Transcript Earthquakes in New Zealand

Earthquakes in New
Zealand
Global Distribution of Earthquakes
What is an Earthquake?
An earthquake is a sudden slipping or displacement of part of
the earth’s 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.
New Zealand
sits on the
boundary
between two
tectonic
plates.
Indo-Australian
Plate
Pacific Plate
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.
Canterbury’s
Unstable
World
Earthquakes
which have
caused damage
in New Zealand
CANTERBURY (2010)
(07/09/10)
Focus = the point of energy release.
Epicentre = the point on the earth’s surface
directly above the focus.
A seismograph or seismometer is an instrument that
measures motions in the ground, including those of
seismic waves generated by earthquakes.
This image represents one day's recording of the seismometer
located in McQueens Valley on Banks Peninsula, Canterbury.
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.
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.
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
• 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
This scale is more useful than the Richter Scale
when describing the impact of an earthquake.
Canterbury
Earthquake
04/09/10
4.35am Modified
Mercalli Scale
Felt Intensities
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.
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
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
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
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
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
The Destructive Effects of Earthquakes
1. Direct shaking
2. Liquefaction
3. Landslides
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.
• 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 >100,000; 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 don’t kill
people, buildings do.’
• 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.
Christchurch Liquefaction Susceptibility
Earthquake-induced liquefaction
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.