Global Seismographic Network

A consortium and a partnership

In 1984 US universities founded the Incorporated Research Institutions for Seismology (IRIS), and the USGS, the National Science Foundation and IRIS began the partnership that built the Global Seismographic Network. The first broadband seismometers were installed in 1986, and the initial network deployment began in 1991. By 2011 it spanned more than 150 stations.

Many stations are hosted by partners in other countries, who keep them running and share the data, making the GSN an international federation rather than an American network.

Broadband and digital

The WWSSN had two instrument types, tuned to short and long periods. The GSN's seismometers are very broadband, recording ground motion from high-frequency signals down to the slow free oscillations of the whole Earth after a great earthquake, with a dynamic range of 140 decibels or more. Data are digitised at the station, time-stamped from GPS clocks and sent over satellite or the internet to the IRIS Data Management Center in Seattle and to the USGS in Golden.

The records locate earthquakes within minutes, feed tsunami warning centres, and are used by researchers to image the interior of the Earth. The data policy is open: anyone can download them without charge.

A new institution

In 2023 IRIS merged with UNAVCO, the geodetic consortium, to form the EarthScope Consortium, which now operates parts of the network with the USGS. The network also contributes to the monitoring of nuclear tests, a link with the origins of the WWSSN, and to research on earthquake sources and the structure of the core.

What a federation needs

The GSN is operated by the USGS and by EarthScope on behalf of partners around the world, and its stations are hosted by institutions in many countries that bear much of the local cost. The arrangement works because the data policy is open, the standards are clear and the host institutions receive the data and the instruments' maintenance in return. When funding is uncertain, the stations are the first to feel it, and the network has had to defer replacement of ageing instruments. A federation needs a stable core to keep standards and archives, and the willingness of the partners to keep stations running.

Instruments and technologies

• Very broadband seismometer A feedback seismometer with a flat response from 0.003 to about 10 hertz, housed in a vault or borehole to cut out temperature and wind noise.
• 24-bit digitiser and GPS clock A data logger that digitises the signal at 20 or more samples per second, with timing from GPS.
• Data management centre An archive that receives, checks and distributes the data, with metadata describing every station's instrument response.

Timeline

• 1984 IRIS is founded and the USGS–NSF–IRIS partnership for the GSN begins.
• 1986 First GSN broadband seismometers installed.
• 1991 Initial network deployment begins.
• 2011 The network spans more than 150 stations.
• 2023 IRIS merges with UNAVCO to form the EarthScope Consortium.

Key lessons for observation today

• Federate by hosting, not by absorbing Partner institutions host stations and share data. A network can be global while its stations remain owned locally.
• Open archives with rich metadata The GSN's value lies in an archive with complete instrument response. Data without metadata cannot be used for precise work.
• Accurate time is part of the data GPS timing makes records from different stations combinable. Timing infrastructure is as important as the sensors.

Sources

• USGS, GSN - Global Seismographic Network
• USGS Fact Sheet 2011–3065, Albuquerque Seismological Laboratory
• Wikipedia, IRIS Consortium (secondary)