SUMMARY We have analysed broad‐band SKS, SKKS, PKS and S data from 12 stations of the IRIS/IDA network for shear‐wave splitting parameters, fast polarization direction ø and delay time δt. These include several stations in the Former Soviet Union (FSU) (GAR, ARU, KIV, TLY, AAK, OBN), as well as SUR (Sutherland, South Africa), ESK (Eskdalemuir, UK), ALE (Alert, NWT, Canada), NNA (Naña, Peru), PFO (Piñon Flat, CA, USA), and ERM (Erimo, Japan). We compare observations from these stations with those from other nearby stations to gain insight into the mechanisms by which the anisotropy registered by split shear waves may arise. Intraplate comparisons exclude absolute plate motion as an orienting mechanism. Alternatively, the anisotropy may be linked to either fossil deformation within the continental lithosphere or to small‐scale convection. The close correspondence with geologic fabric supports the former interpretation. Commonly, the fast polarization direction is oriented parallel to the deformational axes of compressional tectonic regions. In regions of presently active tectonics, we also see fast polarization directions orthogonal to the maximum horizontal stress in the crust. Deformation localized in the margins of tectonic blocks may account for the spatial splitting variability, and can be facilitated by volatile fluxing of the continental lithosphere during the preceding subduction event.
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.
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.
Abstract. A simple method is described whereby stationto-source azimuths are estimated by fitting a plane wave to envelope functions of T-phases observed on a 5-element hydrophone array around Ascension Island, South Atlantic Ocean. When applied to a data set of 55 earthquakes of known location ranging between 2 and 45 degrees distance from Ascension Island, estimated azimuths have a standard deviation of 3.3 degrees from reference azimuths when 3 or more hydrophone elements are used. The standard deviation decreases to 1.8 degrees if T-phase data from all 5 hydrophone elements are used. We also investigate variations in predicted errors for different array geometries and arrival azimuths. This simple method is amenable to automation and can easily be incorporated into a global monitoring system.
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