However, this may be revised when a clearer picture emerges of the extent of the damage not only to buildings but infrastructure such as sewerage and water supply networks. The Canterbury earthquake has been given a MM9 rating. The 1855 Wellington and 1931 Napier earthquakes were both MM10 at their epicentres. The scale quantifies the intensity of ground-shaking and destructiveness of an earthquake from MM1 (smallest) to MM12 (cataclysmic). Mercalli was an Italian seismologist (and priest) who developed his scale for measuring quakes in the early 20th century. The Modified Mercalli (MM) intensity scale is used for measuring the intensity of an earthquake. The 1855 Wellington earthquake had an estimated magnitude of 8.2, the Napier earthquake of 1931 was 7.8, and the recent Canterbury earthquake was of magnitude 7.1. The largest recorded earthquake was the 9.5 Chilean earthquake in 1960. It is a logarithmic scale, which means that a magnitude seven earthquake is 32 times as powerful as a magnitude six quake. The magnitude is a measure of the amount of energy released by the earthquake. The size of an earthquake is referred to as its Magnitude (although the Richter scale is often used interchangeably by the non-scientific community). The Richter scale has been superseded by the moment magnitude scale, which is calibrated to give generally similar values for medium-sized earthquakes (magnitudes between 3 and 7 on the Richter scale) but measures larger earthquakes with greater accuracy. Richter’s method worked well for small to moderate earthquakes, but the magnitudes of very large earthquakes were underestimated. The Richter scale was based on the largest amplitude recorded on a seismograph. In 1935 the American seismologist Charles Richter invented a scale to indicate the strength of an earthquake. Scientists from the Institute of Geological and Nuclear Sciences (GNS) think it is the first time this fault has moved in several thousand years, and it is unlikely that it will move again for another few thousand years. The epicentre was 40 km west of Christchurch city close to the town of Darfield, at a depth of 10 km. It is a previously unknown fault under the Canterbury Plains. The fault that caused the Canterbury earthquake on 4 September 2010 has been named the Greendale Fault. The ‘S’ (secondary or shear) waves follow along at about 10,000 kilometers an hour, and cause the main rolling and shaking effects of an earthquake. The first sign of a quake is often the rumbling sound caused by the ‘P’ (primary or push) waves travelling at about 20,000 kilometers an hour. Earthquakes cause waves of vibration to travel though the ground. The point under the ground where the earthquake actually begins is called the hypocentre or focus, while the place directly above it on the surface is known as the epicentre. Most (though not all) earthquakes occur at faults, which are breaks extending deep within the earth, caused by the movement of these plates. Other areas, where the movement is not constant, are prone to stronger quakes separated by longer periods of time. In some places movement between the plates is happening all the time, causing frequent small or moderate earthquakes. Tension builds up as they scrape over, under or past each other. These plates are in constant motion, travelling at a few centimetres per year. There are ten major plates and many more minor ones. Tectonic plates are large plates of rock, about 100km in thickness, that make up the foundation of the earth's crust and the shape of the continents (like pieces of a giant jigsaw puzzle). New Zealand lies on the boundary of the Pacific and Australian tectonic plates.
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