Research On Very Low-frequency Events In Eurasia Seismic Observation

Low frequency vibration signals are internationally found mainly in the Trough or Subduction zone, such as ETS(Episodic Tremor and Slip), LFEs(Low-frequency Earthquakes), VLFEs( Very Low-frequency Earthquakes). Some studies suggest that there are some relationships between low frequency vibration signals and fault creep, slow slip,earthquake. Research on the low-frequency vibration signals helps us better understand geophysical activities, and is also important to exploration seismic source. Research on low frequency vibration signal on the mainland is still relatively few. This paper tries to explore and research the mainland seismological observation of unknown weak seismic signals.According to the signal characteristics such as signals are weak, rich in signal frequency, etc., we compared and analyzed digital filters and wavelet decomposition, and chose wavelet decomposition as a signal extracting means. We compared three kinds of down-sampling method, the results have shown that: integrated algorithm complexity and effect, mean is the best method, so we use the mean signal down-sampling.This paper studies and analyzes the continuous waveform data of Eurasian seismic stations, has detected low-frequency seismic signals(period 60-120s) whose duration are hours to ten hours, by spectral analysis and wavelet decomposition. These signals are universal seismic observation in the entire Eurasian continent. Compared to other low-frequency signals such as ETS, Tremor, LFEs, VLFEs, Hum, vibration signals induced by ocean storms or cyclones, the results show that these signals(period 60-120s) are different. These signals are named Very Low-frequency Events by us.By analyzing characteristics of Very Low-frequency Events in space, we analyzed and discussed the sources of these signals. Study shows that: Duration of Very Low-frequency Events is up to 6-12 hours or more, Very Low-frequency Events recorded in the coastal and island regions is weak or vacancy, but strong inland areas; the horizontal component is stronger than the vertical component. Period of Very Low-frequency Events is longer than the vibration signals(3-8s) induced by ocean storms or cyclones, and frequency of occurrence is also more. According to the above features we judgment Very Low-frequencyEvents are not from Teleseism, ocean storms or large-scale atmospheric motions, etc. Very Low-frequency Events recorded on many stations of Eurasia is not exactly the same, but similar in multiple time on the stations in a certain area, and signal strength and duration is close to each other. If distance between stations exceeds a certain range, characteristics of Very Low-frequency Events recorded is significantly different. So we conclude that Very Low-frequency Events is from a certain range nearby observation station, and have relationship with regional geographic activity around the station.