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The International Monitoring System (IMS) seismic network is designed to detect and locate underground nuclear explosions with a threshold better than 1kT globally (1 kT is equivalent to a mb 4.0 seismic event occurring in a cratonic region that is well coupled to the earth). The network is composed of 170 seismic stations distributed across the islands and continents of the earth. Fifty of the seismic stations are designated as primary stations and they transmit continuous data to the International Data Centre (IDC) in real time. The remaining 120 stations, known as auxiliary stations, transmit data segments to the IDC upon request.

Most of the events recorded on the network will be earthquakes. Currently 100 earthquakes are located each day, along with the occasional recording from mining areas. Active research is being undertaken to discriminate between the different source types being recorded on the network. Key discriminants such as body-wave to surface-wave magnitudes; depth; hydroacoustic signatures and P to S amplitude ratios are currently being used to differentiate between earthquakes and nuclear explosions.

The seismic stations are made up of array and three-component stations. An array station is a group of three or more identical seismic sensors deployed in an optimal configuration ranging in area from a few square kilometres to a few hundred square kilometres. The outputs of the individual sensors can be summed up to increase the amplitude of the event signal relative to the general background noise and as a result they are more sensitive than stations comprised of one sensor alone. The arrival times of signals at the various sensors in an array are used to determine the distance and direction to the seismic source.

Three-component stations are made up of three seismic sensors at one site arranged such that one sensor detects vertical motion and the other two detect horizontal motion in the north-south and east-west directions. This configuration also allows the station to determine the distance and direction to the seismic source, although the uncertainty is much larger than that calculated by an array.

Nuke or earthquake? Novaya Zemlya nuclear explosion recorded on RSNT seismograph station in USA and a deep Fiji earthquake recorded on ASAR seismic station in Australia. Canberra seismograph recording of 16th August 1995. Note that the significant Lop Nor explosion is swamped by other seismic events on that day. May 28, 1998 Pakistan nuclear explosion recorded on the Australian National Seismograph Network.
Nuke or earthquake? Novaya Zemlya nuclear explosion recorded on RSNT seismograph station in USA and a deep Fiji earthquake recorded on ASAR seismic station in Australia.
© Geoscience Australia
Canberra seismograph recording of 16th August 1995. Note that the significant Lop Nor explosion is swamped by other seismic events on that day.
© Geoscience Australia
May 28, 1998 Pakistan nuclear explosion recorded on the Australian National Seismograph Network.
© Geoscience Australia
Schematic of a seismic array recording a seismic signal. Photograph of CTBT seismic station at Stephens Creek, near Broken Hill, Australia.
Schematic of a seismic array recording a seismic signal.
© Geoscience Australia
Photograph of CTBT seismic station at Stephens Creek, near Broken Hill, Australia.
© Geoscience Australia

 

Topic contact: hazards@ga.gov.au Last updated: July 19, 2011