Research Required to Support Comprehensive Nuclear Test Ban Treaty Monitoring (1997) / Chapter Skim
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Appendix C: Seismic Event Location
Appendix C: Seismic Event Location
Pages 107-112
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From page 107... ...
Examination of the way in which computed arrival times change for perturbations in locations in the vicinity of the "best-fitting" location determines a relationship between the random uncertainty in measured arrival times and the size of the region in which the source is expected to lie. Such uncertainty is conventionally reported in terms of a "90 per cent confidence error ellipse," a type of two-dimensional confidence interval that would contain the actual solution 90 times out of 100 if there were no systematic error (as discussed below)
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From page 108... ...
As a result, event locations based only on regional arrival times or small numbers of teleseismic and regional arrivals are often poor. It is a common experience when locating a moderate or large earthquake with many teleseismic stations that inclusion of regional observations and use of a simple regional crustal model actually degrade the event location (unless the stations are close to the source so that little time is spent in the anomalous region and significant travel time errors do not accumulate)
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From page 109... ...
In many cases it appears that improved locations are obtained, but earthquake monitoring operations have been slow to embrace laterally varying Earth models or station corrections. There is not extensive operational experience with methods 2 and 3 on a global basis, but it is clear that method 1 can achieve global location accuracies at the 1000 km2 level only for quite large events recorded by large numbers of stations.
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From page 110... ...
It has also been demonstrated that complete modeling of the full set of regional waveforms can improve constraints on the source depth and epicenter and provide information about the crustal structure that is difficult to extract from arrival times alone. Systematic efforts to determine details of regional and global structure are being conducted, funded by earthquake monitoring agencies, CTBT research programs, and basic Earth science programs, but there is no concerted effort to integrate these into a global model.
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From page 111... ...
To get accurate locations for the purposes of station calibration, it is possible to use locally recorded large mine blasts and earthquakes whose location becomes well known as a result of rupture of the 111 ground surface, reports of strong ground shaking, or data provided by a good local network or a mining company. This empirical approach would result in a steady cycle of improvement: better locations can lead to better calibration of new stations and better knowledge of Earth structure, which in turn leads back to better locations.
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