Studies on a persistent localized microseism source that produces precursors on noise correlation function observed using stations in central-eastern China

We calculated the cross correlation function(NCF) for each station pair using vertical component of 430 broadband stations in middle-eastern China from 2008 to 2009.Very strong precursors before Rayleigh wave are observed in most NCFs,which are persistent within two years.These signals last for about 50 s and peak from 0.07 Hz to 0.12 Hz in spectrum.The arrival times of these precursors are constant for same station pair but vary with different station pair,which indicates these signals may origin from a persistent localized microseism source.Triangulation location using grid search method indicates that they are generated around Kyushu island of Japan,assuming a homogeneous propagation velocity of 2.7 km/s.This location result is further verified by comparing synthetic NCFs with observations.Special attentions should be paid to minimize possible errors caused by these precursors for studies based on ambient seismic noise.The physical mechanism of this microseism source remains to be studied.