For the territory of Tien-Shan the general agreed directions of the main axes of horizontal deformation and their regional differences based on GPS and seismotectonic deformation (STD) methods to a depth of 25 km for 1997-2019 are estimated. (2021).

  Using 0.2°≈18.5 km interpolation grid (39-44°N, 70-79°E) 5 regions with representativeness of 11-80 comparison pairs of GPS and STD methods data per region were identified. Most of the deformation for both methods is presented by shortening, which, on average, in all comparison data in the near-surface part of the Earth's crust (GPS) has an azimuth 343.1° and in the depth of 5-25 km (STD) – 342.8°. For individual regions, the average directions of the shortening axes of the 2 methods vary in the range of azimuths of 330-358°. The maximal divergence between the average directions of the shortening axes for GPS and STD methods is 11° in the area of the Chuy depression; in other regions the deviations of the axes do not exceed 4°.

Figure. STD for the depth of 5-25 km (blue arrows - axis of shortening, purple - extension) and the rate of modern horizontal near-surface deformation of the Earth's crust according to GPS data (red - shortening, blue - extension). Rectangular regions (number of STD and GPS data pairs): Ch - Chui, I - Issyk-Kul, N - Naryn, A - Alai and T - Tarim. The arrows in circles and degrees are the average rooting directions for the regions by STD and GPS methods.

  According to the complex of geological and seismological data on the northern slope of the Kyrgyz ridge (Tien Shan, the Sokuluk-Kegety interfluve), the area with increased values of signs of seismic hazard was identified (2020).

  In this area, each cell in longitude and latitude 0.25°×0.25° contains up to 360 earthquakes K>6 according to the KNET network data for 1994-2019. This number of earthquakes here is 2-10 times more than seismic events in the cells around. Also in this area there is a high density of faults with increased values of normalized Coulomb stresses and a high probability of their reactivation. According to geological and geodetic data and solutions of focal mechanisms, these faults have thrust and strike-slip kinematics, which indicates the concentration of matter. There are no earthquakes here with a significant level of stress relief as in the area around, which also indicates an increased level of seismic hazard for the selected area.

Figure. Zoning of faults by the magnitude of normalized Coulomb stresses [Rebetsky and Kuzikov, 2016] against the background of the distribution of the number of earthquakes (1994–2019) over the cells ≈20×28 km sq. (0.25°×0.25°). The focal mechanisms of earthquakes with K≥10 are marked in blue. Earthquakes with stress dropped Δσ≥10 MPa are shown as red circles. The pink rectangle is a seismically hazardous area (≈64×30 km sq.).

  In the foothills of the Northern Tien Shan, southern of Bishkek, according to GPS data, zones of increased values of deformation rate of the Rarth's crust were identified, these zones are located on segments of active faults (2019).

  Based on the data of long-term measurements of points in Bishkek local GPS network, an averaged tensor field of horizontal strain rate is constructed. At most calculation points, the main shortening axes have a meridional direction and values in the order of 10^-7 year^-1. Shortening maxima are accompanied by rotation of axial strike in the northeast direction to 25°, and an increase in values on the perpendicular axes of elongation. The coincidence of such zones of suspended deformations with the paths of active discontinuities suggests the presence of segmented left-side shifts against the general background of reverse faults, and the possibility of accumulation of stresses with their subsequent discharge through earthquakes.

Figure. The average tensor field of the horizontal deformation rate based on the data of long-term measurements of the Bishkek local GPS network points (blue rhombuses). Red arrows are the main axis of shortening; blue arrows are the axis of elongation. Brown lines – active faults.

  At the geodetic sites of Bishkek Geodynamic Proving Ground (Northern Tien Shan) along the active fault zone, 4 types of pulsed deformation events were revealed according to linear-angular measurements (2018).

  Weekly measurements of 44 baseline lengths (BL) at 3 geodetic sites spaced 5–13 km along the active fault zone revealed the presence of elastic deformation events (10^-4÷10^-5) with length variations from the first mm to 4 cm: 1 ) isotropic in terms of short-period (6–7 days) synchronous maximum elongations of all BLs at all sites; 2) lengthening of BL of the north direction and shortening of BL of the east direction for 3-4 months: for all directions, the extremes of changes in the lengths of the BL are shifted to the end of the event, in the latitudinal direction there is a delay of 1–3 weeks; 3) BL extensions in the north direction and insignificant BL extensions in the east direction for 4–5 months: in all directions, the extremes of the BL length changes are shifted to the beginning of events; 4) the change in lengths on individual baselines in only one of the directions in the plan.

Figure. Variations in the lengths of the shortest and mutually perpendicular (N – S and B – W) BLs of the Almaly site according to linear-angular measurements with reflection of the main types of pulsed deformation events.

  On the territory of the Bishkek Geodynamic Proving Ground (Northern Tien Shan), a spatial and temporal relationship was recorded between the manifestation of local earthquakes and variations in the deformational environment of the earth's crust (2017).

  The results of linear-angular and GPS observations, in comparison with seismicity and focal mechanisms, made it possible to assess the geodynamic situation during a local earthquake of energy class 10. A few days before the earthquake, GPS data showed a subsidence in this area of the Earth's surface with an amplitude of up to 20 mm. In the period 5 days before and 2 days after the event, an average increase of 8 mm in lengths of lines crossing the Shamsinsky fault was noted by ~ 2 km from the earthquake source according to light-ranging measurements. Such an increase in the fault width corresponds to variation of ~ 7 MPa in the tectonic stresses acting in the fault zone.

Figure. The scheme of study area with sites location (points and their numbers) of linear-angle observations POLYGON, KENTOR AND ALMALY (1), GPS measurement IATA, IAT3, POL2, CHUM points (2), sensing stations using the method of establishing a field in the far zone (3), the focal mechanism of the earthquake source (4). Lines are active faults.