The CTBTO International Data Centre Bulletin
Robert G. North, Center for Monitoring Research, Richard A. Gustafson, Defense Threat Reduction Agency
The Comprehensive Nuclear Test Ban Treaty (CTBT) requires an International Monitoring System (IMS) to provide signatories with the information needed to verify compliance with the treaty. The IMS consists of a worldwide network of hydroacoustic, infrasonic, radionuclide and seismic sensors, connected by a global communications infrastructure to an International Data Centre (IDC) located in Vienna, Austria. The IDC acquires data from the sensor sites, authenticates and archives these data, and carries out automatic processing.
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Date Time Lat
Lon Nph Depth Mag Region Partial list of seismic events recorded by the PIDC for May 11, 1998, including the Indian nuclear test (mag. 5.0). |
IMS seismic stations, distinguishing array/3-C and primary/auxiliary, filled symbol if data received in February 2000 and open otherwise.
The output of that processing is analyzed, revised and corrected, and the results are published in several bulletins and reports. The Vienna IDC was established in 1997. It now has a large multinational staff and substantial computing capability. The IDC began producing regular bulletins in February 2000.
Software used for the IMS is the result of extensive development and testing concentrated at a Prototype IDC (PIDC) at the Center for Monitoring Research in Arlington, Virginia. The ongoing development draws upon the results of an extensive worldwide research program over the last four decades, and the techniques, procedures and software have been thoroughly tested in a series of international experiments devised by the former Group of Scientific Experts (GSE) that met in Geneva from 1976 to 1996 under the auspices of the United Nations Conference on Disarmament. The last of these experiments, the GSE Third Technical Test (GSETT-3), started in January 1995. It involved the transmission of real-time continuous data from worldwide seismic stations to the PIDC, as well as requests for, and receipt of, segments of data from additional seismic stations. At the PIDC, these data were processed and the results analyzed and subsequently distributed in a daily Reviewed Event Bulletin (REB).
The GSETT-3 experiment forms the basis of the seismic component of the IMS, and was expanded in 1996 to include the use of hydroacoustic, infrasonic and radionuclide data. The original GSETT-3 network had a disproportionate number of stations in Australia, North America and Europe. In July 1996, this number was reduced to only those stations that were specified in the Treaty as part of the 50-station primary (continuous data) and 120-station auxiliary (segmented data, on demand) seismic networks. In February 2000, 30 of the primary stations, and 32 of the auxiliary stations provided data to the PIDC. A significant number of the auxiliary stations were installed by IRIS, and several of the primary stations have IRIS affiliations. The map above shows the seismic component of the IMS network.
- the limited size of the network, with a high proportion of very sensitive arrays
- all waveform data used are digitally recorded, and acquired directly by the PIDC within hours of real time,
- initial signal detection and network processing are uniform and completely automated
- processing and analysis includes the direct use of directional information (azimuth and slowness) from both arrays and three-component stations
- the automatic phase readings and event associations are all uniformly reviewed, corrected and supplemented by a small team of analysts who interact directly with a large relational database,
- in northwestern Eurasia, and most of North America, path-dependent travel-time curves for regional phases (Pn, Lg, etc.) based on empirical data are used in event location
- magnitude estimates
(mb, Ms, and "ML" based on
Pn amplitude) are completely automated.
Ms measurements take into account variations in surface wave velocity across the surface of the earth (Steven and McLaughlin, 2000) - bulletins are completed and issued within a few days of event occurrence (2 days through 1996, and from 2-6 days thereafter) by the PIDC. This is considerably faster than the NEIS (~6 months to final bulletin) and ISC (2 years).
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CTBT International Data Centre, Vienna, Austria. |
The use of directional information allows events to be detected and located by only one station, because azimuth can be combined with distance derived directly from slowness, or determined from the relative arrival times of different phases, such as P and S, from the same event at a given station. The PIDC combines this information from as few as three stations to form a more reliable estimate. This means that the PIDC bulletin contains many more small teleseismic (seen to distances of greater than 2000 km) events than the NEIS or ISC, which generally require observations by at least five stations to "form" an event.
The PIDC (and IDC) bulletins include many parameters not included in the other global Bulletins. Event location accuracy is expressed in terms of a "confidence ellipse" rather than standard errors in latitude and longitude. In addition to computing an event average (of station values) magnitude, maximum-likelihood body and surface wave magnitudes that take into account those stations that did not record the event are also computed (Ringdal, 1976). These degenerate to "upper-bound" magnitudes if there are no teleseismic body-wave observations (mb), or no surface waves (Ms) are seen. It is possible to compute these because continuous data are available for the primary stations, and thus noise levels at the predicted signal times can be measured. In addition to the standard station, phase, arrival time, time residual and amplitude/period/magnitude values, the phase reports include azimuth and slowness values and residuals, and signal to noise ratio. The last is a better indicator of the quality of the phase picks than the traditional "impulsive" and "emergent" notations.
Approximately 10% of events in the REB are believedlargely on the basis of their locationto be industrial explosions for mining or other purposes.
An extension of the REB, the "Standard" Event Bulletin or SEB, includes additional measurements specific to the identification of an event as either natural or artificial. These comprise various measures of energy content in different wave types (P, S, Lg, etc) and frequency bands, as well as indicators of waveform complexity. Because event depth is an important discriminant (those more than a few km deep are almost certainly natural) the analysts pay particular attention to finding "depth phases" (surface reflections pP, sP, etc). This task is greatly facilitated by the ability to display waveforms from many events at the same time and look for distinctive patterns of phase signalsa luxury not available when an analyst at a single station reports to an agency such as the NEIS or ISC.
Many earthquakes occur under the oceans or near coastlines, and T-phases (seismic waves coupled into the ocean) are reported in the REB for over 20% of all events. Infrasonic observations are much rarer, but have been reported in the REB in connection with large earthquakes.
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The analyst room at the PIDC, where all the processing and analysis that results in the Reviewed Event Bulletins is carried out. |
Bulletin Distribution
The PIDC bulletins have been widely distributed by the www.pidc.org web page, an email-based AutoDRM system, and through subscriptions, which provide regularly emailed bulletins that can be tailored for the recipient. The PIDC web page also provides access to monthly "Performance Reports"; daily summaries of station and communications link status; hourly summaries of overall IMS network performance in the form of "threshold monitoring"; descriptions of significant system changes, which must be approved in advance; and documentation of software, procedures and formats used.
Summary
The IDC that is an integral part of the CTBT verification system can, as demonstrated by the prototype IDC, contribute significantly to the reporting of earthquakes and other phenomena, both natural and man-made. The rapid processing and analysis of data, and distribution of the results, has already improved the monitoring of earthquakes by other agencies, but will not supplant their role as the recognized authorities on global seismicity, using a much broader range of data sources. Many of the processes and techniques developed for the IDC could be used by these other agencies to improve their products.
Future Access Uncertain
The PIDC openly distributed the REB from January 1, 1995 to February 20, 2000. On February 21, 2000, the IDC in Vienna assumed responsibility for generation and distribution of products such as the REB. As a consequence, the PIDC discontinued daily bulletin production. (Bulletins up until that date are still available from the PIDC.)
At present, the Vienna IDC does not provide open access to the REB. It is expected that this situation will change eventually. Organizations such as the Federation of Digital Seismograph Networks (of which IRIS is a leading member), the International Association of Seismology and Physics of the Earth's Interior, the International Union of Geodesy and Geophysics, the American Geophysical Union, and the Seismological Society of America are actively campaigning for open access to the IMS data. see AGU/SSA Joint Position Statement, p. 21, and Seismologists Campaign for Open Data, p. 19.
Additional Information
For additional information about the Prototype International Data Centre, visit their website at www.pidc.org. The CTBT Organization in Vienna website (www.ctbto.org) provides information about the International Data Centre in Vienna.
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Impact on Other Bulletins The PIDC has substantially increased the completeness of the global bulletins produced by the USGS NEIS and the ISC. Both agencies use PIDC phase readings in the preparation of their bulletins. The figure at right shows that the number of events recorded at teleseismic distances that were reported by the NEIS jumped significantly in 1995 from earlier levels. This is a combination of phase reports being available to locate more such events (about three-quarters of the increase) and body-wave magnitudes being reported by the PIDC for events that would otherwise not have such a magnitude assigned (the remaining quarter). It was noticed that the PIDC magnitudes were somewhat lower than those of other stations reporting to the NEIS, so the NEIS discontinued use of the PIDC station magnitudes in August 1996. The ISC, however, also studied this and decided not to exclude PIDC magnitudes from their bulletins for several reasons (see www.isc.ac.uk/Documents). The PIDC contributed nearly half of all the teleseismic P amplitudes reported to the ISC. While the PIDC reports the amplitude and period of every observed P wave, most other stations and agencies only do so rarely. |
Impact on Other Bulletins Furthermore, the additional capability provided by the PIDC greatly reduced the number of events appearing in the ISC bulletins that had not been previously reported by other agencies. This last effect is particularly pronounced in oceanic regions, where the ISC had, up to 1994, located several hundred "new" events each year. The contribution of the CTBT International Monitoring System to global seismology can be expected to increase as the network is completed and the centralized processing and analysis systems at the IDC are improved.
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