The Site Effect And Shallow Structure Investigations Based On Seismic Noise Analysis In North China Plain

Northern China Plain (NCP) is a sedimentary basin that still has strong tectonic activities in current stage. Several staggered hidden active faults that had occurred large earthquakes in history such as the M-8Sanhe-Pinggu earthquake and M7.8Tangshan earthquake indicate that strong potential earthquake hazards in this area. Controlled by the tectonic faulting, the basin has relative complex structures that would produce strong site effects during the strong earthquakes. But systematical researches about site effects and associate shallow structures are lack in this area mainly because of shortage stromg motion data limited by relative low seismicity and sparse seismic station distributions in region.Using seismic noise data to investigate site effects and shallow structures has been fast developed and applied world wide in recent years for the noise surveys do not depend on seismic events and can cover the whole study region with cheap and flexible temporary observation. In this paper, I intend to investigate the site effects and shallow structures in NCP mainly using seismic noise data. Besides the noise data collecting from North China Seismic array, densely noise measurements were also carried out in several cities in NCP, i.e Baoding, Cangzhou and Beijing. Large three-component datasets were processed with noise horizontal to vertical spectral ratios method (NHVSR). The fundamental resonant frequencies were retrieved from the NHVSR peak frequencies, and further the lateral characters of resonant frequencies and associate shallow interfaces were discussed. With the seismic noise arrays datasets, the shallow velocity structures were inverted and were used to fit or confine the experiment relationship between resonant frequency and interface thickness. To find cheap and fast ways to retrieving shallow velocity structures, more active or passive surface wave exploration methods were experimented and tested in both data collection and data processing.The results show significant shallow resonances commonly exist in NCP. The NHVSRs in mountain front plain belt from Beijing to Tangshan/Tianjing have the characters of single prominent NHVSR peak. The peak frequencies have strong lateral change varying from tens to0.2Hz. The interfaces corresponding to the peak frequencies, with thicknesses fluctuating from near zero to about1km, are probably the base of lately developed basin that started subsiding from Neogene period. Some profiles of NHVSR or interface thickness have obvious clues that connect with the faulting effects of some major seismic faults. The NHVSRs in plain south to Beijing-Tangshan belt have characters with two major NHVSR peaks. The higher peak frequencies mainly around1Hz indicate high impedance contrast existing in sedimentary layer. These interfaces probably correspond to the base of late Pleistocene layer (Q3) with the thicknesses varying chiefly from100-300m and change synchronizing with Jizhong Depression and Cangxian Rift. The lower peak frequencies are below0.5Hz and probably also correspond to the base of Neogene layer. The low frequency peaks are fully discussed in this paper as they are hard to be recognized and more future investigations are need.In total, the results shown in this paper further confirm the possibilities to investigate site effects and relative shallow structures with seismic noise, and also show the feasibilities to identify hidden active faults in basin with large vertical displacement with noise surveys. The study in NCP provides useful information for both seismic engineering and basin research.