Federation of Digital Seismograph Networks (FDSN) and the Global Seismographic Network (GSN): The Global Seismographic Network is jointly operated by IRIS member institutions (primarily the University of California, San Diego) and the United States Geological Survey (USGS). As a founding member of the international Federation of Digital Seismograph Networks (FDSN), IRIS has coordinated its efforts with foreign groups developing similar regional and global monitoring programs. The international cooperation decreases cost, and allows for greatly enhanced coverage. IRIS, in partnership with the FDSN, brings to treaty monitoring an established structure for the collection and distribution of data from a global network of seismic stations.
Since its first installations in 1986, the GSN has seen steady progress toward its long-term goals of global coverage. Over 50 GSN stations are now operating. The locations of these stations, along with other planned and existing open stations are shown in the figure below.2 Many of these stations have been installed under IRIS's Joint Seismic Program (JSP), a cooperative effort between IRIS, the USGS and the former Soviet Union (FSU). Over a dozen stations of the GSN are in operation in Russia, Kyrgyzstan, Turkmenistan and Armenia, with plans for additional stations in Kazakhstan, Tadjikistan, Pakistan, India, Africa, and other locations in the Middle East and southern hemisphere. Networks and arrays, for detailed studies of regional seismicity and with direct relevance to nuclear monitoring, have been operating in southern Russia, Armenia, Kyrgyzstan and Turkmenistan.
Because IRIS stations have multiple applications, they are not necessarily designed to optimize the recording of very small distant events. For example, placing a station in a certain area to provide coverage or analysis of critical earthquake activity will at times be chosen over a quieter site. In addition, the recording of data through a broad range of frequencies can create the appearance of noise to those who are unfamiliar with digital filtering methods. In fact, the broad frequency feature is occasionally cited as a reason for IRIS stations being noisy and therefore inadequate for monitoring. The figure on the next page, however, compares IRIS stations with those stations reporting to the last Conference on Disarmament Group of Scientific Experts (GSE) experiment.3 If you discount the inherently noisy island sites, it can be seen from the figure that on average the IRIS GSN stations are quieter than the stations used for the last GSE experiment.
U.S. National and Regional Seismographic Network: The U.S. National Seismographic Network (U.S. NSN) is being established by the USGS for earthquake monitoring. Data from the NSN stations are sent via satellite to the National Earthquake Information Center in Colorado, where automatic analyses of location and magnitude are performed in near real time. The hardware used in the U.S. NSN is similar to that used in IRIS/USGS GSN and many of the U.S. NSN stations have been installed cooperatively by the USGS and IRIS and designated as joint sites. Data from NSN stations in the U.S. are supplemented by additional data from other national and international stations to produce a series of catalogs by the National Earthquake Information Center - an Earthquake Alert Bulletin within minutes to hours of significant earthquakes; a weekly Quick Epicenter Determination (QED) which lists all located events; and an updated, monthly Preliminary Determination of Epicenters (PDE).
ARPA Arrays: The Nuclear Monitoring Research Office (NMRO) of the Defense Department´s Advanced Research Projects Agency (ARPA) carries out a program in the development of advanced techniques for the monitoring of nuclear explosions. In the past, this program has involved stations with global coverage, but these have largely been transferred to the USGS and the GSN. The ARPA program currently consists of special purpose arrays in Scandinavia and Europe. As part of its array programs, ARPA has developed a specialized system, the Intelligent Monitoring System (IMS), for processing data. ARPA also supports the operation of the USGS stations in China. Data from this Chinese network are contributed to IRIS/USGS GSN.
Atomic Energy Detection System (AEDS) Network: The Air Force Technical Applications Center (AFTAC) currently is responsible for maintaining the U.S. operational capability for monitoring nuclear explosions in the context of treaty compliance.4 The U.S. Atomic Energy Detection System (AEDS) includes more than fifteen special purpose short period arrays which transmit data continuously to the AFTAC at Patrick Air Force Base in Florida. Data from these stations are not available to the general research community. Both the capabilities of the network and the bulletins it produces are classified. An open extension of the AEDS network, the Ancillary Seismic Network (ASN), has been under development in the southern hemisphere since 1978.5 The ASN stations will be operated by the USGS for the AFTAC. The ASN data will be archived in the IRIS Data Management System.
All of these systems have important roles in the non-proliferation regime. The Global Seismic Network provides an open source of continuous seismic data. The data are important not only for detecting and locating seismic events, but also for developing new methods of discriminating earthquakes from explosions at regional distances. Regional networks are useful for characterizing seismicity in areas of frequent earthquakes, and will allow for accurate locations and low-detection capability throughout the area of the network. Special tuned arrays are likely to remain the optimal means for detecting small events at great distances.
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Return to Chapter III: Types of Seismic Stations
Continue to:Chapter III, The Analogy with Earthquake Monitoring