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

We have investigated the potential contributions of improved arrival times (using waveform cross-correlation) and the use of three-dimensional (3-D) v elocity models for seismic event location capability. Our analyses are appl ied to a dataset of nuclear explosions at Balapan, Kazakhstan, for which gr ound-truth locations and some absolute origin times are available. This gro und-truth information allows us to determine excellent origin time estimate s for the remaining explosions. The combination of excellent ground-truth l ocation information and high-quality origin time estimates permits us to (1 ) carry out a detailed examination of the quality of ISC picks, (2) identif y probable timing errors in the digital data, (3) evaluate relative and abs olute location capability using data from a sparse network, (4) assess the influence of event signal-to-noise ratio (SNR) on relative location accurac y, (5) utilize the Balapan events as a source array for 3-D tomography bene ath the test site, and (6) test the influence of 3-D structure (local and g lobal) on relative location accuracy and precision in a "controlled" situat ion. Our principal finding is that improved arrival times are the primary contri butor to improved locations. Joint and individual relocations of Balapan ev ents using the full digital dataset result in average mislocations of less than 1 km and 95% confidence regions of a compatible size. To mimic a CTBT scenario more realistically, we also carry out relocations using very few s tations (4-10 observations). Location accuracy degrades somewhat, but the h igh-quality picks generally result in mislocations less than 10 km, even fo r events with very large azimuthal gaps. Uncertainty is generally underesti mated in these cases. Tests with artificially degraded SNR show that misloc ation increases slowly as SNR decreases. 3-D velocity structure makes a sma ller contribution to relative location accuracy than accurate time picks. T ravel time variations due to global 3-D structure vary little across the so urce region, so that location scatter is not reduced when travel time corre ctions 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 struct ure does yield slight location improvement. (C) 2001 Elsevier Science B.V. All rights reserved.