Global Earthquake Monitoring, Its Uses, Potentials, and Support Requirements (1977) / Chapter Skim
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Appendix A: Research Directions
Appendix A: Research Directions
Pages 65-73
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From page 65... ...
The amplitude data are valuable for the study of both source mechanisms and structures and offer the possibility of improving our knowledge of seismic-energy dissipation. Further improvement in the identification of normal modes will be made possible by the expanding network of broadband stations.
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From page 66... ...
In other words, seismologists want to explain observed motions on a global scale in terms of the source after separating out those effects associated with propagation. The solution of this basic problem is probably essential to an understanding of local strong motions, which, in turn, is needed in testing the dynamic response of critical structures (e.g., nuclear reactors)
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From page 67... ...
This process is time-consuming and not very accurate, and the small dynamic range of analog recordings represents a serious limitation on the magnitude span of events that can be analyzed in this way. Nevertheless, application of numerical methods to time-series analysis has led to significant progress in our knowledge of longperiod characteristics of seismic sources, average properties of the earth, and regional variations in surfacewave dispersion.
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From page 68... ...
An extreme example of the latter is creep. Global data are essential to determine whether any seismic activitiy exists at energy levels between those of creep and of infraseismic events, to examine ultraseismic events more carefully, and to determine how earthquakes begin.
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From page 69... ...
The solution of this basic problem, of explaining observed motions on a global scale in terms of the source and propagation effects, is probably essential to the understanding of local strong motions and, hence, in the general estimation of ground motion of engineering interest. This is especially important to the USGS Earthquake Hazards Reduction Program.
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From page 70... ...
At sufficiently short wavelengths, the raytheory approximation (in which the propagation effects are assumed to be associated with a particular ray path rather than with global properties of the earth) comes close enough to reality that assumption of its validity might lead to useful starting solutions.
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From page 71... ...
It is possible to generate synthetic seismograms corresponding to each of the six elements of the moment tensor and then determine their components by inversion. The advantage of this procedure may be that in a given period range -- say, 30-50 sec -- the body waves will show much less regional variation.
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From page 72... ...
Perhaps the word "global" with respect to seismic instrumentation should be understood not only in the sense of geographical distribution but also in the sense of the frequency band of measurements. We have mentioned earlier the "infraseismic" events, those that are characterized by anomalously high spectral values of the moment rate tensor at very long periods.
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From page 73... ...
73 the necessary zero-frequency response. Measurements of this type are currently planned by NASA, but it seems likely that they should be supplemented by a network of high-class strainmeters that would provide continuous monitoring of the state of stress in the crust.
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