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Manatū Taonga – Ministry for Culture & Heritage
On Tuesday 22 February 2011 at 12.51 p.m. Christchurch was badly damaged by a magnitude 6.3 earthquake, which killed 185 people and injured several thousand. Officially it was one of more than 10,000 aftershocks of the less destructive magnitude 7.1 earthquake that struck on 4 September 2010. ‘I felt the aftershocks were like a home invasion – you close your curtains and you lock your door when you go to bed at night and they enter into your house without an invite.’ –Louise Swatton. Read the full story on QuakeStories.govt.nz Aftershocks, 22 February 2011 to 11 February 2012
Since Christchurch was founded it has been shaken by earthquakes from across the Canterbury region.
Across New Zealand earthquakes have disrupted towns and cities, and caused injury and death. For centuries before Europeans arrived, Maori had experienced ru whenua, which means ‘the shaking of the land’. They described two severe earthquakes, at Taupo and Rotorua. It was said that a Rotorua pa where about 1,000 people lived was swallowed up, and the area became a lake. Historic earthquakes Earthquakes in Māori tradition
Earth’s main features – its continents and ocean floors – are not fixed. The earth’s surface is divided into about 15 major segments, known as plates. Most earthquakes in the world happen where two plates meet. New Zealand sits where the Australian Plate and the Pacific Plate meet. Along plates there are many faults in the earth’s crust. When stress builds up in the crust, it can eventually cause the crust to break suddenly along a fault. This movement releases energy, and is felt on the surface as an earthquake. These predictions are based on the distribution of active faults, how frequently faults have moved in the past, and the location of historic earthquakes. The 2010–11 Canterbury earthquakes are outside the areas of greatest statistical risk of high ground shaking. This shows that large earthquakes may occur anywhere in the New Zealand region, not just in the highest risk areas. The Pacific Plate moves steadily at a rate of 4-5cm per year. Australian and Pacific plate Plate diagram Active faults in New Zealand
The magnitude scale measures the size of an earthquake at its focus. An earthquake generates two different waves.
The P-wave (primary or pressure wave) is a pulse of energy that travels quickly through the earth and through liquids. It forces the ground to move backwards and forwards as it is compressed and expanded. The S-wave (secondary or shear wave) follows more slowly, with a swaying, rolling motion that shakes the ground back and forth perpendicular to the direction of the wave. The waves are recorded on a seismograph and then measured. The seismograph measures the amount of shaking (amplitude) in millimetres. Primary and secondary waves
Seismologists study the way waves travel through the planet to better understand the earth’s interior. It takes around 20 minutes for the seismic waves to travel through to the other side of the planet. S-waves cannot be detected opposite the earthquake as they do not pass through the liquid core. S-P Time is the time (in seconds) between the primary (P) and secondary (S) waves.
A single step in the magnitude scale represents an increase of about 30 times the energy released at the source deep in the earth. A magnitude 5 earthquake is therefore about 900 times bigger than a magnitude 3 earthquake. The Richter magnitude was based on the largest amplitude ‘wiggle’ recorded on a seismograph. It worked well, but magnitudes of very large earthquakes were underestimated. Today, with better instruments, scientists can measure the energy of the different types of earthquake waves. The size of earthquakes is referred to as magnitude, but it is no longer measured on the Richter scale.
Unlike the magnitude scale, which measures the size of an earthquake at its focus – the source deep below the surface, the modified Mercalli scale measures the intensity of shaking felt by people on the ground. From observations gathered after an earthquake, it is possible to draw lines of intensity that radiate outwards from the epicentre. Extent of shaking, Hawke’s Bay earthquake, 3 February 1931
Recent history has not been representative of New Zealand’s actual earthquake risk. Up until22 February 2011 there had not been an earthquake close enough to a major city to cause widespread damage and loss of life since Hawke’s Bay in 1931. It is highly unlikely that it will be another 80 years before the next destructive earthquake occurs. For everything you need to get ready Attribution-NonCommercial 3.0 New Zealand (CC BY-NC 3.0) Email Info@mch.govt.nz Te Ara – The Encyclopedia of New Zealand New Zealand history online QuakeStories.govt.nz