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...