Earthquake recorded off Portland coast

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Portland harbour.

Portland harbour.

AN earthquake was reported offshore south of Mount Gambier on Tuesday night.

But it’s no cause for concern, according to Port of Portland harbour master captain David Shennan.

The 2.7 magnitude earthquake was reported on the Emergency Victoria website on Wednesday and later on the federal government’s Geoscience website.

The earthquake occurred at 6.57pm at a depth of 10 kilometres.

Mr Shennan said it was the first time in five years he had been advised of an earthquake in the port’s vicinity.

“This is the first time someone has said ‘are you aware there has been a 2.7 quake off Mount Gambier,” he said.

He said while it was unusual, it was not concerning.

“We wouldn’t even notice a 2.7 (magnitude earthquake) in the port,” Mr Shennan said.

He said the federal government would advise the port if there were any earthquakes that posed a threat to the area, but there was no warning issued following Tuesday’s incident.

What is an earthquake?

Earthquakes are the vibrations caused by rocks breaking under stress. The underground surface along which the rock breaks and moves is called a fault plane.

The size or magnitude of earthquakes is determined by measuring the amplitude of the seismic waves recorded on a seismograph and the distance of the seismograph from the earthquake. These are put into a formula which converts them to a magnitude, which is a measure of the energy released by the earthquake. For every unit increase in magnitude, there is roughly a thirty-fold increase in the energy released. For instance, a magnitude 6.0 earthquake releases approximately 30 times more energy than a magnitude 5.0 earthquake, while a magnitude 7.0 earthquake releases approximately 900 times (30x30) more energy than a magnitude 5.0.

A magnitude 8.6 earthquake releases energy equivalent to about 10 000 atomic bombs of the type developed in World War II. Fortunately, smaller earthquakes occur much more frequently than large ones and most cause little or no damage.

Earthquake magnitude was traditionally measured on the Richter scale. It is often now calculated from seismic moment, which is proportional to the fault area multiplied by the average displacement on the fault.

The focus of an earthquake is the point where it originated within the Earth. The point on the Earth's surface directly above the focus is called the earthquake epicentre.

Recording earthquakes

Geoscience Australia monitors, analyses and reports on significant earthquakes to alert the Australian Government, State and Territory Governments and the public about earthquakes in Australia and overseas.

Earthquakes are detected by scientific instruments called seismometers. The word seismo originates from the Greek word seismos which means to shake or move violently and was later applied to the science and equipment associated with earthquakes. 

Determining the location of an earthquake

The accurate locations of seismometers are stored in a database accessible by an earthquake monitoring computer system. The system also has access to crustal velocity models which provide approximate information on how fast the various earthquake waves travel through the different layers which make up the Earth in the area between the earthquake and the seismometers. The times at which the differing seismic waves arrive at various seismometers are identified by Seismic Analysts or by a computer system. The arrival times of the seismic waves at the seismometers, together with the locations of the seismometers and the speed at which the seismic waves travel to the seismometers are all used to determine the location of the earthquake. This location is also known as its focus or hypocentre which is represented by the latitude, longitude and depth below the surface.

How Geoscience Australia monitors earthquakes

Geoscience Australia monitors seismic data from more than 60 stations on the Australian National Seismograph Network and in excess of 300 stations worldwide in near real-time, 24 hours a day, seven days a week. Most of the 40 samples per second data are delivered within 30 seconds of being recorded at the seismometer to Geoscience Australia’s central processing facility in Canberra through various digital satellite and broadband communication systems.

Seismic data are also provided by overseas Governments which have national seismic networks. Geoscience Australia uses data provided by the Governments of New Zealand, Indonesia, Malaysia, Singapore and China and has access to data from global seismic networks provided by the USA, Japan, Germany and France. The Comprehensive Nuclear Test Ban Treaty Organisation’s International Monitoring System also provides seismic data for tsunami warning purposes.

The seismic data are collected and analysed automatically and immediately reviewed by Geoscience Australia’s Duty Seismologist.