Nuclear explosion locations at the Balapan, Kazakhstan, nuclear test site: the effects of high-precision arrival times and three-dimensional structure

Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53706, USA
Physics of The Earth and Planetary Interiors (Impact Factor: 2.4). 04/2001; 123(2):283-301. DOI: 10.1016/S0031-9201(00)00215-6

ABSTRACT We have investigated the potential contributions of improved arrival times (using waveform cross-correlation) and the use of three-dimensional (3-D) velocity models for seismic event location capability. Our analyses are applied to a dataset of nuclear explosions at Balapan, Kazakhstan, for which ground-truth locations and some absolute origin times are available. This ground-truth information allows us to determine excellent origin time estimates for the remaining explosions. The combination of excellent ground-truth location information and high-quality origin time estimates permits us to (1) carry out a detailed examination of the quality of ISC picks, (2) identify probable timing errors in the digital data, (3) evaluate relative and absolute location capability using data from a sparse network, (4) assess the influence of event signal-to-noise ratio (SNR) on relative location accuracy, (5) utilize the Balapan events as a source array for 3-D tomography beneath the test site, and (6) test the influence of 3-D structure (local and global) on relative location accuracy and precision in a “controlled” situation.Our principal finding is that improved arrival times are the primary contributor to improved locations. Joint and individual relocations of Balapan events using the full digital dataset result in average mislocations of less than 1km and 95% confidence regions of a compatible size. To mimic a CTBT scenario more realistically, we also carry out relocations using very few stations (4–10 observations). Location accuracy degrades somewhat, but the high-quality picks generally result in mislocations less than 10km, even for events with very large azimuthal gaps. Uncertainty is generally underestimated in these cases. Tests with artificially degraded SNR show that mislocation increases slowly as SNR decreases. 3-D velocity structure makes a smaller contribution to relative location accuracy than accurate time picks. Travel time variations due to global 3-D structure vary little across the source region, so that location scatter is not reduced when travel time corrections for global 3-D structure are applied. Travel time variations due to the local 3-D structure (estimated using source-region tomography) are also modest. Applying travel time corrections that account for the local structure does yield slight location improvement.

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    ABSTRACT: Regional and teleseismic waveforms of underground nuclear explosions (UNEs) at the Balapan and the Nevada test sites are analyzed to understand the nature of shear waves excited from UNEs. The phase composition of wavefields in the source and receiver regions is examined using a source array frequency-wave-number analysis and polarization analyses. It is observed that regional mantle-lid shear waves, Sn, often develop strongly. On the other hand, observed teleseismic shear waves from UNEs are much weaker than those from earthquakes. Source array analyses of the shear waves from UNEs display coherent energy that is polarized in a certain azimuthal direction. The observation of coherent shear energy in different UNEs suggests that the shear energy is excited in a consistent manner at every UNE. We constrain the dominant shear wave excitation mechanism from the observed shear wave features. The observations appear to be consistent with the tectonic release model with conical rock cracking.
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    ABSTRACT: Accurate location of seismic events remains a critical issue for global nuclear explosion monitoring. Herein we present some observations on the nature of kriged P-wave travel-time correction surfaces and their application to improving seismic event location in China. We have adopted the correction surface approach due to ease of implementation and the fact that empirical correction surfaces can be constructed without detailed knowledge of crustal structure. However, correction surfaces can be built on any velocity model, no matter how detailed, so that when such models do become available, surfaces can be recalculated for that model. This, in turn, enables prediction of corrections in regions lacking seismicity. We use the modified Bayesian kriging method to construct surfaces for 76 stations around Asia, analyzing travel-time data from seismic events in the United States Geological Survey Earthquake Data Reports (EDR) and the International Seismic Center (ISC) catalog. Due to limited data, we gather residuals for events throughout the crust (as defined by the 1-D global model employed) whose location accuracies range from 2-25 km. The correction surfaces are used with the EvLoc algorithm to perform regional relocations of several thousand events in the region around China. Correction surfaces dramatically improve the clustering and linearity of regional seismicity and increase the stability of EvLoc. About 50% more events are successfully relocated when surfaces are used. Comparing regional relocations to high-quality ground truth also reveals a significant quantitative improvement in location accuracy. In an effort to further improve our location ability, we are creating a comprehensive merged database for the China region, comprised of EDR, ISC, Reviewed Event Bulletin, and several regional catalogs. This database will provide the most complete record of arrivals for events in eastern Asia, and its location performance will be validated against current databases. One measure of the robustness of the kriged surfaces is their correlation; nearby stations should have similar P- wave correction surfaces. We find that surface correlation is high for nearby stations but drops off beyond about 250 km, implying that, on average, crustal structure varies rapidly across Asia. This length scale may be useful for assessing whether or not surrogates should be used in developing correction surfaces for new stations. Moreover, this correlation length can also be used to constrain the model correlation length parameter in the kriging procedure. We have performed a suite of sensitivity tests to examine the effect of depth and epicentral mislocations on travel-time residuals. These tests were performed using four closely spaced high-quality ground truth events as observed by the 76 stations for which we calculate correction surfaces. Fixing the latitude and longitude of the events and letting the origin time and depth vary, we find that there is about 0.13-sec deviation in residuals for every 10 km of depth error. Epicentral mislocations result in P-wave residual errors of about 0.75 sec per 0.1o. When compared to the root-mean-square residual value of about 1.9 sec, effects due to depth errors and depth averaging are minimal.
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    Bulletin of the Seismological Society of America 02/2008; 98(1):454-462. · 1.96 Impact Factor