- Written by Nadia O’Carroll
The crust of the Earth is divided into a patchwork of tectonic plates which are rigid but float on the hotter, viscous layer of the Earth’s mantle, these plates move continuously and independently. There are 7-8 major plates and many minor ones. The movement of the plates causes friction which builds up stress in the brittle upper layers of the plates, at a certain threshold the rocks break and energy is released causing an earthquake.
An earthquake originates at a point called the epicentre and produces three types of seismic waves, which radiate from this point. The fastest are P waves, compression waves that travel 14,000 – 28,000 kms/hour. Then there are S waves which travel at 9,000 – 14,000 km/hour, these move at right angles to the direction travelled, they cannot travel through liquids and usually cause the most damage. The slowest are surface waves, which can travel around the whole earth until they are too small to measure.
Seismic waves that originate from the largest earthquakes can travel inside the Earth for up to a month, the vibrations create a sound but its tone is so low (1 cycle per hour) that it can only be heard by special instruments.
In order to measure earthquakes two different scales have been developed:
The Richter scale was invented by Californian seismologist Charles Richter; it measures the magnitude of energy released at the epicentre on a mathematical scale. It is calculated by comparing wavelengths of seismic waves 100 kms from the point above the epicentre. Less than 2.0 is a micro quake that could not be felt, 8.6 would be the equivalent of 10,000 atomic bombs, 10+ has never been recorded.
The Mercalli Intensity Scale is a more subjective, descriptive scale that measures the effect on the natural and human environment. MM1 is an imperceptible earthquake with no measurable effect, MM12 causes complete devastation.
The reason that New Zealand has so many earthquakes is because it located over the boundary of the Australian and Pacific tectonic plates, the movement of the plates generate earthquakes. Australia is at low seismic risk because the Australian continent is located at the centre of the Australian tectonic plate and there are no large active fault lines. However no place on Earth is immune to earthquakes and even in the centre of a tectonic plates the stress on the plate boundaries can cause intra plate earthquakes.