Shallow Shear-velocity Structure Beneath The West Lake Area In Hangzhou From Ambient Noise Method

Exploiting the urban underground space resources and reducing geological hazards are the primary issues for Hangzhou city's sustainable development.However,conventional geotechnical methods will meet the challenges of urban environment associated with moving vehicles,dense power grids and so on.Ambient noise method is an environmental-friendly method and suitable for urban underground exploration.Hence,we tested the ambient noise tomography method in Hangzhou city's urban underground space exploration.We chose the West Lake as our study area and tried to apply the ambient noise tomography(ANT)method to obtain the shallow shear-wave velocity in the study area.The results obtained were impressive with the surface waves having a good signalto-noise ratio at 0.35-2 s period after processing continuous ambient noise data from the noisy anthropogenic environment.In addition,we imaged the 3D shallow Svelocity structures down to 1 km depth which is in good agreement with the known geological features.The limestone region that is closer to the western part of the West Lake shows high-velocity pattern.The northeast part of the West Lake and the Baoshi mountain area are covered by rhyolite and volcanic tuff,which displays low-velocity features in the tomographic images.Furthermore,the continual extension of lowvelocity anomaly under the Baoshi mountain is consistent with the location of old magma conduit.The velocity result also reveals the underground undulation of the West Lake synclinorium and the dip direction of one fault.Overall,current research acts as a pioneering application of ANT technique in Hangzhou.This case study justifies that ANT technique are effective to image the underground velocity structures within Hangzhou.The imaging results is of great importance for us to understand the local geological structures,exploit the underground space and prevent geological disasters in the future.