The International Seismological Centre (ISC)

In the early 1960s it was generally recognized that a new organization was needed to bring together all available seismic phase data in order to produce a definitive catalog of global seismicity. After numerous meetings and consultative committees, the International Seismological Centre (ISC) was formed as a direct descendant of the older International Seismological Summary (ISS) based in the United Kingdom and of the Bureau Central International de Seismologique (BCIS) based in France. The Centre's mission was similar to those of the earlier organizations, but the Centre differed im-

portantly in being more adequately funded by supporting members, initially from several countries. Although the original membership was not great, it has grown steadily to include some forty organizations from nearly as many countries.

The objectives of the ISC as set out in its Working Statutes are the collection and analysis of terrestrial seismic events for the advancement of scientific knowledge of earthquakes, their prediction, and their modification. The principal tasks of the Centre are:

1. To collect seismogram readings from station networks throughout the world in cooperation with other seismological data centers and organizations.

2. To edit and preserve the readings for analysis in computer-readable form.

3. To compute focal coordinates and magnitudes for each event for which adequate data are available and to publish monthly a bulletin containing the fullest possible listing of original data and the parameters derived therefrom.

4. To prepare and publish a regional catalog of earthquakes.

5. To undertake such research and development efforts as may be relevant to improving the execution of the abovementioned tasks, and which are within the terms of these statutes.

6. To provide such other services to institutions or individual scientists and engineers throughout the world as are compatible with the execution of the abovementioned tasks, and with the data and facilities available within the Centre.

7. To undertake such other relevant activities as may be approved by the Governing Council on its own motion or on the recommendation of the Executive Committee.

At the beginning it was planned that the ISC would also establish a microfilm bank, and in fact the United States Coast and Geodetic Survey (USCGS) sent the Centre the 1964 output from the Worldwide Standardized Seismographic Network (WWSSN), which included microfilm from the Canadian network, and negotiations were well advanced with the Academy of Sciences of the USSR for copies of Soviet seismograms (J. H. Hodgson, personal communication). However, this activity has not continued owing to lack of funds.

Today the ISC is a lean, well-run organization, collecting some 1,200,000 seismic phase data from over 1,500 seismographic stations distributed around the globe to locate more than 25,000 earthquakes each year. The Centre also reports the earthquake locations estimated by other organizations which individually have access to fewer data as preliminary input, and uses "free search" computer routines to find events that have not been re-

ported. The Centre has neither the staff nor the facilities to study waveform data.

The strength of the ISC catalog of earthquakes and the associated phase arrival data base lies in its completeness and comprehensive global coverage and in the uniformity of its techniques over the years. It cannot, of course, improve on the locations of events that have occurred within dense local or regional networks, but for those parts of the globe without such coverage it provides the definitive focal parameters.

The parameters of large events are now being calculated by seismologists at Harvard and the United States Geological Survey by the analysis of digital waveforms from the available broadband stations. These solutions are routinely published by the USGS in the National Earthquake Information Service (NEIS) monthly summaries. As the global digital networks develop, other organizations also are likely to make estimates of these parameters. The ISC, for its part, will provide the definitive information for intermediate magnitude events (less that magnitude 6.5), particularly in regions poorly monitored by local seismographic stations. The Centre will also, surely, continue to collate, archive, and distribute the definitive cumulative files of phase data and earthquake epicenters. The value of these files grows with time, and studies by Morelli and Dziewonski (1987) and others demonstrate the wealth of information the files contain on the heterogeneous nature of the Earth's interior. The earthquake catalogs themselves also are proving to be increasingly valuable as fundamental input to assessments of earthquake hazard. The importance of the catalogs will, of course, also continue to develop as they span longer periods of time.

In the future, as data from global networks become readily available, it will be natural for the ISC to wish to use such data to refine its estimates of earthquake focal parameters. Such a change from current practice is likely to receive support from the ISC's Governing Council, but the change will not be possible without extra resources, always difficult to acquire.

As the global digital networks develop, an international data center will be needed to collate, archive, and distribute the digital waveforms to the international seismological community. Such a role for the ISC would be fully compatible with its mandate, but again would require additional financial support.