Seismic noise characteristics of broad-band seismic networks in Chinese mainland

Using the vertical continuous waveform recordings of 880 broadband seismic stations in China Seismic Network from January 2014 to December 2015, we calculated power spectral densities （PSDs） and probability density functions （PDFs） over the entire period for each station, and then investigated the characteristics of seismic noise in Chinese mainland. The results indicate that the spatial distribution of noise levels is characterized by obvious zoning for different period bands in Chinese mainland. The high-frequency （0.1−1 s） noise level is lower in the western and northern regions, while higher in the central and eastern regions, especially in the Beijing-Tianjin-Hebei and southeast coastal areas. Densely populated areas have higher noise level than sparsely populated ones, suggesting that high-frequency noise level is related to the intensity distribution of human activities such as transportation, industry. Meanwhile, the high-frequency noise level near the basin is higher than the mountainous areas, which is probably caused by the amplification effect of the sedimentary layer. The microseism energy （1−20 s） mainly results from the coastline of eastern China, and its intensity gradually decreases from the southeastern coastal lines to the inland regions. Based on the zoning characteristics, the 5th and 95th percentiles of PDFs were used to obtain the high and low noise baselines for each network. Compared with the global new high and low noise models, they are much more effective to identify abnormal signals during seismic observations.