4.35 am on Saturday, 4 September,
2010.
Click here to view Paul Nicholls animation.
X
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Canterbury Earthquake 04/09/10 Aftershocks
05/09/10 9.44 pm

• Epicentre
• 20 kms W. of Christchurch
• Magnitude 3.5
• Focal Depth 2 kms

Modified Mercalli Scale Felt Intensities
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Canterbury Earthquake 04/09/10 Aftershocks
06/09/10 2.48 pm

• Epicentre
• 20 kms S.W. of Christchurch
• Magnitude 4.1
• Focal Depth 6 kms

Modified Mercalli Scale Felt Intensities
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Canterbury Earthquake 04/09/10 Aftershocks
06/09/10 3.24 pm

• Epicentre
• 30 kms S.W. of Christchurch
• Magnitude 5.4
• Focal Depth 15 kms

Modified Mercalli Scale Felt Intensities
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Canterbury Earthquake 04/09/10 Aftershocks
08/09/10 7.49 am

• Epicentre
• 10 kms S.E. of Christchurch
• Magnitude 5.1
• Focal Depth 5 kms

Modified Mercalli Scale Felt Intensities
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The Destructive Effects of Earthquakes

• Direct shaking 3. Landslides
• 2. Liquefaction 4. Tsunami

• The level of damage done to a structure depends
  on the amplitude of the seismic waves and the
  duration of shaking.

• The geology of an area can affect the level and
  duration of shaking, but more important are local
  site conditions.

• The amplitudes are largest close to large
  earthquakes and the duration generally increases
  with the size of the earthquake.

• Generally shaking in soft sediments is larger
  and longer than when compared with the shaking
  experienced at a "hard rock" site.

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• Most earthquake-related deaths are caused by the
  collapse of structures and the construction
  practices play a tremendous role in the death
  toll of an earthquake.

Southern Italy, 1909 death toll gt100,000
survival rate 36. More than half the victims
died as a result of building collapse. San
Francisco, 1906 death toll about 700 survival
rate 98.

• Building practices can make all the difference
  in earthquakes, even a small earthquake beneath a
  city with structures unprepared for shaking can
  produce tens of thousands of casualties.
• - Engineers have a saying Earthquakes dont
  kill people, buildings do.

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• After the 1931 Napier Earthquake attention in
  New Zealand was focused on weaknesses in building
  construction, especially poor building standards
  and the lack of any provision for
  earthquake-resistant design.

• This led to a draft by-law in 1931, which was
  incorporated into a building code in 1935. The
  code recommended standards of design and
  construction so that buildings could resist the
  horizontal motions created by ground shaking.

• Building codes in 1965, 1976, 1984 and 1992 have
  added requirements to accommodate changes in
  building materials and design.

• For a major earthquake the goal is to protect
  life by ensuring a building will not collapse and
  people can escape from it, even if the building
  itself is badly damaged.

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Christchurch Liquefaction Susceptibility
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Earthquake-induced liquefaction
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A sand volcano or sand boil is a cone of sand
formed by the ejection of sand on to the surface
from a central point. The sand builds up as a
cone and a crater is commonly seen at the summit.
The cone can range in size from millimetres to
metres in diameter.