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Seismic velocity structure and event relocation in Kazakhstan from secondary P phases

DEPARTMENT OF GEOLOGY AND GEOPHYSICS UNIVERSITY OF WISCONSIN-MADISON, 1215 W. DAYTON STREET, MADISON, WISCONSIN 53706

Abstract

Three-component seismic data from a set of presumed explosions recorded by stations at Bayanaul and Karkaralinsk in Kazakhstan were analyzed in order to model the crustal structure of the region and to examine the use of the arrival times of secondary P phases, primarily PmP, in regional event location. Polarization analysis aided in the identification of the secondary phases. Low-pass filtered data (4-Hz corner) from the first 5 to 10 sec of 13 presumed explosions were modeled with the reflectivity method. The two chemical explosions in 1987 provided a check on accuracy, as their locations and origin times are accurately known. A good fit to the arrival times and amplitudes in the first 5 sec of the P wave (Pn, Pg, and PmP) was obtained in the epicentral distance range of 100 to 300 km. Beyond 300 km, the simple layered model was not adequate to model the PmP arrival.

The crustal P-wave velocity model we derived has an upper crustal velocity increasing fairly rapidly from 4.5 km/sec near the surface to 6.5 km/sec at 15-km depth, then increasing more slowly to 7.05 km/sec at 50-km depth. The observed difference in the arrival times of the phases Pg, PmP, and Pn in the range between 100- and 250-km distance required a relatively sharp transition at the crust mantle boundary. The model is generally similar to previous estimates of P velocity structure in the region, though with a gentler gradient in the upper crust and a steeper gradient in the lower crust. We used the derived crustal model and the primary and secondary P-wave arrival times to relocate events in the Kazakhstan region. Inclusion of the phase PmP substantially decreases the focal depth uncertainty for many of the events. All but one of the events analyzed are concluded to be surface explosions; the identity of the remaining event is uncertain.

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