Analyses Of The Background Weak Signals In The Yunnan Province

Waveform observations recorded by a seismic station not only include earthquake waves, but also the signals related with human activities, weather and other natural phenomenon. A study on the signals except for earthquake waves can help us to understand natural environment.Yunnan locates in the southeast margin of the Tibetan Plateau, where are strong deformed due to the collision between the Indian and Eurasian plates. Geological tectonics here is very complex, and it is one of high seismic hazard region in the southeastern China. Therefore, Yunnan is one focus of the solid earth science in China. Here, we will study the background elastic signals other than earthquake waves recorded in the Yunnan province.Nine-month vertical-component waveform data recorded by 44 seismic stations of Yunnan seismic network during 2008 were analyzed to retrieve inter-station Rayleigh-wave Green's functions by cross-correlation calculation. Before cross-correlation calculation, we processed the original continuous waveforms by removing instrument response, waveform's mean and trend, and then by time domain normalization and spectral whitening, which can removed earthquake waves. The amplitudes of background weak signals obtained from the Green's function referred to dispersion curves retrieved from the Green's functions. Compare the amplitudes of the Green's function at two reverse directions along two stations, we can determine the signal propagation azimuth of the regional background weak signals (or called ambient noises).The Green's functions along two reverse directions between each station pair in Yunnan showed obviously asymmetry, which indicated the source of the background signals heterogeneously distributed in the region. Further analyses on the Green's functions showed a notable azimuthal distribution of signal sources in the region. The background weak signals in a period of 15s propagate from the southeast to northwest of the Yunnan province. The amplitude of the background signals varied with seasons, and is largest between June and September. After comparison with wind data, we did not find any relation between the signals and regional wind. On the basis of the azimuthal distribution of the background weak signals in a period of 15s, we suggested that the signals may be related with oceanic waves of South China Sea or South Pacific.