• Email to friend
• Print friendly

Subscribe

Recording earthquakes

An instrument that detects and records earthquakes is called a seismograph. The earthquake detector is a seismometer. A simple type of seismometer consists of a cylinder with a heavy magnet suspended by a spring from the top of the cylinder and a coil of wire attached to the interior wall of the cylinder. When the earth moves in an earthquake, the cylinder with the attached coil of wire vibrates with the ground it is standing on. The heavy magnet suspended by the spring stays still relative to the cylinder and the coil of wire. The coil of wire moving in a magnetic field generates electricity in the wire. This electricity travels along a cable to a recorder on site, or to a radio transmitter or fibre optic cable that transmits the signal to a remote recorder. The recorder includes a very accurate clock so that we can determine the times at which the earthquake waves arrive at the seismometer. The earthquake trace, called a seismogram, used to be displayed on paper on a rotating drum, but now the digital data usually goes to a computer.

Determining the location of an earthquake

The locations of seismometers are stored in a database accessible by the earthquake location program. The earthquake program also has access to crustal models – 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 earthquake waves arrive at various seismometers are picked either by people or by a computer programmed to do so. The earthquake location program uses the arrival times of the seismic waves at the seismometers, the locations of the seismometers, and the speed at which the seismic waves travel to the seismometers to determine the focus or hypocentre of the earthquake – its latitude, longitude and depth.

How Geoscience Australia monitors earthquakes

Geoscience Australia monitors seismic data from over 60 stations on the Australian National Seismic Network and over 130 stations worldwide. This is done in near real-time, 24 hours a day, seven days a week, where the majority of the 40 samples per second data are delivered within one minute of their being recorded at the seismometer. The data are delivered by various digital satellite and broadband communication systems to our central processing facility in Canberra.

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

The seismic data are collected and analysed automatically and then immediately reviewed by Geoscience Australia’s Duty Seismologist. Geoscience Australia monitors, analyses and reports on earthquakes to alert the Australian Government, State and Territory Governments and the general public about earthquakes in Australia and overseas.

As part of the Joint Australian Tsunami Warning Centre (JATWC), Geoscience Australia’s Duty Seismologists have the responsibility to analyse and report, within 15 minutes of the origin time, on earthquakes that have the potential to generate a tsunami. An earthquake alert is then sent to Geoscience Australia’s partner in the JATWC, the Australian Bureau of Meteorology, to determine tsunami advice and publish tsunami bulletins.

The parameters of all other earthquakes over magnitude 3.5 are generally computed within 30 minutes. The analysis includes the origin time and date of the earthquake, its hypocentral location (latitude, longitude and depth) and its magnitude. Smaller earthquakes that are not detected by many seismometers are difficult to locate in real-time and are thus computed by Seismic Analysts later.