Ambient Noise Surface Wave Tomography Using A Dense Array And Its Applications

Compared with traditional seismic tomography,ambient noise tomography has the advantages of not relying on seismic signals,no need to consider the error of the source locations and the time of the earthquakes,and the good recovery of short-period surface wave signals.In addition,this method is safe and easy to obtain data with low cost,because it does not require the artificial sources.Therefore,the ambient noise surface wave tomography method has been widely used and developed in the past decade.Most of the researches on ambient noise focused on global and regional structures,and their research depth ranges from tens to hundreds of kilometers.With the development of mobile seismic network deployment,ambient noise tomography using dense seismic arrays has also been rapidly developed and applied.The resolution using dense arrays is gradually increasing.The research area and imaging depth are developing in a smaller and shallower direction,up to nearly ten kilometers.And ambient noise tomography can be effectively applied to the researches of urban underground structures.Based on the advantages of ambient noise tomography and the resolution advantage of dense seismic arrays,this paper carried out the following four researches:(1)Fine fault structure of Xinfengjiang reservoir area from high frequency ambient noise tomographyThe Xinfengjiang(XFJ)water reservoir in Guangdong,China locates in yanshanian granites.After the impoundment of the reservoir in 1959,numerous earthquakes occurred,including the M6.1 one in 1962.XFJ water reservoir is one of the most active seismic zones in Guangdong.However,the underground structure of this area is not yet well understood,and there are uncertainties about the mechanism of reservoir earthquakes.To investigate the shallow crustal structure near XFJ water reservoir area,we used ambient noise surface wave tomography to invert Rayleigh group velocity and 3D shear wave velocity beneath RZSF area.Our results show that for such dense spacing stations,high frequency(0.2-1.2 s)dispersion signal can be well retrieved,which are important to resolve shallow crustal structure(<1km).Shear wave velocity maps show obvious low velocity anomalies beneath RZSF,and this slow anomaly extend to both sides of the fault in the south.The two sides of the RZSF show relatively high velocities,which may correlate with surficial geology.Our results show fine structures of the distribution and geometry of fractures near the RZSF,and provide important constraints in understanding the geological tectonics and seismicity in XFJ water reservoir area.(2)Topography effect on ambient noise tomography using a dense seismic arrayWith the rapid increase of dense seismic array deployment,more and more ambient noise studies have been applied on short period surface waves tomography.For arrays with inter-station distance of several hundred meters,the effect of surface topography has to be considered.In this study,we investigate topography effect on ambient noise surface wave tomography using synthetic data from different topographic models.Our inversion results suggest that topography does affect subsurface shear wave velocity inversion.If topography is not considered,although the pattern of the structure may be recovered reasonably well,the depth distribution of velocity structure can be distorted.The maximum distortion depth is generally correlated with the relief of the topography and the amplitude of the velocity anomalies.Finally,our example of real data inversion in a mountain area demonstrates good correlation between shear velocity and the geological settings,as well as the core sample in that area.(3)Sub-surface structure and site effect extracted from ambient noise in metropolitan Guangzhou,ChinaIn this paper,ambient noise is used to investigate near-surface structures and site effects in metropolitan Guangzhou.We inverted three-dimensional shear wave velocity structures of Guangzhou area via ambient noise tomography.Results show low velocity near the Guangzhou-Conghua Fault(GCF),Shougouling Fault(SF),and beneath the alluvial plain in the south region.Meanwhile,high-velocity anomalies are beneath the mountain area in northeastern region.Moreover,we obtain the sediment thickness and sub-surface shear wave velocity structure around the Shougouling Fault(SF)by horizontal to vertical spectral ratio(HVSR)method.HVSR results show a significant shift in the thickness of the sedimentary layer across the SF.Shear wave velocity derived from HVSR curves has a consistent trend of variation with the sediment thickness.Our results provide a better understanding of sub-surface structures of metropolitan Guangzhou,and can be served as a reference model for geological disaster migration prediction in the city.We have verified that the ambient noise tomography method can be successfully applied to small-scale researches,and its spatial resolution is greatly improved with the deployment of mobile dense seismic arrays.However,this method still has a lot of research space and potential to investigate,which can be further expanded.For example,Rayleigh wave and Love wave can be used to jointly invert three-dimensional subsurface structures to improve the shallow resolution;using high-order surface wave dispersion and more inversion constraints to invert underground structures,which can improve accuracy of the surface wave inversion;adding reflection interface constrains based on body waves extracted from ambient noise to improve deeper spatial resolution.