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 5_int_shake/ 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 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 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 16 th ’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 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.