Geodynamic processes in seismically active areas of the Tien Shan from monitoring data with the use of nuclear explosions

Recent geodynamic processes in the Tien Shan region are studied by the analysis of time series of effective velocities and traveltime delays relative to the IASPEI-91 traveltime curve of the weakly refracted wave P _n from nuclear explosions at the Semipalatinsk test site over the period of 1968–1989. The time series were constructed for 10 seismic stations located at distances of 800–1200 km from the test site in the regions of the Northern, Central, and Southern Tien Shan. The twenty-year period of observations at stations in the North Tien Shan showed a significant decrease in traveltime delays by 0.20–0.76 s, which corresponds to a 0.2–0.7% increase in seismic velocities. An opposite pattern is observed at stations of the Central and Southern Tien Shan: traveltime delays increased by 0.2–0.5 s and, accordingly, seismic velocities dropped by 0.2–0.5%. These results suggest the predominance of compression processes in the crust and upper mantle during the period of observations in Northern Tien Shan and extension processes in the Central and Southern Tien Shan. The series of velocities and traveltime residuals are characterized by the presence of rhythmic oscillations of various amplitudes and periods against a linear trend. A correlation between variations in kinematic parameters and yearly numbers of earthquakes is observed at all stations. Diagrams of the spectral time analysis reveal rhythms with periods of 2–3 and 5–7 yr. The data obtained in this study are consistent with results of studying the stress-strain state of the Tien Shan crust from focal mechanisms of earthquakes and the velocities of recent crustal movements from GPS data. It is found that the amplitude of variations in kinematic parameters of the P wave at stations located in seismically active regions (the Tien Shan, Kopet Dagh, the Caucasus, Altai, and Sayany) is two to five times higher compared to aseismic regions (the Russian and Kazakh plains).