Coherence between the fields of low-frequency seismic noise in Japan and California

Аннотация

The relationship is considered between the statistics of the field of low-frequency seismic noise which was synchronously recorded by two broadband seismic networks in Japan (78 stations) and California (81 stations). The analysis is based on the data for seven years of observations (2008–2014). For each network, the daily time series of the median values are constructed for five parameters of seismic noise: kurtosis (excess), minimal normalized entropy of the distribution of the squared wavelet coefficients, generalized Hurst exponent, support width of the singularity spectrum, and index of linear predictability. The median values for each parameter were calculated on a daily basis over all the stations of the networks and resulted in a time series containing 2557 data points of the integral characteristics of the noise with a daily time step. The use of the median values of the noise parameters avoids considering the effects of the gaps in recording by individual stations and provides the continuous time series as the integral characteristic of the whole network. Next, for each network, the aggregate signals were calculated for the obtained five-variate time series. By construction, the aggregate signal is a scalar signal which maximally accumulates the most general variations that are simultaneously present in all the analyzed signals and, at the same time, rejects the components that are only characteristic of a single process. The final step of the analysis consists in estimating the evolution of the quadratic spectrum in the moving time window with a length of one year. It is shown that during the considered interval of the observations, the coherence is characterized by the increasing linear trend, which independently supports the previous conclusion about the enhancement of the synchronization between the parameters of the global seismic noise.