National Academies Press: OpenBook

Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements (1977)

Chapter: New Developments and Capabilities

« Previous: The Worldwide Standardized Seismograph Network
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
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Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
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Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
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Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
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Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
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Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
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Page 28
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
×
Page 29
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
×
Page 30
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
×
Page 31
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
×
Page 32
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
×
Page 33
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
×
Page 34
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
×
Page 35
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
×
Page 36
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
×
Page 37
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
×
Page 38
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
×
Page 39
Suggested Citation:"New Developments and Capabilities." National Research Council. 1977. Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements. Washington, DC: The National Academies Press. doi: 10.17226/18566.
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Page 40

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NEW DEVELOPMENTS AND CAPABILITIES Two important new developments over the past decade or so are (l) improved methods for recording earthquakes and (2) improved technical capability in seismic instrumentation. Great improvement of magnetic-tape recording through the conversion of signals from analog to controlled digital format and the availability of small, compact, and inex- pensive recorders could revolutionize the analysis of seismic signals from earthquakes, as it already has in seismic exploration for petroleum using signals from con- trolled sources. These developments allow high-speed- computer analysis of spectra, improvement of signal-to- noise ratio, and more complete analysis of seismograms. Combined with the vastly improved dynamic range (several orders of magnitude) and substantial new developments in seismic instrumentation, this great advance in our capa- bility to analyze seismic signals can add greatly to our understanding of the physical properties and structure of the earth and provide new insights into many earth-related problems that are of major significance to society (re- sources, earthquake-related hazards, and others). Among the significant advances in instrument technology in recent years, improvement of the quality of long-period data stands out. Long-period seismographs are more sensi- tive to their operating environment, especially to small changes in temperature and pressure, and resolution is often limited by instrument-related noise. Moreover, until quite recently the structure of earth noise in the long- period band was not well known. The long-period seismo- graphs used in the WWSSN, for example, cannot resolve earth noise in the 20- to 40-sec band, and long-period signals from many events are not detected although they have energy well above background levels. These problems have largely 23

24 been overcome by advances in technology, and new observing systems have resulted. HIGH-GAIN LONG-PERIOD SYSTEM By careful design and control of the operating environ- ment, scientists have been able to increase the operating sensitivity of long-period seismographs by more than an order of magnitude for periods above about 20 sec as compared with the WWSSN system. This work resulted in the installation of the high-gain long-period (HGLP) seismo- graph system, which includes digital recording to increase the dynamic range. Eleven HGLP systems have been installed as part of a study of very long-period seismic waves spon- sored by the Defense Advanced Research Projects Agency (DARPA). This system lowers surface-wave detection thresh- olds substantially. SEISMIC RESEARCH OBSERVATORIES Problems with long-period-data recording were not all re- solved by the HGLP system, however. Wind-generated seis- mic noise often masks small earthquakes recorded on the HGLP system, particularly the horizontal components because of their sensitivity to ground tilt. However, wind- generated earth motion is attenuated rapidly with depth, and thus attention soon focused on the development of a borehole seismometer that could be used for long-period sensing at depths in the hole where wind-generated noise is reduced (see Figure 8). The sensors had to be small compared with conventional seismometers, with low internal system noise yet sensitive enough to resolve long-period earth displacements measured in angstroms. Miniaturization was achieved by using three orthogonally oriented short- period seismometers as the sensing elements, and instrument noise was reduced to acceptable levels by employing the latest electronics technology and evacuating the air from the sensor modules. The signal output from each sensor is broadband and is proportional to earth acceleration from 0.02 to l Hz. The broadband data are filtered in a well- head terminal to produce long-period and short-period outputs, which are recorded separately. Operated in a borehole at a depth of l00 m, this new seismometer is unaffected by wind noise, even during periods in which the noise obliterates data from conventional seismometers operated near the surface.

25 FIGURE 8 SRO borehole seismometer being pre- pared for installation. (Photo U.S. Geological Survey, Jon Peterson.) Improvement of long-period sensing using the advanced borehold seismometer described above was one goal in the development of the network of Seismic Research Observa- tories (SRO). Another equally important objective was to achieve a capability to accommodate a large range of signal amplitudes. Most conventional seismographs, like the WWSSN instruments, record photographically on drum recorders to produce the typical 24-hour seismogram. The recording range is quite limited, about 44 dB, or a little over two orders of magnitude (xioo)• A digital recording system of the type used in the SRO project provides an excellent al- ternative, with 66 dB of resolution plus 60 dB of automatic gain control for a total of l26 dB of recording range, over 6 orders of magnitude (xl,000,000) . Three long-period data channels and the vertical-component short-period data chan- nel are recorded on visual recorders to produce conventional seismograms. The same data are digitally recorded, with

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27 the long-period data sampled once each second and the short-period data 20 times each second. The station pro- cessor is a l6-bit minicomputer with 8000-core memory and peripheral controllers. In addition to controlling the recording operations, the station processor edits the short-period data so that only a short segment of data prior to each event, and the event itself, are recorded on tape (see Figure 9). The station operator communicates with the processor and controls the system through a teletypewriter. The system appears to be reliable. A prototype SRO has been operating for l8 months at Albuquerque with very few failures of component parts. Sites for l3 SRO stations were selected jointly by DARPA and the U.S. Geological Survey on the basis of recommenda- tions from the Committee on Seismology of the National Research Council. Most will be operated in conjunction with WWSSN stations. Specific considerations were seismic noise levels as determined from existing data, geological setting, and the interest of prospective host organizations. The general geographical siting of the SRO stations was influenced by the location of existing HGLP stations. ABBREVIATED SEISMIC RESEARCH OBSERVATORIES In a project closely associated with the SRO's, five of the HGLP seismograph stations will be furnished with vertical-component short-period seismometers and abbrevi- ated versions of the SRO recording system. These modified HGLP stations (designated ASRO) will have functions, soft- ware, and a digital data format identical to those of the SRO stations; the principal difference is that the ASRO stations will continue to use conventional surface long- period seismometers. Figure l0 shows the locations of the HGLP, SRO, ASRO, and IDA (see p. 34) stations. Figure ll shows the oper- ations characteristics of the WWSSN, HGLP, and SRO systems. ARRAYS Since the early l960's, large arrays of seismometers have been installed by the United States as research systems in Montana, Alaska, Norway, Korea, and Iran. Great Britain and other countries are operating medium-sized arrays. DARPA is the U.S. governmental agency that has primary responsibility for the U.S. program of installation and

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29 6. o UI UI _ a. CO a DC wwss A Ms 6.0 at 30° 30 40 50 60 70 80 90 )00 PERIOD (seconds) FIGURE ll Comparison of the recording ranges of the WWSSN, HGLP, and SRO systems for long- period waves. (From J. Peterson and N. Orsini, ESS, Trans. Am. Geophys. Union 57, 548-556, l976, copyrighted by American Geophysical Union.)

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3l operation of the arrays in conjunction with the host country. The aperture of a given array may be 50-l00 km, and the number of seismometers in the array may be l00 or more (see Figure l2). The data from these units have been useful in signal enhancement, phase identification, and earth-structure studies in general. Considerations in using array data must involve the large amount of data available for a given event and the ensuing processing time and costs. DATA STORAGE AND DISTRIBUTION The principal purpose of the new stations (HGLP, SRO, and ASRO) and of the WWSSN is to provide data for research, and the success with which this purpose is met depends on accessibility of their data to researchers. For years, conventional seismograms have been sent to the National Oceanic and Atmospheric Administration's Environmental Data Service (EDS) or to its predecessors for microfilming and distribution of copies to subscribers. This service will continue, but for the new digital stations, the re- production of seismograms or film chips from the digital tapes is being considered as an alternative to the col- lection and processing of station seismograms. The repro- ductions are superior to the conventional seismograms in several respects; for example, they have greater dynamic range and can be plotted at any magnification. A comple- mentary data-management system is being established by DARPA to process, store, and disseminate data collected from the digital-recording network and from the seismic arrays in Montana, Alaska, Norway, Korea, and Iran. The short-period array data will be analyzed by automated event-detection and event-association processors to pro- duce a daily summary of events and their associated param- eters. The event summaries, associated waveforms, and raw array data will be placed in a mass data-storage device to be augmented later by SRO network data as tapes are received from the stations. Access to this data bank will be provided through the DARPA-sponsored Seismic Data Analysis Center (SDAC) in Alexandria, Virginia, and, as is the practice with WWSSN data, the data will be made avail- able to the international seismological community.

32 INTERNATIONAL DEPLOYMENT OF ACCELEROMETERS Another development of the past decade has been the improve- ment of instruments for measuring very-long-period (VLP) ground motions—those with periods of more than l00 sec. The LaCoste-Romberg gravimeter, with electrostatic feed- back, has demonstrated long-term stability unattainable with systems utilizing conventional mechanical suspensions. High-quality seismic data for periods greater than 40 sec are currently being recorded digitally on cassette tapes from a small global network of stations equipped with the modified LaCoste instrument (see Figure l3). This is the FIGURE l3 (a) The IDA system electronics rack. Included from the top are the feedback and bias control chassis, the lock-in amplifier, the tide and mode filters, the cassette recorder, two monitor strip chart recorders, and the power supply with battery charger. (b) Donald Miller attends the La Coste-Romberg gravimeter in its constant- temperature container. The micrometers on the baseplate are used for initial leveling and subsequent tilt calibra- tion. (Photos courtesy Jonathan Berger.)

33 i0 )2 It )6 )8 20 22 21 FREQUENCY [MHZ] FIGURE l4 Fourier power spectra (in dB) of the IDA records at Sutherland of the Guatemalan earthquake, February 4, l976. The lower spectrum represents the earth's back- ground noise plus instrument noise for the 24-hour period before the earthquake. The upper spectrum represents the earth's normal mode activity for the first 24-hour period after the earthquake. For frequencies above 2 mHz the S/N is about 35 dB. The 0-dB point corresponds to a power level of 5 x l0~16 m2 sec~3. (Courtesy Ray Buland.) International Deployment of Accelerometer (IDA) project, whose station locations are shown in Figure l0. In early l976, there were 5 of these stations, and plans call for a total of about l5 eventually. Digital recording, in conjunction with active filters, provides a dynamic range of over l00 dB for periods between 2 and 60 min. The cassette tapes, after processing by minicomputer, produce VLP data that can be used for earth-tide studies and for investigations of earth structure and earthquake mechanisms using free oscillations and surface waves (see Figure l4). POTENTIAL FOR VERY-LONG-PERIOD MEASUREMENTS The nominal long-period band for the SRO instruments is l0-l00 sec. At longer periods, the active filters have a

34 decrement of 20 dB/decade. Yet, preliminary results for the Solomon Islands earthquakes of July 20, l976, as recorded at Albuquerque, show that the spectrum of the vertical SRO instrument contains free-oscillation peaks at very long period (for example, the mode os!3 with a period of 473 sec is present with a signal-to-noise ratio of about l0 dB). Therefore, it may be possible to use SRO instruments to study very-long-period phenomena, especially source mechanisms. However, to realize this possibility requires that broadband, very-long-period (T > l00 sec) active filters be developed, evaluated, and calibrated for these instruments. It is also worthwhile to investigate the long-period characteristics of the ASRO and HGLP sys- tems. Together, the l5 IDA stations along with the very- long-period channels of the other instruments can provide very good global coverage at very low frequencies. Be- cause of the potentially important relationship between low-frequency spectra to source mechanisms and precursory phenomena, the evaluation of the very-low-frequency char- acteristics of the instruments of the combined networks should have a high priority. ADMINISTRATION AND SUPPORT OF NETWORKS All of these networks (HGLP, SRO, ASRO, and IDA) are composed of research stations that make their data avail- able to all researchers. The results thus far have been rewarding for special studies, and much additional basic information about the earth can still be derived from the observations of these stations. Yet these stations, which have cost perhaps $5 million or more to install, may be lost to us before the end of this decade, after only a few short years of operation, because of termination of support—even though operational and maintenance costs are relatively small, averaging about $l7,000 annually per station. Considering the large investment in installation and the substantial new knowledge that could be expected from continued operation of these stations, it is clearly within the national interest to provide long-term support for them. The Panel, therefore, submits the following recommendations: A. Sufficient support should be provided to the U.S. Geological Survey under a stable budgetary arrangement to assure continuing operation, maintenance, and improvement

35 of the new networks (HGLP, SRO, ASRO, and IDA), with reviews of results and performance at 5-year intervals. B. A plan should be formulated for an orderly trans- fer of responsibility to the U.S. Geological Survey for funding the long-term continued operation and maintenance of seismic networks and arrays, including global networks. C. A plan should be formulated for the National Oceanic and Atmospheric Administration to expand funding and increase facilities for the long-term storage and distribution of observations from seismic networks and arrays. D. The responsibility for formulating the plans dis- cussed above should be vested in an interagency initiative definitely including the Defense Advanced Research Projects Agency, the U.S. Geological Survey, the National Oceanic and Atmospheric Administration, and the National Science Foundation. THE NEED FOR PORTABLE INSTRUMENTS In addition to the networks of permanent stations, the availability of a system of portable broadband, digital instruments to be used on a worldwide basis would be of great importance to studies of aftershock sequences, mapping of upper-mantle discontinuities and regional structure, studying waveforms of phases diffracted or re- fracted on or near the core-mantle boundary, regional surface-wave dispersion, identification of geothermal sources, and many other similar studies. The instruments could be located with several of these purposes in mind and the data made available to the seismology community. The transmission of data from such a portable system by means of satellites may be economically feasible and would offer advantages in terms of real-time monitoring of station performance. The Panel recommends that portable broadband, digital instruments be obtained for a variety of fundamental in- vestigations for which high-density instrumental coverage is necessary for a limited time. SPACE-RELATED STUDIES A set of geophysical stations is being established by the National Aeronautics and Space Administration (NASA) in a

36 program of using space-related techniques to study crustal strain and lithospheric-plate motion. Lunar laser-ranging stations are operating at McDonald Observatory at Fort David, Texas, and Haleakala Observatory on Maui Island, Hawaii. Geophysical instrumentation consisting of long- period and short-period seismometers, tiltmeters, strain meters, and gravimeters are being installed at these sta- tions. The data from these instruments will be recorded digitally and will be made available to interested sci- entists. Similar instruments will be operated at sites to be occupied beginning in the late l970's by a transportable lunar laser-ranging facility now being constructed. These sites will probably include Goddard Space Flight Center in Maryland, Haystack Observatory in Westford, Massachusetts, and a location in the Pacific Northwest. Similar lunar laser-ranging stations are also being constructed in Australia, Japan, France, and the Soviet Union. The Panel urges that the data derived from this effort be supplied to interested investigators through NOAA's Environmental Data Service. OCEAN-BOTTOM SEISMOGRAPHS Ocean-bottom seismographs (OBS) can be used both to aug- ment the global network and to provide more precise in- formation about the regional structure of the oceanic crust and upper mantle. Together, the oceans represent a unique tectonic province that has never been directly sampled with seismic methods, except for the most shallow struc- tures. At present, recordings of surface and body waves generated by earthquakes in the oceans must be made on continental platforms or on oceanic islands that are different geophysically from typical oceanic regions. Major differences between continental and oceanic structure have been known for some time, and the hypothesis has been advanced that some of the more extended differences persist to great depths. A detailed understanding of upper- mantle structure beneath the oceans is required if the composition and dynamics of the entire mantle are to be fully understood. To obtain this information certainly requires that the global network include ocean-bottom seismographs that can remain in operation for a long time. The technology of self-contained, free-drop, OBS cap- sules has advanced to the state that deployment times of as long as a year can now reasonably be considered (see

37 Figure l5). Stations of a more permanent nature, utilizing tethered buoys, acoustic data links, or oceanic telephone and telegraph cables appear to be feasible. These promis- ing developments now make it possible to consider long- term experiments on the ocean bottom with the assurance that enough earthquakes will be recorded to provide good digital and analog data bases. Ocean-bottom seismograph sensors now in use, or soon to exist, are sensitive to very long, including tidal, periods as well as to short- period ground motions. That is, the modern vertical seis- mograph is also a sensitive gravimeter, and the horizontal seismograph is a sensitive tiltmeter. Thus, these instru- ments will enable scientists to extend greatly their studies of gravity and of load and tilt tides, as well as other infraseismic phenomena. Global tidal observations would be useful in conjunction with computer solutions of the Laplace tidal equations, to mention but one interrelation- ship of geodesy, oceanography, and seismology. The existence of such a geographically comprehensive data base would create new areas of opportunity and expand existing ones as seismologists continue to improve their knowledge and understanding of the earth and earthquakes. The Panel recommends a comprehensive research effort to determine the feasibility of an extensive, long-term pro- gram in ocean-bottom seismology. Such a program might include portable arrays of several broadband OBS instru- ments and a few permanent installations. CALIBRATION To use amplitude information effectively in studies of structure and source mechanisms requires that the instru- ments be well calibrated. The nonlinear as well as the linear response functions of the instruments must be known. All instruments are nonlinear for sufficiently large signals, and both the SRO and IDA instruments ex- hibit obvious nonlinearities in their responses to Rayleigh-wave packets (R) for large earthquakes (M > 7). This nonlinearity causes the frequency spectrum of the input (the sensed ground motion) to be convoluted with itself and weighted with the instrumental nonlinear re- ponse function. The result is a distorted output spectrum that can lead to erroneous interpretations. Therefore, a general procedure, such as cross-correlation, should be developed, evaluated, and applied to the calibration of

38 FIGURE l5 (a) View of assembled digital ocean-bottom seismograph. Tripod at bottom couples to the sea floor and is left on the bottom after release. The 22-inch inside-diameter sphere is posi- tively buoyant. An acoustic link, whose transducer is mounted at the top, allows accurate location of the system on the sea floor and permits capsule diagnos- tics and release commands to be exchanged with the mother ship. The flashing light and radio beacon are used to locate the capsule on the surface. (b) Interior components of digital ocean-bottom seis- mograph. One-second "Ranger" seismometer is at bottom with electronics assembly suspended by shock mounts above. l2-bit data are recorded in serial format with commercial recorder on top. The system is entirely digital and incorporates CMOS, low-power technology throughout. A three-component, CMOS microprocessor, long-period capsule is currently under construction. (For details see W. A. Prothero, l976. A free fall seismic capsule for seismicity and refraction work, Offshore Technology Conference Paper No. 2440. Photos courtesy of Marine Seismology Group, Scripps Insti- tution of Oceanography.) (a) SRO, IDA, and other instruments. The broadband linear response function, quadratic response function, and, if necessary, cubic response function should be determined to good accuracy. In this way, the user could decide which signals to accept and whether to apply a relinearizing postprocessing procedure. Calibration of the instruments is obviously important and should be given high priority if users are to receive the benefits of broadband digital data. Weakly nonlinear signals can be deceptively attractive but seriously mis- leading. They cannot be identified unless the instruments are well calibrated. Certainly, more consideration will have to be given to what is needed before regular site visits for calibration can be made.

39 The Panel recommends the establishment of a program to provide better instrument calibrations that include non- linear as well as linear amplitude effects. THE NEED FOR ON-SCALE RECORDING Another important concern is the need for global on-scale recording of major earthquakes. The SRO systems clip at earth accelerations of 4 x l0~5 msec"2, equivalent to an M 6.8 earthquake at a distance of 30 degrees. Signals re- corded by most WWSSN stations are partically off-scale for major earthquakes. Therefore,

40 The Panel recommends that some instruments at these stations be run at very low gain to ensure that all data possible are obtained. In addition, if near-field effects predicted from far-field observations are to be verified, it is absolutely essential to have strong-motion instru- ments at WWSSN stations located within or close to seis- mically active areas.

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