Transcript Why study fault lines - opotikicollegeearthscience

• Fault lines
• Nature of Science
• Scientists' predictions are based on their
existing science knowledge. By examining
fault lines, scientists are able to predict
where earthquakes are likely to occur and
the likelihood of when they might occur.
• A nervous laugh often accompanies the
statement, “Our house is on the fault line”.
We all know that it is not a great place to
be during an earthquake, but what actually
is a fault line?
• Cracks or fractures in the earth’s crust are
known as faults and, because there may be
more than one rock fracture, an area is called a
fault line. This might be an inactive fault where
scientists can see where past movement has
been, or one which continues to be active even
after millions of years. All but the very deepest
earthquakes occur on faults. Faults might be
only metres or up to a thousand kilometres.
• When tectonic plates move, the rocks at
the junction between the two plates are
not able to simply glide past each other,
due to friction and the rigidity of the rock.
Instead, stress builds up in the rock until
eventually it breaks, and the two rocky
blocks move relative to each other along a
fault line.
• Go to
http://www.teachersdomain.org/asset/ess0
5_int_shake/
and model the ways that plates can move
• Faults are categorised into three groups
• Normal fault – the blocks of earth are pulled apart and
follow normal gravitational pull, and one block slips
downwards. The exposed upward block forms a cliff
known as a fault scarp.
• Reverse (or thrust) fault – the fault blocks move towards
each other. In reverse to the normal fault, one block rides
up to overlap the other.
• Strike slip fault– the blocks shift past each other in a
horizontal movement.
• Most faults are a combination of fault types.
What about Wellington?
• Wellington is located on six active fault lines –
many are a combination of reverse and strike
slip faults. As with any coastal settlement, there
is also a threat of tsunami which can be caused
by vertical fault movement under the ocean.
• Geologists study a fault trace to build up a
history of its movement and work out the timing
between each movement. The probability that a
fault within 40km of Wellington will break within
the next 50 years is estimated 40–45 percent.
• Wellington is sitting on the relatively light continental crust of the
Australian Tectonic Plate which is riding over the dense oceanic
crust of the Pacific Plate. The main boundary between the two
plates (the subduction interface) slopes westward down beneath the
North Island and is about 25-30 km below Wellington City.
• At Wellington the two plates are moving against
each other at a rate of about 3.5 cm per year.
This slow collision puts immense pressure on
the crust and has broken it up into several large
pieces, separated along fault lines – including
the Wellington and Wairarapa faults, and the
subduction interface.
• When the strain between these blocks of crust
overcomes the resistance that locks them
together, they move relative to each other and
we experience the jarring, shaking jolt of a large
earthquake
• Scientists know now that the fault lines in
the South Island and the North Island are
not connected, so an earthquake in one
island is not likely to carry over into the
other island.
South Island Land Movement
• In the South Island of New Zealand, the
boundary between the Australian and
Pacific tectonic plates can be seen on
land. This is unlike the North Island
boundary, where a subduction zone is
under water off the east coast.
The Alpine Fault from Space
Alpine Fault movement
• The Alpine Fault is called a strike slip or transform fault.
The Australian plate is sliding horizontally towards the
north-east, at the same time as the Pacific plate is
pushing up, forming the Southern Alps. The mountains
are rising at 7 millimetres a year, but erosion wears them
down at a similar rate.
• The horizontal movement along the fault is not smooth,
as both sides are locked together. When tectonic forces
overcome this locking, the fault slips, jumping up to a
distance of 8 metres at a time. These large earthquakes
don’t happen very often – the last one was nearly 300
years ago.
Why are New Zealand
researchers interested?
• Researchers are studying the Alpine Fault
to investigate past earthquakes, mountain
formation and the structure of the Earth’s
crust.
• A lot of research is being done to find out
about earthquakes in the past as they may
help indicate when to expect one in the
future.
• The work involves scientists from several disciplines working
together, using different methods, such as:
• detailed mapping and satellite surveying
• digging trenches to find buried evidence, such as landslides
• dating trees buried by landslides using radiocarbon dating and tree
growth rings. Some trees survive landslides, but the event is marked
by unusual growth rings. Whole forests that have grown back after
an earthquake can be dated, too.
• Using techniques like these, scientists have found out that major
earthquakes happened on the central Alpine Fault in 1100, 1450,
1620 and 1717. This is not a regular pattern, but enough to suggest
there is a high probability of a large earthquake in the next 50 years.
South Island Rocked by
Earthquake
Is this the big one that was coming?
More reports of damage from July 16th’s earthquake in the South Island
are expected to filter in.
The 7.8 magnitude quake at 9.22pm was centred 100 kilometres
northwest of Tuatapere at a depth of 12 kilometres. It is the biggest New
Zealand has experienced since the Napier earthquake in 1931. The first
aftershock was recorded at 9.41pm and measured 6.1 on the Richter
scale. The second struck at 1.50am and measured magnitude 5.9.
• New Zealand scientists record around 14,000
earthquakes a year, of which around 20 top
magnitude 5.0.
• The last fatal earthquake in our geologically
active country, caught between the Pacific and
Indo-Australian tectonic plates, was in 1968
when an earthquake measuring magnitude 7.1
killed three people on the South Island's West
Coast.