Characteristics of seismic ambient noise in Sichuan region

Based on the three-component continuous waveform data recorded by sixty permanent seismic stations in Sichuan seismic network from January 1, 2015 to December 31, 2018, this paper calculated the noise power spectral densities and corresponding probability density functions, then gave the statistical characteristics of noise power spectral density at different frequencies, and finally analyzed the characteristics of noise level at different regions and frequencies. The results show that the high-frequency seismic noises of most stations are affected by the nearby human activities, production mode and lifestyle, which has obvious seasonal and diurnal variations. The noise level increases during summer and decreases during winter with the lowest level during the Spring Festival in the whole year; and the geographical distribution is not obvious. For double-frequency microseisms, the noise level increases during winter and decreases during summer, and has obvious seasonal variation with an average of 1−5 dB, which has obvious geographical distribution characteristics. The average noise level in eastern Sichuan is the highest, followed by Panxi region, and the lowest in western Sichuan Plateau. The microseism peaks have different amplitudes and occur at different frequencies in summer and winter, with the peaks shifted by 1−2 s toward longer periods in the winter. Compared with the double-frequency microseism band, the noise energy at primary microseism band is weaker, the seasonal variation is not obvious, and the difference of noise level in geographical distribution is significantly reduced. While the long-period （＞20 s） noise level has no obvious seasonal variation and no difference in geographical distribution. In addition, installing seismographs in caves and borehole can effectively reduce the influence of noise sources, temperature and pressure on high-frequency band and long-period observations, therefore the noise level is lower than that of shallow installations.