| Recently,the understanding of ambient noise has been significantly deepened,and a large number of researches related to ambient noise have been launched,including seismic ambient noise tomography,dynamic changes of seismic velocity and media anisotropy,furthermore,detecting time clock of continuous waveform with ambient noise.Traditional seismic methods depend on earthquakes or artificial sources transitionally.However,Seismic ambient noise method,without these limitations of earthquake spatial and temporal distribution,has become an important technology in seismology,and promoted the development and progress of seismology greatly.This paper is mainly based on the seismic ambient noise method,conducted related researches on measuring the clock drift of seismic waveform data and monitoring dynamic variations of velocity.The first aspect of this paper was the clock drift detection of continuous waveform data.Accuracy of the clock is one of the most fundamental requirement for seismological observation,and it has a very critical influence on data processing of seismology.In reality,however,due to various factors,whether it is a fixed seismic station or a mobile station,there is a possibility that the clock is inaccurate.In response to the clock problem of Yunnan mobile seismometers,we detected data clock drifts of four mobile stations using ambient noise method in Yunnan from 2017 to 2019,and found that one of the stations had clock drift in multiple time periods.And has two typical clock drift modes: clock skipping and linear drift.The clock skipping amplitude was up to 1.75 s,and the linear drift rate is about-0.03s/d.It was found that another station had a clock drift problem within the initial 5 months.Furthermore,the above results are verified by using the repeated signals from the Fixed Airgun Signal Transmission Stations(FASTS)in Binchuan Yunnan province.It is found that the two results have good consistency.Besides,we also found cross correlation of ambient noise has a high sensitivity of 0.1~0.2s to the clock drift of seismic waveform?The another aspect of this paper was analysis time-varying features of seismic velocity with ambient noise.Seismic velocity variation was related to stress or physical property changes of media,and could provide support for applications such as monitoring stress field and earthquake forecasting.In this paper,we processed ambient noise data in west region of Northern Tianshan,Xinjiang from 2015 to 2018 to study long-term time-varying characteristics of velocity in this region and the law of velocity variation near the source area before and after the earthquake occurrence.The results illustrated there was a clear seasonal cycle of long-term seismic velocity changes in some places.It was speculated that the temporal and spatial distribution of noise sources and temperature fluctuations are the possible reasons for these changes features in dv/v.Besides,we found dv/v of station-pair near earthquake area decreased during earthquakes occurred,and it increased after earthquakes.The research in this paper illustrated that we can study different seismological problems with ambient noise method.Which can effectively detect the clock problem of waveform,prevent the waveform from being misused,and provide reference information for the subsequent clock correction of waveform.On the other hand,using ambient noise cross-correlation to monitor feature of long or short term velocity variation continuously in some regions conduce us to analyze long-term tendency of stress filed and learn about short-term variation of media physics property during earthquake occurrence.We can test these features with corresponding geophysical researches to improve accuracy of earthquake forecasting and prediction work. |
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