What causes Earthquakes?
Earth's outer layer is broken into pieces called tectonic plates which are about 100km thick and are constantly moving towards, away from or past each other. For example, the plate containing Australia and India is moving north at the rate of 7cm a year, causing an intracontinental collision with the Eurasian Plate in the Himalayas. That is why these mountains are so high. Because continents are part of these plates, they also move. An earthquake occurs when the rocks break and move as a result of stresses caused by plate movements.
Most earthquakes occur on the boundaries between plates, where one plate is forced under another such as happens off island chains such as Japan, Indonesia or the Solomon Islands, or past another as occurs in California and New Zealand. Some regions have more earthquakes than others with 80 per cent of all recorded earthquakes taking place around the edge of the Pacific Plate, including New Zealand, Papua New Guinea, the Solomon Islands, Vanuatu, Japan, Canada, USA and South America.
In areas where plates collide, earthquakes can occur down to depths of 700km. In areas where plates slide past each other, such as California or New Zealand, earthquakes are shallower. Shallow earthquakes also occur where plates are pulling away from each other along under sea ridges, and the oceans are growing bigger, like the plate margin between Australia and Antarctica.
Earthquakes that do not occur on plate margins are called intraplate earthquakes. All earthquakes on mainland Australia and Tasmania are intraplate. On studying these intraplate earthquakes in various continents, seismologists have found that most of them are caused by thrust faulting due to the rocks being squeezed or compressed. It seems that the movement of the tectonic plates causes the rocks away from their margins to be compressed. Intraplate earthquakes are not as common as those on plate margins, but major earthquakes with magnitudes of 7.0 or more do happen occasionally.
Molten rock, called magma, is stored in reservoirs under volcanoes. As this magma moves upwards, it can fracture the rock it squeezes through, causing earthquakes, usually with magnitudes not much greater than 5.0. Sometimes the magma collects in a high level reservoir prior to a volcanic eruption and as it moves around it causes bursts of continuous vibration, called volcanic tremor. Because of these precursors, seismographs (earthquake recorders) are very useful for monitoring volcanoes to give warning of an impending eruption.
Foreshocks and Aftershocks
Foreshocks are smaller earthquakes that may occur in the same area as a larger earthquake that follows. They are caused by minor fracturing of rocks under stress prior to the main break that happens during the largest earthquake of the series, called the mainshock. Foreshocks can start up to a year before the mainshock, as was the case before the three large (magnitudes between 6.3 and 6.7) earthquakes near Tennant Creek in January 1988. Not all earthquakes have foreshocks, and sometimes a series of similar sized earthquakes, called an earthquake swarm, happens over months without being followed by a significantly larger mainshock. This limits the usefulness, at this stage, of foreshocks in earthquake prediction.
Aftershocks are smaller earthquakes that may occur after the mainshock, in the same area. They are caused by the mainshock area readjusting to the fault movement, and some may be the result of continuing movement along the same fault. The largest aftershocks are usually at least half a magnitude unit smaller than the mainshock and the aftershock sequence may continue for months or years after the mainshock. Not all earthquakes have aftershocks – the magnitude 5.6 Newcastle earthquake in 1989 only had one aftershock, which was very small with a magnitude of 2.1. Occasionally, small earthquakes with magnitudes between 3.0 and 3.5 have aftershocks. This has been observed in the Dalton-Gunning area, north of Canberra.
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