High‐frequency seismic observations in eastern Kazakhstan, USSR, with emphasis on chemical explosion experiments

Abstract

Two temporary three‐station seismic networks, deploying surface and 100‐m borehole high‐frequency seismometers, of the order of 200 km from the Kazakh test site in the USSR and the Nevada test site in the United States are discussed, with emphasis on chemical explosion experiments. Seismograms attained from the detonation of three buried explosions (10 t, 20 t, 10 t) in eastern Kazakhstan at distances between 156 and 637 km are examined in the frequency band of 1–80 Hz. Observed signal‐to‐noise (S/N) ratios were high, reaching a maximum of 400 for Pg waves and 200 for Lg waves. Good signal‐to‐noise levels persisted to high frequencies; S/N = 2 at about 50 Hz for Lg waves about 250 km from the source, and at about 14 Hz at 680 km distance. For Pg waves, S/N = 2 at about 50 Hz 270 km from the source. Shapes of displacement amplitude spectra were similar, characterized by a broad maximum in signal‐to‐noise levels between 4–8 Hz, and a decay at higher frequencies (e.g. above 10 Hz) of about f−3.5 − f−4.1 for Lg waves, and f−3.1 − f−4.5 for Pg, unconnected for distance. Magnitudes estimated from Lg time domain amplitudes for the 10 t explosion are between 2.8 and 3.3, depending on the magnitude relation used. Spectral characteristics are used to put some constraint on Lg Q. Pg Q is poorly constrained by the data. A similar experiment in southern Nevada showed much lower Pg and Lg signal‐to‐noise levels above 1 Hz, although Kazakh and Nevada absolute noise levels are comparable.