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IMPROVED WORLDWIDE STANDARDIZED SEISMOGRAPH NETWORK The preceding discussion has reviewed the characteristics, findings, and future potential of the WWSSN and other net- works. It has become clear that an integrated global system that combines and takes advantage of the special individual capabilities of all of these networks could produce results far beyond those that can be expected from them separately. Such an integrated system, with digital recording capability and augmented by ocean-bottom seis- mographs and by portable instruments for short-term field studies of particular earthquakes, is essential for anti- cipated studies of the internal structure and dynamics of the earth, of regional structure, and of earthquake source mechanisms. (See Appendix A.) DESCRIPTION Ideally, the improved worldwide network would have broad- band instruments sited in an optimal geographic configura- tion. The locations of the 25 HGLP, SRO, and ASRO stations currently supported by DARPA are shown in Figure l0. These instruments provide three components of the long-period band and, except for the HGLP, the vertical component in the short-period band. At present, the band of l0-l00 sec is recorded continuously, and an event detector is used in recording the short-period band. Different active fil- ters could be used to extend the long-period band. For large events, Ms > 1, the signal-to-noise ratio (S/N) for SRO stations appears to be favorable for periods as long as 500 sec. It would also be desirable to provide an additional broad passband, possibly event triggered, to include recording of intermediate-period (5-l0 sec) data. The IDA program, sponsored by NSF and private funds, involves the deployment of l5 very-long-period LaCoste- 4l
42 Romberg gravimeters. The band extending from 40 sec to tidal periods (approximately l2 hours) is continuously recorded digitally on cassettes. These stations (see Figure l0) should be included in the improved global net- work, and the data should be collected by EDS. It is desirable to select about 50 current WWSSN sta- tions for upgrading, the selection to be made primarily on the basis of geographical location, and to provide digital-recording facilities for at least the vertical components. The long-period band, yet to be defined ex- actly, would be recorded continuously, and the short-period band could be supplied with an event detector, in the same manner as for the SRO and ASRO instruments. It would also be desirable to record some data from intermediate periods. In areas where key geographical coverage is lacking, it may be necessary to upgrade reliable non-WWSSN stations to WWSSN status with digital-recording capability. All together, we have identified about 90 installations for the basic integrated network, thus permitting the system to provide sufficient data for the study of source mechanisms, dispersion characteristics of fundamental- and higher-mode surface waves, amplitudes of long-period body waves, and frequencies and line widths of the earth's free oscillations. Regional and global earth structures could be better delineated, and the temporal and spatial char- acteristics of sources could be better defined. It should be emphasized that this digital-data recording system is not intended to replace the current WWSSN recording capa- bility but to complement it. Existing visible or analog recording facilities of the WWSSN are essential to travel- time studies, in particular, and should be continued. On remote islands, for example, these may be the only practical means of recording, and many foreign countries do not have the facilities to make full use of a digital data base. Moreover, some researchers may be limited to the use of analog data by lack of training in digital processing techniques, by a lack of data-processing facilities, or by a lack of research funds for computing purposes. It is therefore essential that the United States continue to acquire analog data from as many stations as possible and to ensure that geographical coverage is as complete as possible. To further augment the improved WWSSN, the data from the NASA geophysical stations (described above) should be collected by EDS (NOAA) for the use of investigators. A great deal of valuable data are also available from
43 seismic arrays such as the Large Aperture Seismic Array (LASA) and the Norwegian Seismic Array (NORSAR). All the array data could easily overwhelm the envisaged digital- data collection and distribution facilities. One alter- native is to include only selected data from arrays, such as subarray beams, array beams, or subarray center elements; another alternative is to maintain a separate data center for the array data. Including a network of strainmeters would also be valuable, especially for the study of ultra-long-period phenomena. Benioff (quartz rod/tube) meters (now installed in stations at Ogdensburg, Boulder, Isabella, and Nana), laser strainmeters (in Albuquerque, Boulder, Pinon Flat, and California) and Sacks-Evertson volumetric meters, al- ready in operation in North America, South Africa, and Japan, could form the basis for a strainmeter network. Wire strainmeters (Great Britain and Iran) might also be considered. By recording the output of these meters dig- itally and by making the data available in a simple format at a central site such as EDS, a working network of strain- meters could quickly be established. To augment the fixed stations of the global network to provide a sufficient number of the broadband observations for regional studies, we envisage a system of portable broadband digital instruments that could be used for a variety of purposes that require high-density instrumental coverage for a limited time. These instruments could be quickly emplaced in the aftershock regions of large earth- quakes to provide needed information for studies of earth- quake source mechanisms, regional upper-mantle properties, diffracted waveforms, geothermal sources, and regional surface-wave dispersion. Stations on oceanic islands help to improve the geo- graphical coverage; seismographic stations operating on the ocean bottoms must also be given serious consideration and support. Many of the technical problems associated with the deployment and long-term, high-gain recording of the OBS have been solved. However, some research and development still remains to be done in order to produce an instrument package for long-term emplacement on the bottom together with a suitable method for sampling the data. A field program with three-component broadband sensors extending to very long seismic-wave periods could be undertaken probably by the end of l978. About five institutions in the United States are capable of carrying out this program, either individually or jointly.
44 DATA EXCHANGE By properly coordinating the recordings, archiving, and distribution of data from the improved WWSSN, the potential power of this network can be made available to all inter- ested users. And we can anticipate that data-exchange agreements with other countries will further augment its effectiveness. It is especially desirable to have better coverage of the large continental areas from which we do not now receive data. Such areas include West Africa and the People's Republic of China. SUMMARY There now exists the basis for a global system having digital recording capability for anticipated studies of global structure, regional structure, and source mechan- isms. It would consist of 25 installations from the pro- grams of HGLP, SRO, and ASRO stations; l5 very-long-period gravimeters deployed under the IDA program; and about 50 stations of the WWSSN improved with digital recording. Ideally, this global earthquake observation system would have broadband instruments sited in an optimal geographi- cal configuration. The addition of a number of portable broadband digital instruments would be of great importance to many studies, including aftershock sequences, areas of geothermal interest, and source mechanisms. Selected data from seismic arrays could be added. Including digitally recording strainmeters would also be valuable. The NASA program for installation of geophysical instruments glob- ally could also contribute elements to the integrated global system. The development and deployment of ocean- bottom seismographs could also enhance the effectiveness of the proposed global network. All together, a broad- band integrated digital system using the data of over l00 installations is envisaged. In addition, data-exchange agreements with other countries can provide better cover- age of large continental areas from which we do not now receive data. To assure that the global network system functions at its maximum capability, the organization, collection, storage, and dissemination of digital data must be up- graded accordingly, and special regional research centers should be established to support users of digital data. (This is discussed in detail in Chapter 8 and referred to in recommendations B and C, below.)
45 To achieve a new level of seismic research capability far beyond that possible with present network facilities, the Panel recommends the following: A. Stable budgetary support for the improved WWSSN should be provided to the U.S. Geological Survey so that this system can provide a modern data base for the seis- mological community. The improved WWSSN would consist of the following components: HGLP, SRO, ASRO, IDA, selected WWSSN stations, OBS, and other appropriate instruments and facilities. B. A system for collecting, organizing, archiving, and distributing digital data to the seismological community should be established. The USGS Albuquerque Seismological Laboratory is recommended as the facility to carry out basic quality control of incoming digital seismic data and merging onto magnetic tapes, and the NOAA/EDS is rec- ommended as the facility for archiving and distributing the data on magnetic tapes. C. Regional research centers should be established that would provide a library of digital seismic data, access to modern computer facilities, required software for processing the data, and a support staff.
