Research On Very Low-Frequency Seismic Signal In Sichuan-yunnan Tectonic Area

In the analysis of continuous waveform data of China mainland broadband digital seismic signal, a very weak low-frequency seismic signal can be universally observed, usually lost in the background signal. Period of the signal is between 30-120 s, and its duration is from several hours to ten hours; temporal change of the signal has a certain degree of regularity; the signal has strong horizontal component performance, but weak in vertical component. With lower frequency and wider band, it is obvious different from very low-frequency earthquakes mentioned internationally in subduction zone; we call it very low-frequency seismic signal(VLFS).This paper focuses on VLFS in Sichuan-Yunnan area and carries out a detailed analysis. Firstly, main features of VLFS are described. By a variety of time-frequency processing methods including FFT, 1 Hz sampling seismic data in long period has been analyzed. Through analysis with digital band-pass filtering and wavelet transform techniques, the result shows that its main spectral component is in the detail sections d5 and d6 of wavelet processing, which is equivalent passband in period of 30-120 s. In comparison to seismic signals generated by local regular earthquakes, teleseismic, oceans, gravity waves, large scale weather events such as storms and cyclones, it is different in lots of aspects. Considering the features of VLFS, the part containing main cycle is extracted. By comparing VLFS in several seismic stations, we found that they exhibit certain repetitive over a long period. Its frequency of occurrance is relatively high, with duration of several hours each time. And the consistency shows in several stations, with a little time difference. On this basis, we try to search the generating source of VLFS for a deep understanding. Taking into consideration that VLFS is weak low-frequency signal, and has long lasting time, and the arrival time is also relatively difficult to determine, in large observation area and sparse station network density conditions, based on the source scanning algorithm, we adopted multiple data processing methods, and effectively extracted feature ?seismic phases? of continuous waveform data recorded in Sichuan-Yunnan area for positioning research.We choose different wave propagation speed to scan Sichuan-Yunnan area with signals of period between 60-120 s in several stations. The result of locating indicates that the source of these VLFS is more likely from area near the stations.