On Tuesday 22nd of February 2011 there was a 6.3 magnitude earthquake at 12:51pm. The damage in Christchurch was far more substantial than the damage caused in the 7.1 earthquake only 5 months before. 185 people were killed in the February earth quake and thousands more were injured. The epicentre of the earthquake was very close to Lyttelton and only 10 kilometres out from Christchurch Central city. The fault line that ruptured and caused the earth quake was the 15 kilometre fault along the southern edge of the city from Cashmere to Avon Heathcoat estuary.
The build-up of stress or pressure in the rocks below Christchurch The earth is made up of many tectonic plates. It is the movement of these plates that has created the geographical features of many countries. New Zealand lies on the boundary between two of these plates: The Pacific Plate and the Australian Plate. This boundary is most obvious along the Southern Alps. From this main fault there are many smaller ones spreading outwards. Some of these spread into Canterbury. Before the 2010 and 2011 earthquakes it was not thought that there were any significant fault lines near Christchurch. Now we know there are. Stress is not caused by the movement of tectonic plates, but more by when the edges of the plates jam against each other and stop moving. The pressure builds up behind the part that has jammed. Eventually the pressure is so great that the plate moves suddenly and this causes earthquakes.
The build-up of stress and the sudden movement when the stress is released shakes the rocks deep under Christchurch. The fault lines under and near the city shake, causing the rocks to move. This causes the major earthquakes.The outer core of the earth is layer of molten rock. On top of this is the mantle. On top of the mantle is the earth’s crust that we live on. The mantle gets very hot and in thin places molten rock rises to its top. Because of this hot and cold relationship, the crust on top is broken into tectonic plates. The movement of molten rock causes energy to be moved upwards into the crust. When the crust can no longer hold this energy, it moves along the boundaries of the tectonic plates. These boundaries are the weakest parts of the crust and will therefore be the first bits to break and move.
Release of stress
The Port Hills fault line that caused the 6.3 magnitude earth quake in February was 15km long and stretched east to north east along from Cashmere to the Avon Heathcoat estuary. The fault line that caused the Feb 22 quake was one that was previously unknown close to the Port Hills. This fault line did not break the earth’s surface, so there was nothing visible for scientists to go and look at. Using instruments that measure earthquakes and earth movement scientists were able to determine exactly where this fault was and that the bit that actually moved, causing the earthquake was 15km long. Fault lines are the weakest part of the earth’s crust. When there is stress caused underneath by movement, the fault lines are the first part to move. It is a bit like a piece of glass: if there is a crack in the glass that will be the first place where the glass will break. So pressure is released under the earth’s crust. The earth’s crust will actually hold a lot of this pressure. When the pressure becomes too much for the crust to hold, the fault lines are the first bits to move. One of the ways scientists measure and record the fault movement was through the displacement of GPS stations, whose displacements were used to create the fault slip model.
http://www.gns.cri.nz/Home/Our-Science/Natural-Hazards/Recent-Events/Canterbury-quake/Hidden-fault The transmission of energy through the earths crust
The energy released when a fault line moves is like a stone being dropped in a puddle of water. It causes ripples to move out from the centre. The ripples are worst nearest the centre and they get smaller as they get to the edge of the puddle. The centre of an earthquake is the epicentre. When energy is released at an epicentre, waves of energy move outwards, causing the ground to shake. As the epicentre of the Christchurch earthquakes was quite close to the city, the energy that caused the ground to move under Christchurch was violent. If the epicentre had been further away the quakes would have felt smaller.There was movement up, down and sideways during the February quake. The block of land on the south side of the fault line rose about 2.5 metres – this included the Port Hills. The land on this side of the fault also moved westwards. The land on the north side of the fault moved eastwards towards the sea.
However, much of this movement was underground. The land has risen as much as 40 centimetres around the western side of the Avon-Heathcote estuary.The amount the Port hills has risen varies a lot , under Lyttelton Harbour it has risen about 5 centimetres but at the base of the hills near the Heathcote valley, it has risen to a maximum of around 25 centimetres. As a result of the fault slip, areas like Bexley, Aranui, Wainoni, Avondale and New Brighton have sunk, mostly by less than 15 centimetres. Northern, northeastern and central areas of Christchurch have also sunk, but generally by less than 5 centimetres. Earth quakes / fault movements are measured with a sensitive instrument called a seismograph. Earth quakes generate seismic waves and the seismograph is used to detect them.
The magnitude of an earthquake depends on how big or small the fault is and the amount it has slipped. The seismograph shows the recordings made on the seismographs detected at the surface of the earth to determine how large the earthquake was. A short wiggly line that doesn’t move very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. Through the seismograph we get the Richter scale. The Richter scale is a scale used for expressing the magnitude of an earthquake based on the height of the largest shake 100km from the epicentre. There are different types of seismic waves that can be detected; there are P waves and S waves (primary and secondary) P-waves are travel at the greatest velocity through the earth making them the first waves to be recorded on a seismograph during an earthquake. S waves are transverse waves that travel slower than P waves but are more destructive because of their large amplitude.
The effects on man-made structures and aftershocks
Liquefaction is a substance where strength and stiffness of a soil is reduced by tectonic plates shifting or other rapid movement. Liquefaction occurs in sodden soils. The water applies a pressure on the soil particles this controls how tightly the particles are pressed together. Before an earthquake, the water pressure is relatively low. However when the tectonic plates start shaking it can cause the water pressure to build up to the point where the soil particles can readily move. Some Areas of Christchurch are more prone to others as some suburbs have up to 9cm of liquefiable substances such as sand and soft soil. These substances turn into liquefaction easily as they mix well with the water underground. Costal areas are also more prone as the soft sand mixes well with the water also. There was more damage caused by liquefaction in the central city and eastern suburbs than other places simply because it depends on the soil. Since Christchurch is built on marsh land our soil is very susceptible to liquefaction and there is very little we can do to stop it. Some Areas of Christchurch are more prone to others as some suburbs have up to 9cm of liquefiable substances such as sand and soft soil.
These substances turn into liquefaction easily as they mix well with the water underground. Costal areas are also more prone as the soft sand mixes well with the water also. Most buildings that survived the Christchurch earth quake were wooden because when wood is shaken it is able to bend and twist with the movement, whereas for bricks and stone they can’t move at all and just have to absorb the shock of the shaking which generally leads to buildings collapsing all together E.g. Christchurch Cathedral, CTV building and Knox church. Buildings like the Christchurch Women’s Hospital survived through the September and February earthquake because its foundations are base isolated. Base isolation is when a building is separated from the ground that it is built on. Large plates separate the ground and the building. One plate is secured to the ground and the other is secured to the bottom of the building. During an earth quake these plates only move a little bit, absorbing most of the shock from the earthquake minimizing damage to the rest of the building.
Also depends on the soil under the building, if it is prone to liquefaction or not. Some houses had liquefaction coming up through their floors damaging the foundations so much they had to leave their homes, and basements of office buildings or apartment blocks were flooded with liquefaction, making them un-use able.A tsunami didn’t occur after the Christchurch earth quake because the epicentre was in land and no seismic energy was released under the sea. Tsunamis only happen when the epicentre is in the sea, making the energy from the earth quake ripple the water on to land. The bigger the earth quake is, the bigger the tsunami.
After shocks are a lots of smaller earth quakes that happen after the initial earth quake. They occur because the fault line that ruptured needs to settle down again and release stress, aftershocks are the fault lines way of releasing remaining energy. According to http://www.christchurchquakemap.co.nz/february there have been 8673 after shocks since February 22nd 2011 as of Monday 19th May 2014. Including after shocks that have been so big they are classed as an earth quake of their own like the 13th of June 2011 and the 23rd of December 2011. After shocks do not occur in the exact same place, but they do occur along the same fault line and in the same region. The main earthquake has already happened and released most of its energy already, but further along the fault line the rocks are now out of place and the grinding between them causes more earthquakes (aftershocks).