Spatial and Temporal Structure of Global Low-Frequency Seismic Noise

Аннотация

The paper aims to study in detail the structure of seismic noise before and after the Sumatra mega-earthquake with M = 9.1 of December 26, 2004. The records by the IRIS seismic stations in different regions of the world, equipped with STS-1 seismometers providing ground motion velocity recording in a broad range of periods from 0.2 to 360 s with the use of standard equipment, form the empirical base of the research. In the records of each station, intervals free of earthquakes, interference from manmade impacts, and noise enhancement due to cyclone propagation were selected. The amplitude of steady seismic noise in the range from 40 to 360 s, not complicated by the recordings of earthquakes and effects of meteorological origin, is in the order of 20 to 40 nm, which characterizes the actual resolution of a broadband seismic station. The noise bursts in the ranges 40–80, 80–160, and 160–320 s differ by the shape and time of occurrence suggesting different sources of their generation. The absence of the correlation between the noise recordings at the neighboring seismic stations spaced 10²–10³ km apart indicates the influence of local processes. The noise reflects turbulent processes in the Earth’s atmosphere, whereas the exponential growth of the noise with the increase of the oscillation period is consistent with A.N. Kolmogorov’s theory of locally isotropic turbulence in the atmosphere. The noise amplitude after the Sumatra mega-earthquake with M = 9.1 of December 26, 2004 increased by a factor of 1.5–2 in January 2005 compared to January 2004, irrespective of the location of the seismic station.