Earthquakes, Active Faults and Disaster Preparedness in New
Zealand
Earthquakes and Faults
A fault line is a fracture along which the crust
has moved. Stresses in the crust along New Zealand’s plate boundary have broken
it into separate fragments or blocks that move relative to each other along
fault lines.
The relationship between earthquakes and faults was
first established by nineteenth century geologists following the 1855
Wellington Earthquake in New Zealand.
As far as seismologists now understand, all but the
very deepest earthquakes (deeper than 600km) occur on faults. Seismic waves are
generated when the two sides of the fault rapidly slip past each other.
For most earthquakes, the faults do not break the
surface, so the faults can be "seen" only through analysing the
seismic waves. Faults can be anywhere from metres to a thousand kilometres
long.
Seismologists still have much to learn about the
mechanism that causes the deepest earthquakes. At 600km, the earth is probably
too warm for faults to be brittle like glass, so some sort of chemical change
might occur very rapidly.
Different
types of Faults
A close look at faults helps geologists to
understand how the
tectonic plates have moved relative to one another.
Types of movement of crustal blocks that can occur
along faults during an earthquake:
2. Where the crust is being compressed, reverse
faulting occurs, in which the hanging-wall
block moves up and over the footwall block – reverse slip on a gently inclined
plane is referred to as thrust faulting.
3. Crustal blocks may also move sideways past each
other, usually along nearly-vertical faults. This ‘strike-slip’ movement
is described as sinistral when the far side moves to the left, and dextral,
when the far side moves to the right.
4. An oblique
slip involves various combinations of
these basic movements, as in the 1855 Wairarapa Fault rupture, which included
both reverse and dextral movement.
Faults can be as short as a few metres and as long
as 1000km. The fault rupture from an earthquake isn’t always a straight or
continuous line. Sometimes there can be short offsets between parts of the
fault, and even major faults can have large bends in them.
When is a Fault "Active"
Geologists believe that if a fault shows evidence
of having moved at least once in the past 100,000 years, it should be regarded
as a potential source of earthquakes.
If it has moved at least once in the past 5000
years, then it should be considered a potential source of damaging earthquakes
to any settlement within a radius of 50km.
Once a major fault has formed, future earthquakes
are generated along the same line, and after hundreds of thousands or million
of years of movement, increasingly large vertical and horizontal displacements
of land occur.
Repeated earthquakes and their associated fault
movements have formed the major mountain ranges of New Zealand.
There are major fault lines running the length of New Zealand. Many of the larger faults are oblique strike slip faults, having a combination of sideways and vertical movement. To explore in detail a map of New Zealand's known active faults go to our active faults database and click on the link near the bottom of the page.
 |
| The Awatere Fault cuts a clear line across the hills. It last ruptured in the 1848 Marlborough Earthquake. |
There are major fault lines running the length of New Zealand. Many of the larger faults are oblique strike slip faults, having a combination of sideways and vertical movement. To explore in detail a map of New Zealand's known active faults go to our active faults database and click on the link near the bottom of the page.
The North Island Fault System is a major plate
boundary feature in the North Island, under constant stress from movement
between the Pacific and Australian plates.
Running in a continuous line from the Bay of Plenty
southwards to the Wellington coast, this system has pushed up a line of
mountain ranges including the Kawekas, Ruahines, Tararuas and Rimutakas. In the
Wellington area several major faults are spread out parallel to each other,
including the infamous Wellington Fault.
The Taupo Volcanic Zone also has many active faults
associated with rifting and extension of the crust in the area.
In the South Island, the Marlborough Fault System
is another series of major parallel faults. These join together further south
to form the Alpine Fault which carries most of the total plate
boundary strain.
This is a very distinct feature along most of its
length because of the Southern Alps that have been uplifted along its eastern
side, making it clearly visible from space. It is considered to be at high risk
of producing a major earthquake in the next 50 years.
However, there are still many other relatively
minor faults across the South Island, which rupture less frequently, such as
those which have caused the Canterbury and Christchurch
earthquakes.
Earthquake
- What to do?
A damaging earthquake can occur at any time. If you have a plan, an emergency water supply and a well stocked emergency kit, you may save yourself and your family from a lot of avoidable distress!
Before an
Earthquake
There are many things you can do to prepare your
house to make it a safer place should an earthquake occur.
 |
| Footpath damage from March 1987 earthquake, Bay of Plenty. |
Secure heavy furniture to the wall
or floor
or floor
· Put heavy items near floor level
· Put strong catches on cupboard doors
· Check that your chimney is secure
· Secure your hot water cylinder
· Check your house is well secured to its foundations
The Civil Defence website has all
of the important information you need to prepare your own emergency plan, your
household survival kit and a suitable first aid kit.
You should also consider that three days is the
likely minimum time that it will take for help
to reach you from outside, and you may wish to stock up a larger supply of food
and water.
In addition don't forget to think about including
other items that could be important for you such as :
· A first aid manual
· Family documents such as birth and marriage
certificates, insurance policies, drivers' licences, passports
· Family photos
When the shaking begins
Drop, take cover, and hold on. Move only a short distance to a nearby safe place. Stay indoors until the shaking stops and you're sure it's safe to go outside. Stay away from windows, chimneys, and shelves containing heavy objects.
In bed - hold on and stay there, and protect your head and body with a pillow
and blankets.
Outdoors - find a clear spot away from buildings trees and
power lines. Drop to the ground.
In a car - slow down and drive to a clear place (as above).
Stay in the car until the shaking stops.
In a lift - stop it at the nearest floor and get out.
Do not look for your pets until the shaking stops.
When the
shaking stops
 |
| Main rifts from Edgecumbe earthquake. |
Protect yourself from further danger by putting on
long pants, a long-sleeved shirt, sturdy shoes, and work gloves.
· Check those around you and offer help if necessary.
· Put out small fires and eliminate fire hazards.
Evacuate the building if you are unable to control the fire. Turn off the gas
if you think it's leaking.
· Listen to the radio for instructions from Civil
Defence. Some people may need to be evacuated.
· After a big earthquake expect aftershocks - they
can go on for weeks or even months. Each time you feel one, drop, take cover,
and hold on.
· Check your home or workplace for damage. Get
everyone out if the building is unsafe.
· Don't go sightseeing - you'll add to the congestion
and hamper relief efforts.
Don't touch downed power lines - treat all power lines
as alive.
If a
tsunami threatens
 |
| Gisborne Wharf after a tidal surge due to a tsunami |
Be prepared to evacuate. Listen to your radio for
instructions.
· Leave the area immediately if a strong earthquake
strikes and you are close to a beach or near a river
· Go at least 1 kilometre inland or 35 metres above
sea level
· Take your disaster survival kit and any important
documents with you
· Tsunamis sometimes 'draw up' water before they come
inland. If you see the water suddenly retreat (after an earthquake), it is
probably a tsunami. Leave the area immediately.
· DO NOT go to a
beach or river to watch the waves come in. If you see the tsunami, it will
probably be too late to get to safety.
Related Posts:
http://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/Major-Faults-in-New-Zealand
http://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/Earthquake-What-to-do/Before-an-Earthquake
http://www.gns.cri.nz/Home/Learning/Science-Topics/Earthquakes/Earthquake-What-to-do/When-the-shaking-begins
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