Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
THE WORLDWIDE STANDARDIZED SEISMOGRAPH NETWORK ORIGIN AND DESCRIPTION The Worldwide Standardized Seismograph Network (WWSSN) was begun in the early l960's as a system designed to greatly expand routine observations of seismic events, to provide a standardized, well-calibrated response, and to provide versatile research capabilities. Responsibility for the installation and management of the network, supported by Department of Defense funds, was assigned to the Coast and Geodetic Survey of the Department of Commerce, a federal agency with years of experience in the operation of con- ventional seismic observatories. In l972, responsibility for operation and maintenance of the WWSSN was transferred to the U.S. Geological Survey. The Coast and Geodetic Survey was guided in the design and installation of the WWSSN by a special committee of the National Research Council. The committee's strategy for the network encompassed several main points. The in- struments were to be of a reliable, proven type that could conveniently monitor a broad portion of the seismic spectrum and that required no extensive further develop- ment. To suppress the high level of background micro- seisms at periods of about 3 to 9 sec, it was decided to separate the frequency spectrum into two bands. Six seismometers are operated at each standard station: one vertical and two orthogonally oriented horizontals for monitoring the short-period spectrum, and a similar set for the long-period spectrum. The free periods of the pendulums are l sec and either l5 or 30 sec. The periods of the galvanometers are 0.75 sec for the short-period and l00 sec for the long-period instruments. Recording is photographic, with drum rates of 60 mm/min for short- period and l5 or 30 mm/min for long-period records. Special ll
l2 care is exercised to maintain synchronous records through accurate timing. A crystal clock accurate to l part in l07 controls the recording-drum rate and the time-marking device. In addition, radio time from the standard time broadcast is impressed automatically on the records every l2 hours. Storage batteries will operate the system for 8 hours in the event of power failure. The electrical system is designed to operate over a wide range of input voltages at frequencies of 50 to 60 Hz (see Figure 3). A unique feature of the system is its standardized response. A simple switching mechanism can vary the FIGURE 3 Components of the WWSSN station. Encased long- period seismometers are shown in the upper left and long- period photographic recording equipment in the lower left; encased short-period seismometers are shown in the upper right and short-period photographic recording equipment in the lower right. The instrument rack in the center contains a crystal-controlled clock for timing, the cali- bration instrumentation calibration system, and other electronic gear. The contact printer used to duplicate seismograms is shown as a rectangular box at the base of the rack. (Photo U.S. Geological Survey, Jon Peterson.)
l3 magnification of the short-period system, operating at a period of l sec, from 3l25 to 400,000 in 6-decibel steps without significantly changing the shape of the response curve. Similarly, the long-period magnification for the l5-sec period ranges from 750 to 6000. A simple calibra- tion pulse impressed at the beginning and the end of each record permits the seismologist to determine the exact frequency response of each instrument and to make quanti- tative comparisons among seismograms from anywhere in the network. This refinement had never before been widely achieved in the seismometry of network systems. One hundred and twenty-three stations were established originally throughout the world, primarily at institutions that had demonstrated a continuing interest in seismology or where there appeared to be great promise for the devel- opment of a research program; some stations were established at geographic locations of special interest or of low back- ground noise. The present configuration of the WWSSN is shown in Figure 4. Concurrently with installation of the recording equip- ment, facilities for copying and distribution were estab- lished in Washington, D.C. This responsibility, and the facilities, were later transferred to Asheville, North Carolina, and eventually to Boulder, Colorado, the present location of the Environmental Data Service (EDS) of the National Oceanic and Atmospheric Administration (NOAA). The size and shape of the seismograms required the devel- opment of a special microfilming system that has performed very effectively and reliably to date. CAPABILITIES AND ACCOMPLISHMENTS The WWSSN soon became the most widely used seismological research network in the world, by both U.S. and foreign investigators. Within the seismological community there is general agreement that the WWSSN has been an outstanding success as a research tool (see Appendix B) as well as an effective stimulant to seismic studies at a number of institutions previously without modern equipment. Data from the WWSSN played a key role .in seismology's contribu- tion to the testing and development of the concepts of sea-floor spreading, continental drift, and plate tectonics, as well as the detection and identification of underground nuclear explosions. During the l960's, the number of earthquakes routinely located, and the precision of those locations, increased by a factor of 3 to 5. This was an
l4
l5 important development, which greatly improved knowledge of seismicity and made correlation of geology and seismicity much more precise and informative. The resulting consis- tent global pattern of seismicity and the nearly continuous, narrow major seismic belts outlining the stable areas de- fine the boundaries of the lithospheric plates postulated by the concept of plate tectonics (see Figure 5). The simple fault-origin model of earthquakes predicts a definite pattern of radiation of seismic waves, and observations of this pattern can be used to derive the orientation of the fault plane and direction of relative motion (focal mechanism). Data from the WWSSN are vital to the determination of focal mechanisms, since data from stations covering a large part of the earth's surface are needed for this purpose. When WWSSN data became available, inconsistencies in focal-mechanism determinations dropped from about 20 percent to less than l percent. Focal- mechanism determinations for shallow shocks in island arcs, ocean ridges, and fracture zones provide the major part of the seismological data in support of plate tectonics. These data have shown, for example, that the transform- fault hypothesis, which predicts a direction of relative motion along ocean fracture zones exactly opposite to that of the conventional interpretation, is indeed correct. A key finding in the problem of the driving mechanism of the plate motions came from a comprehensive study of the focal mechanism of the intermediate and deep earthquakes in island arcs throughout the world. It was found that the pattern of stresses at intermediate depths is con- sistent with sinking of the lithospheric slab under its own weight beneath island arcs, with resistance to further sinking at greater depth (see Figure 6). Recently, several new techniques have been used to determine parameters of the seismic source, such as seismic moment, fault dimension, stress, and stress drop. In the exploration of this promising new direction, data from the WWSSN have been and will continue to be crucial. The uniform response of the WWSSN instruments has made possible studies of wave propagation, and WWSSN data were used to map areas in which plates of lithosphere were judged to be continuous or discontinuous on the basis of the efficiency of propagation of certain types of seismic waves through those areas. With few exceptions, the re- sults support the simple model of plate tectonics. Before the existence of the WWSSN, there was no possibility of a global study and, hence, of interpreting the fundamental global pattern of wave propagation.
l6 I 8 US 8 3 8 8 8BB33SB9 a lp
l7 INCREASING STRENGTH HIGH STRENGTH FIGURE 6 Model showing plausible distributions of stresses within slabs where gravitational forces act on excess mass within the slabs. A filled circle represents down-dip extension, an unfilled circle represents down-dip compression, and the size of the circle qualitatively in- dicates the relative amount of seismic activity. In (a) the slab sinks into the asthenosphere, and the load of excess mass is mainly supported by forces applied to the slab above the sinking portion; in (b) the slab penetrates stronger material, and part of the load is supported from below, part from above; the stress changes from extension to compression as a function of depth. In (c) the entire load is supported from below, and the slab is under compression throughout. In (d) a piece has broken off. A gap in seis- micity as a function of depth would be expected for (b) and (d), whereas no deep earthquakes occur beneath (a). The horizontal dashed lines in the figures indicate possible phase changes in the upper mantle near 350-400 km and 650-700 km. (Figure after B. Isacks and P. Ward, Rev. Geophys. Space Phys. 9, No. l, February l97l, copyrighted by American Geophysical Union.) Free oscillations of the earth have been detected and their characteristic frequencies measured from the spectra of WWSSN recordings even at periods as long as l500 sec. These oscillations are largely unaffected by local struc- ture, so that observations of their spectral energy dis- tribution gives information about the radial distribution of elastic parameters and density with depth, averaged over the whole earth. Despite the poor response and small signal-to-noise ratio of WWSSN instruments in this very- long-period range, the data from a great many WWSSN stations used simultaneously as a global array has increased the number of observed free-oscillation modes to over l000.
l8 Thus, it can be seen that the WWSSN has provided cru- cial information on seismicity, focal mechanisms, wave propagation, and other characteristics of the earth. This information has been valuable to the geophysical community in the broadest sense. The WWSSN has continued to provide data for numerous studies of geologic phenomena within the earth, partic- ularly for those studies that bear on the seismicity of the earth and on its temporal variations. These studies are vital, moreover, to our understanding of the serious earthquake hazard confronting a large portion of our na- tion, and the need for this understanding grows as the nation's population density and the complexity of society grow (see Figure 7). Destructive earthquakes originate with forces and processes deep within the earth, and the WWSSN is a vital tool in our efforts to understand these forces and processes. A better appreciation of earth- quake characteristics such as depths, dimensions, and stresses is useful, if not essential, to an understanding of the conditions that exist in fault zones. Hence, the potential for developing a capability for meaningful earth- quake prediction depends significantly on data from the WWSSN. In another area of current major concernâthe location and delineation of energy resourcesâthe Network may be useful in many ways, for example, analysis of de- lays in the arrival times of seismic waves as determined from the network may help in the location of geothermal sources. Knowledge bearing on the siting of nuclear re- actors is also an important product of studies based on data from the Network. In plate tectonics, the Network has provided data that have made possible an integrated earth-science approach to an understanding of basic earth processes and have contributed toward a new dimension, geodynamics, that must be included in our search for mineral and energy resources. In recent years, significant progress has been made in the theoretical and numerical aspects of the study of earthquake mechanisms and in the investigation of earth structure. Observations used in such analyses have been derived almost exclusively from the WWSSN, and now the technology is available to improve and to augment existing instrumentation for these and newer studies. First, there are requirements for data in digital form, which is now accomplished by manual conversion of analog WWSSN records. This process is time-consuming, not very accurate, suffers from a small dynamic range, and in terms of manpower requirements alone severely limits the research potential.
<o VO
20 Second, the WWSSN system does not adequately record the ultra-long periods normally used in free-oscillation studies. Useful observations of the earth's free oscil- lations were only identified after considerable processing and then for only a few infrequent great earthquakes. SUPPORT Although the WWSSN provides social benefit and scientific knowledge of a value far greater than its cost, in the past decade it has had a history of perilously inadequate and unstable funding and has followed an uneven and round- about course to its present status. By l967, the WWSSN had essentially reached it present configuration. Project VELA Uniform had contributed about $9 million and various U.S. institutions more than $2.7 million for the purchase of seismographic and photographic equipment and for its installation, general maintenance, and repair. Foreign scientific organizations had contrib- uted perhaps more than $6.5 million. The operating budget was less than $l million per year and provided for regular yearly visits to the stations for nonroutine maintenance and calibration. The program was running quite well, but a serious funding problem then began to develop. Project VELA Uniform began to phase out this activity, and Depart- ment of Defense (DOD) funds were no longer available for the Network. Adequate support for this program was not made available to NOAA and its predecessor agency ESSA. After near failure, the network was saved, temporarily at least, through a most welcome but patchy arrangement. Part of the maintenance and calibration activity was cut back, some support of U.S. stations was undertaken by local institutions, limited funds were made available by the agency responsible for the operation and maintenance of the networkâthe Environmental Science Service Adminis- tration (ESSA), and the National Science Foundation (NSF) granted partial, but crucial, support to the foreign seg- ment of the network. Through l972, NSF continued its partial support of the network at an annual level of $283,500, and ESSA supported 3l stations operated by domestic agencies at a comparable level. In l973, the responsibility for foreign-station support and data quality control was transferred from the National Oceanic and Atmospheric Administration (NOAA), the successor to ESSA, to the U.S. Geological Survey (USGS), but NOAA retained responsibility for record copying and data
2l distribution. The NSF support for the WWSSN was also transferred to the USGS, at the same level, i.e., $l84,l00 for foreign-station support and $99,400 to be passed to NOAA for EDS Data Center support. Through fiscal year l976, the total of awards by NSF in support of the WWSSN was approximately $2.5 million. The NSF commitment to the WWSSN for Data Center support is scheduled to drop to $60,000 in fiscal year l977 and to zero in fiscal year l978. In spite of the fact that 75 percent of the WWSSN stations have not been visited in the last five years for calibration and maintenance, the network has, surprisingly, managed to survive and to continue to serve as a primary data source for seismological research. But the inevitable progressive deterioration of record quality to be expected under such conditions is now becoming apparent. The Panel considered whether certain stations of limited utility should be closed but agreed that the small annual cost of operation of each such station is warranted by the extended geographical coverage that these stations provide. Where does support for the WWSSN stand today? The USGS sought direct funding for the WWSSN in its budget request for fiscal year l976, but this item was deleted from the USGS budget by the Department of the Interior. In fiscal year l977 budget requests, support for the WWSSN was given a high priority and was part of the "within-target" budget increase requested by the Department of the Interior, but the Office of Management and Budget (OMB) deleted the re- quest from the budget. Fortunately, NOAA has provided a long-term solution to part of the problemâfunding for record copying and data distribution. After fiscal year l977, this activity will be supported wholly by NOAA. In short, current funding commitments are seriously in- adequate for proper operation and maintenance of the WWSSN. The Network is in a critical state, and its very survival is still threatened. A directed effort should be made immediately to improve the WWSSN, which is still very much needed for studies that will provide large practical benefits to the nation and the prospect of substantial scientific advances. The major accomplishments made by scientists using data from the WWSSN indicate that much new knowledge of the earth's systems, processes, and structures awaits the proper tools to discover it. This is frontier science and will yield large dividends from a relatively small investment. Now is the time to make that investment.
22 In view of the WWSSN's history, achievements, and potential for new discovery, the Panel recommends that adequate and continuing funding be assured through a stable budget (a) to operate the WWSSN as a basic research facility for U.S. investigators; (b) to provide adequate maintenance and suitable upgrading of the WWSSN; and (c) to provide suitable data organization, storage, retrieval, and distribution facilities. We estimate that funding in the amount of not less than $860,000 per year will be needed.
