Orientation of the Principal Stress Axes as a Possible Cause of Geoacoustic Radiation Anisotropy During Earthquake Preparation

The results of long-term investigations carried out in Kamchatka show that an increase in intensity of geoacoustic radiation and amplification of its anisotropy can be considered as an indicator of deformation process activation during earthquake preparation. One of the main characteristics of the stress-strain state of rocks is the orientation of the principal stress axes, and in particular of the maximum compression axis. The present paper deals with the experimental studies and mathematical modeling performed to detect the relation between the orientation of the axis of maximum compression of rocks and geoacoustic radiation anisotropy during earthquake preparation. In 2008–2016, using the vector-phase method, an analysis was made of the direction of geoacoustic radiation, recorded at the "Mikizha" site in Kamchatka within a three-day interval before earthquakes. There were detected two main orthogonal maxima of radiation (average azimuths 312.5°±15° and 42.5°±15°), with one of them, in the authors’ opinion, perpendicular to the maximum compression axis. In an elastic approximation of a medium, modeling has been performed for the orientation of the axis of maximum compression of rocks during the preparation of strong Kamchatka earthquakes in 1976–2020. According to the results, the axis was lying within the sector with the azimuths of 330.5 and 84.2°, the modal interval was 22.5 up to 45.0°, and the average value was 32.7°. The model-based average orientation of the axis of maximum compression agrees with the value determined from experimental observations of geoacoustic radiation at the "Mikizha" site. The results of the study confirmed the assumption that the direction of geoacoustic radiation maxima occurring during earthquake preparation is determined by the orientation of the axis of maximum compression of rocks at the observation site. The obtained results can be used to study geodeformation process characteristics in seismic regions.