Manifestation of earth deformation processes by high-frequency seismic noise characteristics

Abstract

During recent years special investigations of high-frequency seismic noise (HFSN) have been conducted in order to study the nature of temporal regularities of this process. Some correlation between HFSN level and slow changes of stress field have been observed under different geological conditions. These phenomena were interpreted as being due to the presence of a seismic emission component in HFSN which is characterized by a very strong “tensosensitivity”, i.e. sensitivity to the stress changes. Later observations of HFSN carried out at different places using highly sensitive resonant seismometers show that a “modulation” phenomenon, i.e. correlation between HFSN changes and Earth's tides, has been seen with a different degree of confidence both in time and space. This result is explained by the spatial variations in emission activity. It is thought that HFSN is influenced by different kinds of geodynamic processes which change the tensosensitivity and the emission activity with time and with the presence of external seismic noises. Detailed inspection of HFSN behaviour shows that its response to external action is more complex than was previously thought. According to observations held on a Russian platform, the seismic emission level is found to be modulated by changes in the intensity of 4–6 s microseisms connected with the Baltic Sea storms. In the Ashkhabad region where no strong effects of external sources are known HFSN amplitude correlates with changes of tilt caused by moon-solar tides. Temporal change of correlation between these processes is observed. Well observations detected noticeable variations of the HFSN intensity with depth and the dependence of intensity and spectral composition on structure and composition of rocks. This is direct proof of the endogenic nature of observed HFSN. Periodic changes of HFSN level observed after the strong Gasli earthquake of March 19, 1984 ( M = 7.2) near the city of Ashkhabad are discussed as a new “self-excitation” type of HFSN. Two opposite points of view on the nature of HFSN, either as telluric processes highly sensitive to stress conditions or as a noise which is not sensitive to the stress conditions, are discussed and explained by both the large variance of HFSN behaviour and by the methods and conditions of observation.