Subdaily Ambient Noise Monitoring In Parkfield

The monitoring method of the temporal variation of underground seismic velocity based on ambient noise cross-correlation(C~1)has attracted the attention of researchers and has been widely used because of its low cost,high adaptability and continuous and reliable results.However,this method still has its shortcomings,that is,the low temporal resolution.Among the published results,the subdaily temporal resolution can be achieved with the exception of a few studies using stations from the mobile dense array and on the high sensitive basis.For most studies based on long-term observation stations,the resolution was only a few days to tens of days,because stable stacking cross correlations waveform can be obtained through thus long-term stacking,and then reliable seismic velocity changes can be obtained.In order to improve the resolution of monitoring,in recent years,researchers have proposed two kinds of data processing strategies:noise reduction of interrelated waveforms,such as singular value decomposition and Wiener filtering(SVDWF)denoising method;Increase available data sets,such as seismic velocity measurement by multicomponent noise correlation.Based on these methods,this study introduces the correlation of the coda of correlations(C~3)temporal change monitoring method to study the underground medium seismic velocity changes before and after the 2004 Parkfield M6.0 earthquake.In this study,using the continuous seismic records of 13 borehole stations of the High Resolution Seismic Network in Parkfield area,after the cross-correlation of nine-component ambient noise was calculated,the appropriate C~1coda window was selected,and then C~3was constructed by prestack processing technique.The SVDWF noise reduction was carried out on the current empirical Green function obtained by C~1method and C~3method,and then the moving window cross-spectral technique was used to calculate the relative travel time offset in the frequency band of 0.7-1.4 Hz.Finally,the 4-hour high-resolution temporal variation monitoring results in Parkfield area were obtained.This was an unprecedented high temporal resolution noise-based seismic velocity measurement result in Parkfield area,and it was also the highest time resolution result obtained by using fixed seismic network in published literature so far.The study found that the co-seismic velocity of the Parkfiled M6.0 earthquake in 2004decreased by about 0.1%,and the seismic velocity decreased again when strong aftershock of M5.0 occurred 24 hours after the main shock.Within a few days after the earthquake,the seismic velocity first recovered rapidly,then slowly,showing a logarithmic seismic velocity recovery process.In two months after the earthquake,the seismic velocity recovery degree exceeded 50%;There was no reliable abnormal change of seismic velocity before the main earthquake.It was determined that the upper bound of detecting the background disturbance of preseismic velocity before earthquake at 4-hour resolution was 0.0105%,which was lower than the previous measurement result at 1-day resolution using the same data(0.0265%).Analyzed the physical mechanism of seismic velocity changes in combination with existing studies,we concluded that the physical mechanism of coseismic velocity reduction was dominated by the structural damage of fault zone caused by the earthquake and the drop of the shear modulus of the shallow crustal medium,and the physical mechanism of postseismic velocity recovery was mainly the afterslip.