Regional seismic event location with a sparse network: Application to eastern Kazakhstan, USSR

Abstract

Three‐component data from a sparse three‐station seismic network in eastern Kazakhstan, surrounding the Soviet nuclear test site, have been analyzed to determine location estimates for regional events recorded by two or three stations. Included among these events are the September 1987 chemical explosions whose locations are known. Locations are calculated using arrival times of P and S phases and arrival azimuths from first P. Location uncertainties are estimated using a combination of a priori and a posteriori data uncertainties. A layered P wave velocity model adapted from Soviet Deep Seismic Sounding surveys is employed for calculating travel times, and two S models are tried. Location results for the chemical explosions are excellent, even if only two stations are used: absolute location errors are less than 10 km, and estimated 90% confidence uncertainties are only a few kilometers. The data are also adequate to determine correctly their depth (i.e., focus at the surface). The other regional events include numerous suspected mine blasts and two earthquakes from the Tien Shan. The calculated locations of the latter events agree well with a teleseismic location for one of them, falling within a belt of regular seismic activity. Nearly all of the presumed blasts can be associated with mapped mines, and we have been able to identify the source areas for two sets of blasts in high‐resolution satellite images. “Before and after” photographs allow us to identify specific active mines. Our location estimates agree quite well with the observed active mines.