I. A. Sanina

Velocity properties of the lithosphere in the ocean-continent transition zone in the Kamchatka region from seismic tomography data

Arrival times of P and S waves from local earthquakes in the Kamchatka area of the Kurile-Kamchatka Island Arc are used for calculating a spatial model of the elastic wave velocity distribution to a depth of 200 km. The lithosphere is shown to be strongly stratified in its velocity properties and laterally heterogeneous within the mantle wedge and seismic focal zone. A lower velocity layer (an asthenospheric wedge) is identified at depths of 70–130 km beneath the Eastern Kamchatka volcanic belt. The morphology of the Moho interface and the velocity properties of the crust are studied. The main tectonic structures of the region are shown to be closely interrelated with deep velocity heterogeneities. Regular patterns in the statistics of the earthquakes are analyzed in relation to variations in the elastic wave velocities in the focal layer. A mechanism of lithospheric block displacements along weakened zones of the lower crust and upper mantle is proposed.

Mikhnevo: from seismic station no. 1 to a modern geophysical observatory

The Mikhnevo seismic station was founded in accordance with directive no. 1134 RS of the Council of Ministers of the Soviet Union of February 6, 1954. The station, installed south of Moscow, began its operations on monitoring nuclear tests in the United States and England in 1954. For dozens of years this station was the leading experimental base for elaborating new technical solutions and methods for monitoring nuclear explosions, equipped with modern seismological instruments. At present, the focus of activities has been moved from military applications to fundamental geophysical research. The station preserves its leading position in seismological observations due to the development of national high-performance digital instruments and creation of the small-aperture seismic array, the only one in the central part of European Russia, which is capable of recording weak seismic events with ML ≥ 1.5 within a distance of 100 km.

Modern Geodynamically Active Zones in the Central East European Craton

To identify the modern geodynamic processes in the structures of the East European Craton (EEC), the complex approach is used. The results of visual and automated deciphering of the LANDSAT satellite images are analyzed combined with the data of preci� sion measurements for horizontal components of motion rates of GPS reference points. The approach used allowed us to consider the modern activity of geo� logical structures and to distinguish the zones of higher geodynamic instability. To analyze the morphostructures and geodynamic setting of the studied territory, we constructed a space� tectonic scheme of the central EEC using the LESSA software packet [1] and data on the deep structure of the Earth’s crust [2] (Fig. 1). In the scheme, the sys� tems of extended lineaments of different directions are marked; these lineaments shape the integrated struc� tural pattern describing the complex geological evolu� tion of the region. Deep faults in the basement are often expressed on the surface as localized deforma� tions and clustered cracking zones. The rheological properties of sedimentary rocks predetermine the “scatter” of boundaries (faults) between the basement blocks [3]. The orientation of extended lineaments and linea� ment forms in the central EEC reflects the block struc� ture of the basement and partially reveals the buried structures that were activated at the neotectonic stage. Figure 1 shows that the major lineaments are oriented in the SW–NE and SE–NW directions and are related with the primary regmatic division of the crust trans� formed by tectonic processes. Sublatitudinal and sub� meridional lineaments are of secondary value [4]. The distinguished extended Moscow–Ryazan lineament zone of SE–NW strike consists of a series of subparal� lel extended lineaments and corresponds to the transi� tion zone boundary at the layer of sedimentary cover

Application of a three-component seismic array to improve the detection efficiency of seismic events by the matched filter method

It is found that the best results, in terms of the efficiency of detecting regional seismic phases from blast sources, are achieved by using the method of waveform cross correlation applied to the data from a small-aperture array of three-component sensors

Detection of ultraweak signals on the Mikhnevo small-aperture seismic array by using cross-correlation of waveforms

Original Russian Text

Using a matched-filter technique at the Mikhnevo small-aperture seismic array

The Mikhnevo small-aperture array (SAA) was designed as an instrument for various regional seismic studies, including the compilation of a detailed catalogue of industrial blasts in the East European craton. This array includes 12 observation points arranged in three circles and equipped with SM3-KV shortperiod seismometers. The Institute of Geospheres Dynamics launched the array in 2004, and since that time it has been detecting up to 1000 industrial blasts per year. The Mikhnevo SAA uses beam formation for array processing. The stacking of individual waveforms reduced to a reference point allows the suppression of microseismic noise and improving the signal-to-noise ratio (SNR) relative to a three-component station. An improved SNR for a given signal is equivalent to a reduced detection threshold: much weaker signals can be detected with the use of a beam-formation technique. In turn, much more signals from small industrial explosions are detected. Weak signals are difficult to identify because of the higher uncertainty in the estimates of such characteristics as azimuth, slowness, and amplitude. Having a ten-year catalog of industrial blasts and the archive of raw digital records for this period, we apply a waveform cross-correlation (matched filter) technique, which has an extremely high relative location accuracy and thus identification capability. We have created a set of master events with relevant waveform templates for automatic data processing and creation of an accurate catalogue of industrial blasts.

Remote detection of aftershock activity as a new method of seismic monitoring

A weak seismic event with body wave magnitude 2.1 was detected near the epicenter of an underground explosion conducted by the Democratic People’s Republic of Korea (DPRK) on September 9, 2016. The event occurred approximately two days after the test. Using the method of waveform cross-correlation, two array stations of the International Monitoring System (IMS), USRK and KSRS, recorded the arrival of Pn waves. At the same time, this event was not recorded during the standard automatic processing at the International Data Center. The location and other characteristics of this weak event indicate that it is likely to have been an aftershock of the preceding explosion. Based on the successful automatic detection and construction of the event, we conducted an extended investigation, which consisted in searching for secondary seismic phases at the nearest stations that do not belong to the International Monitoring System. The final solution, which is based on waveform cross-correlation, includes stations MDJ (China) and SEHB (Republic of Korea), the aftershock epicenter is located at ~2 km northwest of the epicenter of the DPRK test.

Detection of regional phases of seismic body waves using an array of three-component sensors

A small-aperture seismic array consisting of seven three-component seismometers carried out continuous measurements of regional seismicity in a selected area of the Nizhni Novgorod nuclear power plant during four months of 2013. Automatic signal detection using beamforming was applied separately for each motion component. Two horizontal components were transformed into radial and transverse components for the given values of the velocity and azimuth of the plane wave front. We have investigated the dependence of the coherence of microseismic noise on frequency, azimuth, and slowness, as well as determining the level of cross-correlation between signals on separate channels in order to estimate expected improvement in the signal-to-noise ratio, which is crucial for signal detection. Most signals detected by the seismic array from regional sources are associated with quarry blasts. Using repetitive explosions at seven quarries, we have quantitatively estimated and compared the increase in detection efficiency of regional seismic phases using a three-component small aperture seismic array and a subarray of vertical sensors. Horizontal sensors showed a higher efficiency in the detection of transverse waves, while the subarray of vertical sensors missed S-waves from certain events. For one of the nearby quarries, the vertical subarray missed up to 25% of events (5 of 20). The results of the investigation point to the need for the use of three-component seismic arrays for the study of regional seismicity.

The ADSS-3 broadband stand-alone digital seismic station

The SSD-3 three-channel seismic recorder and the ADSS-3 three-component broadband standalone digital seismic station based on the SSD-3 together with SM-3E seismic sensors were developed. The main advantage of this equipment in comparison with foreign and domestic analogs is simplicity and convenience while maintaining high technical characteristics. The structure and operation of the seismic sensor and seismic recorder are considered, and their main technical characteristics are given. Laboratory, bench, and comparative tests of the seismic recorder and station demonstrated their working capacity and compliance with the development goal. Based on the test results, the ADSS-3 seismic station was commissioned as a three-component broadband observation point of the Mikhnevo small-aperture seismic array. The data obtained using the ADSS-3 made is possible to study the structure of the crust and upper mantle of this region using the receiver function method.

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 Pn 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).