Nuclear explosion locations at the Balapan, Kazakhstan, nuclear test site: the effects of high-precision arrival times and three-dimensional structure
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: From a local high-resolution base catalog at Parkfield, California, 5076 earthquakes (M 0.2 to 6) are used to study the comparative performance of a correla-tion detector and standard energy detector on the sparse regional network of continu-ously operating stations. Eighty-six percent of the events detected by a standard energy detector can also be detected by cross correlation. Correlation detection is able to find additional events by lowering the detection threshold by about 1 unit beyond what standard processing detects for Parkfield, a factor of 10 increase in number of events such as those predicted by Gutenberg–Richter. Most event separation distances for events that correlate at Parkfield are less than 1 km. The distribution of magnitude differences for events that correlate at Parkfield is not distinguishable from the input magnitude distribution. More robust measures to quantify reduction in detection threshold are introduced. Detection magnitude threshold reduction of about 1 unit holds for large-scale application to the 18,886 events in China and 5,076 events in Parkfield with false-alarm rates of a few percent. Large and small events are seen to correlate well enough for detection. Two examples are shown with magnitude differences as large as 2.3 and 3.3 units. The correlation detector also finds two cases of buried aftershocks in the coda of mainshocks that were previously unreported in the Annual Bulletin of Chinese Earthquakes (ABCE).Bulletin of the Seismological Society of America 12/2010; 100(6). DOI:10.1785/0120100042 · 1.96 Impact Factor
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ABSTRACT: Seismic coda is composed of scattered waves originated from various kinds of heterogeneities, whose locations are not confined in the great-circle ray-path direction between the source and receiver. We devise a technique to image laterally varying regional heterogeneities using single-station seismograms for clustered events. We analyze regional seismic records of the Borovoye seismic station for the Balapan nuclear explosions in Kazakhstan. Scattered waves that arrive coherently over the source arrays, are extracted by directional beamformings. The locations of scatterers responsible for the scattered waves are mapped from the beamforming direction, travel time and slowness (equivalently, phase velocity). The illuminated locations of heterogeneities are highly correlated with the structural variations in surface topography, crustal thickness and sedimentary thickness. The influence of the structural variations on seismic waves is quantified in terms of scattering intensity and quality factor. The scattering properties are observed to vary with phase due to the difference in frequency content and phase velocity. The proposed technique appears to be useful for a study of active tectonic regions with limited monitoring stations.Physics of The Earth and Planetary Interiors 02/2008; 166(3-4):188-202. DOI:10.1016/j.pepi.2008.01.001 · 2.40 Impact Factor
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ABSTRACT: Seismic coda is composed of scattered wavelets originated from various heterogeneities. The phase composition of regional seismic coda still remains un- known, despite its use for several decades. This is caused partly because the ray paths of scattered wavelets in coda are not on the great-circle path between a source and receiver. We examine the constituent original phases of regional coda with the help of a source-array analysis. A set of uniform sources that are essential for a source-array analysis is organized with underground nuclear explosions. Strong Rg-origin energy is observed in the coda at frequencies of 0.2-0.8 Hz, and it lasts more than 700 sec until the end of records. The coherent energy in the coda reduces with frequency. It con- stitutes about 20% of the total coda energy at frequencies of 0.2-0.4 Hz, and 12% at frequencies of 0.4-0.8 Hz. The other 80% of coda energy in 0.2-0.8 Hz is mixed with complex phases from various untraceable origins. The Rg energy is the most influ- ential component in the construction of low-frequency regional coda. On the other hand, the coda at higher frequencies, 0.8-3.2 Hz, is observed to be mixed with com- plex phases that cause the wave field to be diffused. The observation of Rg-origin energy at the regional coda suggests that scattered energy from phase coupling of Rg is not significant compared to Rg-to-Rg scattered energy.Bulletin of the Seismological Society of America 02/2008; 98(1):454-462. DOI:10.1785/0120070121 · 1.96 Impact Factor