Seasonal variations of cross correlations of seismic noise in Israel

Abstract

The long-period microseismic noise was recently found to carry deterministic information about the crust and upper mantle structure in the cross-section between the two station sites in terms of the surface wave Green function, which is theoretically proportional to the noise cross-correlation function (NCF) for the long-term observations at the pair of the BB stations. We performed daily-long cross correlations for the period of 2–3 years between the 7 BB stations, distributed in the Eastern Mediterranean, and stacked them for every month of a year. A preprocessing of the broadband waveforms included whitening of the direct and inverse DFT of the waveforms to avoid influence of earthquakes recordings and to equalize energy of different types of microseisms. As the result, we have found that the NCFs obtained exhibit clear seasonal variations within period band 2–20 s persistent from year to year. For the best description of these variations, we applied seasonal diagrams, which presented the distribution of the NCF maximal amplitudes in three narrow frequency bands, with respect to the month of a year. The diagrams helped in determining that these variations can be split into four types. The different types of the seasonal NCF variations are assumed to be attributed to the four certain remote deep ocean regions, which are responsible for the increased microseismic activity due to the specific interaction between the ocean waves and the bottom during ocean storms.