Study On Strong Motions Characteristics Of Seismic Unfavorable Sections Based On Ki K-net Data

The shortage of construction land resources is an important problem faced by our country’s social development.A lot of strong motion observation results have proven that ground motion characteristics and seismic damage effects are significantly influenced by near-surface sediments and various topography.Nowadays,it is of great practical significance to study the site effects of various seismic unfavorable section with earthquake disasters occurring frequently.Based on the collection of a large number of topographic and geomorphological data,this study carried out relevant research on the characteristics of strong motion in the seismic unfavorable areas:(1)In this study,the borehole data of the Ki K-net station in Japan was collected and sorted.The 696 Ki K-net sites are grouped based on its equivalent shear wave velocity according to the Chinese site classification standard.We collected and sorted out Japan geomorphic maps to extract relevant topography and landforms.Based on Japan’s highprecision DEM(Digital Elevation Model)data to conduct digital terrain analysis related to this study.According to the Chinese relevant regulations about seismic unfavorable section,based on borehole data,topographic features,height difference,slope,distance to slope shoulder and other terrain parameters extracted by DEM,a standardized process for the classification of seismic unfavorable section was formulated,and the Ki K-net stations in Japan were subjected to the classification of seismic unfavorable sections:favorable and ordinary,slope,protrusive and soft sections.(2)Utilizing more than 40,000 sets of weak vibration observation data from 545 Ki K-net stations,the fundamental period of the site was identified.Combined with the classification results of seismic unfavorable section,it is shown that affected by local topographic effects,the fundamental period of the site in the slope section and the protrusive sections is slightly smaller than that in the seismic favorable section.Affected by the soft soil layer,the fundamental period of the soft site is obviously larger than that of seismic favorable section.For same classification sites,the difference in magnification corresponding to the fundamental period of the seismic unfavorable section and the seismic favorable section is not obvious.And based on the strong motion and weak motion records of 696 Ki K-net sites,this paper study the average magnification of the Fourier spectrum in each seismic section,Its exhibits the similar variation characteristics as the fundamental period.(3)Utilizing 54,962 sets of acceleration records that PGA is between 5-20 gal and1984 sets of strong motion records that PGA is larger than 100 gal from 696 Ki K-net stations,the average magnification of the acceleration response spectrum in each seismic unfavorable section under different site classifications was compared.We calculate the difference in magnification to estimate the amplification factor of seismic unfavorable section.The research shows that,as a whole,the slope section,as the transition between the general section and the protrusive section,has similar amplification characteristics as the protrusive section in terms of period and magnification.The softer the site,the greater the period corresponding to the amplified peak ratio of the response spectrum in the unfavorable section is significantly larger than that of the general section,and this amplification characteristic will lead to a significantly wider width of the response spectrum platform.The result is basically consistent with the amplification factor of 1.1-1.6 times specified in the seismic design code.(4)The study is based on 22 arrays of adjacent surface Ki K-net and K-NET stations.Stations with highly consistent surface topography and soil shear wave velocity have highly consistent instrumental seismic intensity in each earthquake.The instrumental seismic intensities recorded at sites with slightly softer soil layers are significantly greater than those recorded at reference stations with slightly harder soil layers.This study proves that the instrumental seismic intensity above 4 degrees is a stable index for evaluating the amplification effect of the site,which can better reflect the difference of instrumental seismic intensity between the unfavorable seismic section and the general section.Studies have shown that for unfavorable sections such as slope shoulders and near water bodies,the instrument intensity increment caused by the site topographic effect is about 0.2-0.6,and the PGA amplification is about 1.2 times.The intensity increment caused by the soft soil layer is increased by about 0.4-0.8,and the PGA is increased by about 1.3-1.7 times.(5)A total of 16,714 sets of ground motion records with instrumental seismic intensity greater than or equal to 4 degrees in Japan from 1996 to June 2021 were selected.Under the Chinese site classification method,the relationship between the seismic intensity increment of the surface and downhole instruments in each unfavorable seismic section and the effective peak acceleration EPA,peak acceleration PGA and peak velocity PGV magnification of the acceleration records are calculated respectively.Statistics show that,on the whole,the correlation coefficient between the seismic intensity increment and the fitting of the EPA,PGA and PGA amplification coefficients is generally above 0.8.From the perspective of the types of seismic sections,for the same type of sites,the difference between the seismic unfavorable、favorable and ordinary sections is not obvious.