The Distribution Of Noise Source Both In Space And Time And Its Influence On Noise Cross-correlation Functions

With the wide application of the technique of ambient noise corss-correlations in seismological research,the noise cross-correlation function(NCF)has become an important basic data for understanding the waveform in NCF.Because the spatial and temporal inhomogeneous distribution of noise sources is not satisfying the theoretical assumption of random seismic wave field,the waveform in NCF will be affected by the inhomogeneity of noise sources.Therefore,a better understanding of the origin of noise sources and their spatial and temporal distribution is needed for improving the accuracy of seismic research based on NCF.Using the continuous vertical records of 322 broadband seismic stations which are evenly distributed in a square in southern part of Chin Array Phase II,we calculated the NCFs from September 2013 to June 2016.We analyzed the dominant azimuth distribution and seasonal variation of ambient noise sources in different frequency bands in the studying area.It draws the following conclusions:1.Using the normalized background energy flux(NBEF)method,we analyzed the seasonal variation and azimuthal anisotropy for period 4-8 s,8-12 s and 12-20 s respectively.Clear seasonal variation and directional anisotropy are observed for all periods and the characteristics of different periods are different.Overall,the ambient noise energy is strong in winter and weaker in summer.This is consistency with global oceanic activities which is stronger in winter and becomes weaker in summer in each local hemisphere.Moreover,strong precursors are observed in NCF for period 10-20 s and they present clear azimuthal dependency in the NCFs having similar inter-station distance and different azimuths.The precursors are strong in winter and weak in summer.By analyzing its seasonality and arrival time,it should be cause by one local noise source located in northern Atlantic Ocean.The precursors may generate strong signals just before the Rayleigh wave and distort the dispersion measurement.2.Using the NCF-based slowness beamforming technique,we carried out direct slowness observations at period of 4-30 s and studied on the composition of ambient noise signal and its distribution in space and time for different periods.The result shows,in the different frequency band,the distribution of ambient noise sources components is different.At higher frequencies the component of body wave is observed while at lower frequencies the body wave signal is weaker and surface wave signal becomes dominant.In addition,the main source direction of surface wave energy in different periods is different,and the seasonal variation characteristics of noise energy in different directions are not same.Besides,the seasonality and frequency dependence of noise sources distribution is wellcorrelated with variations of the ocean activity itself,which is consistent with the results from the previous method.This study shows that the influence of uneven noise source distribution,both in space and time,should be considered for noise cross-correlation studies,especially when the waveforms are critical for related studies.