Broad Band Estimates of the Seismic Source Functions of Nevada Explosions from Far-Field Observations of P Waves
- Steven R. Taylor,
- Howard J. Patton and
- Paul G. Richards
Published Online: 18 MAR 2013
Copyright 1991 by the American Geophysical Union.
Explosion Source Phenomenology
How to Cite
Douglas, A. (1991) Broad Band Estimates of the Seismic Source Functions of Nevada Explosions from Far-Field Observations of P Waves, in Explosion Source Phenomenology (eds S. R. Taylor, H. J. Patton and P. G. Richards), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM065p0127
- Published Online: 18 MAR 2013
- Published Print: 1 JAN 1991
Print ISBN: 9780875900315
Online ISBN: 9781118663820
- Underground nuclear explosions—Detection—Congresses;
One widely used method of obtaining information on the seismic source functions of underground explosions is to compute seismograms using models of the earth, source and seismograph and try and match these to the observed. This process is essentially one of testing the compatibility of the models with observations. An alternative method of obtaining source information is to analyse broad band P seismograms corrected for anelastic attenuation. The main pulses contributing to the signal can be observed on such seismograms and the parameters of the pulses measured. In this paper examples are shown of broad band P seismograms for Nevada explosions. The data are from 39 Nevada explosions (38 fired at the Nevada Test Site proper and one, FAULTLESS, fired at Hot Creek Valley to the north of the test site) recorded at Eskdalemuir, Scotland, supplemented for a few of the explosions with seismograms recorded at stations of the Long Range Seismic Measurements network.
To correct for anelastic attenuation requires an estimate of the variation of t* with frequency, t* being the ratio of travel time to specific quality factor. Two models are investigated: for one, t* = 0.35 s and is independent of frequency; for the other t* is frequency-dependent. The frequency-dependent model, however, is rejected as it appears to result in overcorrection of frequencies around 2 Hz.
The deconvolved seismograms derived using the frequency-independent t* show significant variability between explosions. However, most of the explosions at the Pahute Mesa fired at depths greater than 819 m are broadly similar and show a roughly systematic move to longer duration with increasing depth—which presumably correlates with increasing yield (ignoring the explosion SCOTCH which was overburied). The shallowest explosion of this sequence, HALFBEAK, has a yield of 365 kt and the largest yield of the deeper explosions is greater than 1000 kt. From this yield information it would appear that the duration scales roughly as (yield) 1/5 which is the scaling law predicted by the Mueller-Murphy (M-M) model. The rise times of the P pulses are, however, larger than those predicted by the M-M model and scale roughly as (yield)1/3 although the scatter in the observations is large.
Most of the seismograms show P closely followed by an arrival (ApP) of negative polarity which might be interpreted as pP. Following ApP there is usually a positive arrival which may be larger than P and which has been attributed to the effects of spalling at the free surface above the source. The ApP−P time for most explosions is significantly longer than predicted from the known depth of firing. The deconvolved seismograms show that the P radiation from explosions is not well described by idealised point compressional source models and that a dynamic model of the explosion source needs to be developed that takes account of the observations from such seismograms.