ИСТИНА

A new method for investigating the coherence field of the noise component of high-frequency GPS time series is proposed. The method is applied to the main territory of the United States, which is characterized by a dense network of near 6000 GPS stations. The data are presented in steps of 5 minutes from February 28, 2013 until June 29, 2019 on the Nevada Geodetic Laboratory website. The proposed method estimates the spatial distribution of the mean values of multiple coherence, calculated within nodes of a regular grid, between GPS coordinates of a given number of nearest operable stations and the periods at which the maximum values of coherence are reached. The two-dimensional probability density of the positions of places where the coherence maximum is most often realized is estimated. These estimates can be obtained for the entire history of observations and also in a sliding time window of a given length, which makes it possible to trace the dynamics of changes in time in the coherence field of the earth's tremor. The entropy of the two-dimensional probability density of places of concentration of maximum values of coherence allows us to distinguish seasonal changes in the structure of the coherence field of GPS noise. To study the temporal dynamics, we use the auxiliary time series of changes in the maximum multiple coherence at 50 reference points located throughout the study area. The study of the coherence properties of this auxiliary 50-dimensional time series (“secondary coherence”) in a 180-day sliding time window highlighted a series of synchronization bursts of earth's surface tremors. The research presents continuation of the recent investigations of background processes in the Earth’s crust, based on estimating coherence measures of geophysical fields random fluctuations [Lyubushin, 2018(a,b)]. The work is supported by the Russian Foundation for Basic Research, project no. 18-05-00133 References Lyubushin A (2018a) Synchronization of Geophysical Fields Fluctuations. In: Tamaz Chelidze, Luciano Telesca, Filippos Vallianatos (eds.), Complexity of Seismic Time Series: Measurement and Applications, Elsevier 2018, Amsterdam, Oxford, Cambridge. Chapter 6. P.161-197. https://doi.org/10.1016/B978-0-12-813138-1.00006-7 Lyubushin A (2018b) Global coherence of GPS-measured high-frequency surface tremor motions. GPS Solutions. October 2018, 22:116. https://doi.org/10.1007/s10291-018-0781-3