Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements (1977) / Chapter Skim
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New Developments and Capabilities
New Developments and Capabilities
Pages 23-40
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From page 23... ...
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 inexpensive recorders could revolutionize the analysis of seismic signals from earthquakes, as it already has in seismic exploration for petroleum using signals from controlled sources.
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Miniaturization was achieved by using three orthogonally oriented shortperiod 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.
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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 Observatories (SRO)
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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 abbreviated versions of the SRO recording system.
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28 1 O > M-l H co w O rH -H <0 -P O |
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00 PERIOD (seconds) FIGURE ll Comparison of the recording ranges of the WWSSN, HGLP, and SRO systems for longperiod waves.
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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)
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The LaCoste-Romberg gravimeter, with electrostatic feedback, 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)
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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.
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Geological Survey under a stable budgetary arrangement to assure continuing operation, maintenance, and improvement
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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 refracted on or near the core-mantle boundary, regional surface-wave dispersion, identification of geothermal sources, and many other similar studies.
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Together, the oceans represent a unique tectonic province that has never been directly sampled with seismic methods, except for the most shallow structures. 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.
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These promising developments now make it possible to consider longterm 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 shortperiod ground motions.
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A free fall seismic capsule for seismicity and refraction work, Offshore Technology Conference Paper No.
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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 recorded by most WWSSN stations are partically off-scale for major earthquakes.
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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 instruments at WWSSN stations located within or close to seismically active areas.
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