An analysis of P times reported in the Reviewed Event Bulletin for Chinese underground explosions

Douglas, A; O'Mongain, A; Porter, D; Young, JB. 2005 An analysis of P times reported in the Reviewed Event Bulletin for Chinese underground explosions. Geophysical Journal International, 163 (2). 559-566.

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Analysis of variance is used to estimate the measurement error and path effects in the P times reported in the Reviewed Event Bulletins (REBs, produced by the provisional International Data Center, Arlington, USA) and in times we have read, for explosions at the Chinese Test Site. Path effects are those differences between traveltimes calculated from tables and the true times that result in epicentre error. The main conclusions of the study are: (1) the estimated variance of the measurement error for P times reported in the REB at large signal-to-noise ratio (SNR) is 0.04 s2, the bulk of the readings being analyst-adjusted automatic-detections, whereas for our times the variance is 0.01 s2 and (2) the standard deviation of the path effects for both sets of observations is about 0.6 s. The study shows that measurement error is about twice (∼0.2 s rather than ∼0.1 s) and path effects about half the values assumed for the REB times. However, uncertainties in the estimated epicentres are poorly described by treating path effects as a random variable with a normal distribution. Only by estimating path effects and using these to correct onset times can reliable estimates of epicentre uncertainty be obtained. There is currently an international programme to do just this. The results imply that with P times from explosions at three or four stations with good SNR (so that the measurement error is around 0.1 s) and well distributed in azimuth, then with correction for path effects the area of the 90 per cent coverage ellipse should be much less than 1000 km2—the area allowed for an on-site inspection under the Comprehensive Test Ban Treaty—and should cover the true epicentre with the given probability.

Item Type: Publication - Article
Digital Object Identifier (DOI):
Programmes: BGS Programmes 2008 > Earth hazards and systems
ISSN: 0956-540X
Additional Keywords: Chinese explosions, P times
NORA Subject Terms: Earth Sciences
Date made live: 01 Oct 2009 15:59 +0 (UTC)

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