Detailed Structure of Attenuation Field in the Western Tien Shan Based on Short-Period Coda Waves

Abstract

The seismic coda waves attenuation characteristic in the western Tien Shan region were studied. The short-period coda waves of the local earthquakes, recorded by the frequency-selective seismic stations in 1960–1984, and digital seismogram data of 150 earthquakes that had been occurring in the western Tien Shan region since 2005 until 2017, recorded at the AAK station (KNET network), were analyzed. The attenuation field is represented by the zones (blocks) having high Q-factor and being close to isometric in plan and by the linear zones of strong attenuation (weakened zones), which coincide with faults. It was revealed that the structure of attenuation field agrees with the structure of S-wave velocity field and the velocity anomalies of P-waves: low-velocity anomalies correspond to low-Q zones. The most expressed conducting zones, revealed from magnetotelluric data, coincided with the low-Q zones, indicating their significant fluid saturation. Sources of the strongest earthquakes with K > 16.0 were confined to the zones of maximum attenuation contrast, to the boundaries of blocks and weakened zones, where the mobility of blocks was determined by reduced viscosity of low-Q layers and small thickness of high-Q ones. The relationship between the features of seismic processes and spatial inhomogeneities of the attenuation field in the background of variations in the Earth’s rotation speed was also considered.