COMPLEX ELECTROMAGNETIC MONITORING OF GEODYNAMIC PROCESSES IN THE NORTHERN TIEN SHAN (BISHKEK GEODYNAMIC TEST AREA)

Abstract

At the Bishkek geodynamic test area in the Northern Tien Shan, complex electromagnetic monitoring was conducted, including a wide frequency range by magnetotelluric sounding in various modifications and induction sounding with a controlled impulse source (method of sounding in the far zone, SFZ). The geoelectrical sections of the lithosphere were constructed for the study area. Based on the geophysical monitoring data and the profile MT moni‐ toring data on the Baitik depression, the main faults belonging to the Northern Tien Shan fault system were detected: Shamsi‐Tyundyuk (Predkirgizsky), Baitik, Chonkurchak, and Issyk‐Ata. From the geoelectrical data, we obtained new information that is independent of other geophysical methods used for studying the deep structure of the Earth – the hidden fault structures and the geoelectrical segmentation of the study area were revealed. The latter reflects the main elements of the block structure of the junction zone of the Chuya basin and the Kirghiz ridge. This information needs to be taken into account when constructing a comprehensive model showing the geological, geophysical and geodynamical features of the development of the Tien Shan, a vivid example of an intracontinental orogen. The MTS, FS and SFZ data from the Ak‐Suu and Chon‐Kurchak electromagnetic monitoring stations were interpreted. Based on the representative array of experimental electromagnetic data, we analyzed the effective depths of field penetration, which are most sensitive to changes in the electromagnetic parameters of the medium for the stationary and routine observation sites with respect to the clusters of seismic events. This paper discusses the development of the azimuth MT monitoring technique and the analysis of the time series of electromagnetic parameters, which were used for de‐ termining the contribution of each component of the impedance tensor to the informativeness of the monitoring stu‐ dies. The data from the 2004–2016 KNET Catalogue of the Research Station of RAS were analyzed in order to investi‐ gate the relationship between the space‐time seismicity distribution and the electrical conductivity variations that correspond to the features of the seismicity pattern at depth. Examples discussed are the time‐frequency series of MT monitoring in 2007 and 2016 at the Ak‐Suu and Chon‐Kurchak stations that identified anomalous electromagnetic parameters corresponding to the fluid redistribution model for the pore‐cracked medium: syn‐phase decreasing and increasing values of the parameter by the orthogonal azimuths. Thus, a phenomenological model has been tested. It relates the change in the stress‐strain state of the medium with the redistribution of fluids between the systems of fractures, which causes variations in the active and reactive components of the electrical resistance.

Keywords

• electromagnetic monitoring
• tien shan
• deep structure
• electrical conductivity
• geoelectrical section
• tectonic faults and fractures