Analysis of the Time Structure of Strain Processes in the Ashgabat Fault Zone (Northern Kopet Dag)

Abstract

The paper presents the results of long-term (55 years) ground-based geodetic observations with increased spatial and temporal detail (the distance between reference points is 0.3–0.5 km, and the intervals between measurements are 1–2 months) in the Ashgabat fault zone (Northern Kopet Dag); local (with a width of 0.5 km) anomalous deformations of the Earth’s surface with average annual deformations rates of 1–3 × 10^–5/year are detected. The results of comparing the velocities of vertical and horizontal displacements of the Earth’s surface in the Ashgabat fault zone with data on the modern kinematics of interaction of the Turan and Iranian plates, obtained from GNSS measurements according to the Iranian Geodetic Network, are discussed; it is shown that the rate of reduction of the Kopet Dag based on ground geodetic measurements is almost 100 times less than in GPS observations. This proves that local vertical displacement anomalies occur under conditions of a quasi-static regional stress field. Spectral analysis of long-term series of leveling observations was carried out, and the prevailing periods of anomalies of vertical displacements of the Earth’s surface were revealed. It is demonstrated that the annual harmonic dominates in the block parts of the leveling profiles. The maximum periods in the fault zone do not correspond to the annual component and have a shorter duration. It is established that the annual harmonic is associated with seasonal thermoelastic strain and local changes in the fault zone are caused by periodic changes in the pore pressure associated with the precipitation regime. A mechanism of parametric excitation of local vertical displacements of the Earth’s surface is proposed, when changes in the internal parameters of the medium (bulk elastic modulus) occur under stationary regional loading.