Earthquakes happen every day in New Zealand. Instruments record the ground shaking from over 14,000 earthquakes in and around the country each year. Most are too small to be noticed, but between 150 and 200 are big enough to be felt.
Plate movement and faults
Earth’s seemingly solid outer surface – the continents with their submarine shelves, and the rocky floor of the deep ocean – is in fact divided like a jigsaw puzzle into huge sections called plates. Driven by convection currents deep within the earth, these plates move a few centimetres per year across the surface of the planet – the rate at which your fingernails grow.
Earthquakes are most frequent in regions where two moving plates meet and press against each other. New Zealand is in such a region – it straddles the boundary between the Pacific Plate, which covers almost a quarter of the earth’s surface, and the Australian Plate.
Where plates collide, the brittle top layer of the plate – the crust – slowly distorts, and stress builds up over many years until the crust ruptures.
Earthquakes usually occur along faults, which are existing fractures in the crust. Sometimes the blocks of rock on either side of a fault abruptly shift to a new position in just a few seconds. This sudden release of energy sends out waves, which are felt on the surface as an earthquake. The strength of the quake depends on the area of fault that has shifted and the amount of movement.
Goodbye to the Richter scale
In 1935 the American seismologist Charles Richter invented a scale to indicate the strength of an earthquake. The Richter magnitude was based on the largest amplitude ‘wiggle’ recorded on a seismograph. Richter’s method worked well for small to moderate earthquakes, but the magnitudes of very large earthquakes were underestimated. Today, with better instruments, scientists can measure the energy of different types of earthquake waves. The size of earthquakes is referred to as magnitude, but it is no longer measured on the Richter scale.
The most powerful New Zealand earthquake on record occurred in the Wairarapa in 1855. During this quake, land moved along at least 140 kilometres of the Wairarapa Fault. Where the movement was greatest, the land shifted horizontally more than 18 metres along the fault, and part of the adjacent Rimutaka Range rose more than 6 metres.
Not all of the movement may occur during the initial earthquake. Subsequent quakes, called aftershocks, occur nearby as the earth adjusts to the dislocation of land along the fault. Aftershocks may occur for weeks, months, or even years after a large earthquake.
One measure of the energy released by an earthquake at its source is earthquake magnitude. Magnitude is commonly determined from the shaking recorded on a seismograph. Each unit of magnitude on the scale represents a substantial increase in energy. A magnitude 5 earthquake, for example, releases more than 30 times more energy than a magnitude 4, and a magnitude 6 more than 900 times more energy than a magnitude 4. The powerful 1855 Wairarapa earthquake – the largest recorded in New Zealand in the last 200 years – had a magnitude of about 8.2.
Bedrock is better
The damage caused by earthquakes depends in part on the type of ground beneath buildings. Bedrock generally shakes the least, while soft sediment or artificial fill can amplify shaking. Some loose, water-saturated soils may even liquefy, producing geysers of sand or mud. Buildings on liquefied soils may settle or list like ships. Maps showing zones with different ground-shaking amplification, based on ground conditions, are available for several New Zealand cities.
Measuring earthquake shaking: the Modified Mercalli scale
As they travel outward from the earthquake source, earthquake waves lose energy and the shaking they cause gradually diminishes in intensity. Shallow earthquakes, originating near the surface, cause more severe shaking and damage than deep earthquakes of the same magnitude.
To study the effects of a particular earthquake in different parts of the country, the intensity of shaking at a number of localities is determined using the Mercalli scale. This scale ranks the severity of shaking by its effects on people, buildings and the environment – from barely perceptible tremors to ground movements that can topple large buildings. The amount of damage to buildings caused by a given amount of shaking, however, varies from country to country, depending on the way the buildings are constructed. New Zealand uses a modified version of the Mercalli scale that is based on the local building types.