Earthquakes and an earthquake in Australia

Introduction

Earthquakes are a sudden shaking of the ground caused by the movement of tectonic plates. Many people think that Australia never has earthquakes. There have, however, been a number of earthquakes on the Australian continent in recorded history. In fact, Australia's level of earthquake activity is moderate to high compared with other intraplate regions, or regions in the centre of a tectonic plate. See image 1

How earthquakes happen

Earthquakes are created by the movement of tectonic plates. Most earthquakes occur at or near convergent and transform plate boundaries, although in the case of Australian earthquakes, they also occasionally occur in the centre of tectonic plates. This is because the Indo-Australian Plate, like all tectonic plates, is moving. This movement causes stress within the plate. Tectonic plates are constantly moving (though too slowly for the human eye to see) so there are constantly small earthquakes that occur around the world every day. The majority of these small tremors are not felt by humans.

Earthquakes originate in the depths of the Earth. The point from which the earthquake originates within the Earth is known as the focus point. The point on the surface directly above the focus point is known as the epicentre. Earthquakes that are closer to the surface generally have more destructive power than those with deeper focus points. Shallow focus points are less than 70 kilometres (km) deep. Deep focus points are over 300 km deep. The deepest earthquake ever recorded had a focus point of just over 700 km. See image 2

Fault movements are used to describe the different ways that the tectonic plates move during an earthquake. Earthquakes occur as a result of one of four different types of plate movement: slip-strike; dip-slip; reverse; or thrust. The strike-slip fault occurs when two plates move laterally (side by side) to one another at a transform plate boundary. Generally, only one plate moves while the other remains stationary. The dip-slip fault, also known as the normal fault, occurs when one side of the fault drops below the other side. The reverse fault is the opposite of the normal fault. Instead of one side dropping down, one side is pushed up. This type of movement is evident in rock layers that no longer match on either side of the fault. The thrust fault is similar to a reverse fault except that one side of the fault is pushed up and over onto the other side. This occurs at convergent plate boundaries.

What happens in an earthquake

When the tension between plates is relieved, an earthquake occurs. The movement at the focus point sends waves of energy that radiate up to the epicentre and outwards. These waves of energy are known as seismic waves. These seismic waves make it possible for people who are many kilometres away from the epicentre to feel the tremor. The distance that seismic waves travel outwards from the epicentre depends on the magnitude of the quake and the type of materials the waves must pass through. Seismic waves can travel both under the surface on the Earth (body waves) or on the surface of the Earth (surface waves).

Tsunamis are a result of either volcanic or seismic activity. Earthquakes, especially when the epicentre is in the ocean, can create tsunamis. Powerful volcanic explosions either under the ocean or on an island can also create tsunamis. The volcanic explosion at Krakatau, Indonesia, blew apart the island and sent shock waves, in the form of a tsunami, across the ocean. The 2004 Indian Ocean earthquake produced shockwaves, in the form of a tsunami, across southern Asia. This is known as the Asian, or Boxing Day, Tsunami. See image 3

Magnitude measures the amount of energy released during an earthquake. It measures how strong the earthquake was at its focus. The most widely recognised and used form of measurement for earthquakes is the Richter scale. The Richter scale uses a mathematic scale to measure an earthquake's energy. This scale has no definite starting or ending value. Earthquakes up to 2.0 are considered micro quakes, and are not felt on the surface of the Earth. Earthquakes above 9.0 are considered massive quakes and cause extensive damage to surrounding areas.

Intensity measures the effects of an earthquake in any specific region. The intensity of an earthquake is measured by the reaction of various people. Intensity is dependent upon distance to the earthquake epicentre, the depth of the focus, the type of geology in that region, the type of buildings in that region and individuals' reactions. The Modified Mercalli Intensity Scale is a series of Roman numerals from one to twelve that describe the effects of the earthquake on the surrounding environment, with I being very weak and not felt at all, and X-XII being devastating, with many buildings destroyed.

Preventing earthquake damage

There are still no ways to truly predict an earthquake. Scientists have been studying data collected from seismographs, or devices that measure earthquake magnitude, in the hope of finding a pattern. By studying how earthquakes work, scientists can devise ways to help prevent extensive damage to the surrounding environment. See image 4

In many places around the world there are standards that require buildings to have certain earthquake-resistant elements. These standards include building designs that are able to withstand and move along with earthquake waves and resilient materials that resist breaking under stress from earthquakes.

An Australian earthquake: the 1989 Newcastle earthquake

On 28 December 1989, an earthquake measuring 5.6 on the Richter scale hit Newcastle. The epicentre was 15 km south-southwest from the city centre. This earthquake caused widespread damage, damaging 50 000 buildings and killing 13 people. Damages are estimated to have cost $1.5 billion. The death toll from the earthquake could have been a lot worst. Fortunately, school was not in session at the time, as many schools were badly damaged. See image 5

Although 5.6 is generally considered to be only a moderately strong earthquake, many buildings were destroyed. This was due to a number of factors. Newcastle was considered to have a low seismic risk, so buildings were not expected to adhere to earthquake standards. Many of the damaged buildings were built on alluvial land, or land deposited by water, which is affected more strongly by earthquake waves. The shaking went on for a very long time, 35-40 seconds. Finally, the focus was very shallow at only 11 km below the surface of the Earth. While the earthquake's measurement on the Richter scale was moderate, its measurement on the Mercalli scale would have been much higher.