Investigation Of Low-frequency Ground Motion Amplification In Sedimentary Basins In NE China Using Seismic Ambient Noise

Low velocity/density sediments inside sedimentary basins are known to amplify ground motions caused by earthquakes.Such amplifications can cause loss of lives and damage of buildings.Hence,accurate estimation of ground motion amplification at different sites is of great importance for mitigating earthquake hazards.To estimate site amplification,we computed power spectral density(PSD)of ambient noise data recorded by a large-scale broadband array deployed in the northeast China.The NECESSArray(North East China Extended Sei Smic Array)consists of 127 broadband stations with a station spacing of 80 km and an aperture of 800 km and 1200 km along the NS and EW directions,respectively.The broadband array covers the Songliao Basin,Sanjiang basin,and their surrounding mountain ranges.We selected several bedrock reference stations located in the Great Xing'an Range which is in the west of Songliao Basin and in the Zhangguangcai Range which is in the east of Songliao Basin.Then we calculated the average PSD of two bedrock reference stations separately located in the Great Xing'an Range and Zhangguangcai Range as the reference PSD.After averaging we calculated the spectral ratios of stations in the Songliao Basin and Sanjiang Basin to quantify amplification factors at 0.1 Hz?1 Hz.For Songliao Basin,resonant frequencies of most stations are in the range between0.1Hz and 0.6 Hz according to noise results.Stations with highest amplification factors are located in the central and southern region with thicker sediments.But spectral ratios of earthquake demonstrate that stations with highest amplification factors are located in the north part,which is the deepest region of Songliao Basin.This difference is probably caused by attenuation of 0.1 Hz?0.6 Hz ambient noise propagating from coast to inland.Most stations inside the Sanjiang Basin have resonant frequencies between 0.4 Hz and1 Hz,and amplification factors calculated by ambient noise data are smaller than that of earthquake data.For stations located in the Songliao Basin,spectral ratios obtained by the way of averaging are highly consistent with that of earthquake.More specially,resonant frequencies and magnitudes of spectral ratio obtained through average method are very similar to that of earthquakes in the horizontal direction and vertical direction.This indicates that the average method can effectively correct the influences of attenuation caused by low-frequency ambient noise propagating from coast to inland.Moreover,area with apparent amplification is strongly correlated with known geological structure in the horizontal direction.With the growing depth of sediments,the amplification factors also become larger.In a nutshell,it is feasible to get amplification characteristics of Songliao Basin through the average method.However,for Sanjiang Basin,the applicability of this methed is not fit.Finally,we concluded that for a large and deep sedimentary basin,in the circumstance of lacking high quality attenuation data and existence of complex coastal line,one can selected different bedrock stations according to their distance from coast line as one reference station to calculate spectral ratios,which can probably be useful to get a realistic and accurate results.