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Study on the characteristics of microseisms in the Yunan region, China

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Abstract

The Green's function between two stations can be extracted by cross correlating long time range ambient noise recorded by these stations, thus can be used for subsurface velocity inversion as well as time lapse monitoring of velocity variation. The knowledge of the variation of ambient noise direction and strength is important to improve the precision of these researches. Using the vertical component records from the Yunnan regional broadband seismic stations from 2008 to 2010, we calculated the noise cross correlation functions between each stations, then used the normalized background energy flux measurement to study the characteristicss of ambient noise in Yunan region, especially for secondary microseism period in 5~10s and primary microseism in 10~20 s. A significant direction variation of 5~10 s noise energy is observed, whose azimuth is 180°~210° in the Summer and moves to 70°~100° in the Winter. The direction of 10~20 s ambient noise is more stable covering 180°~210°. The strength of ambient noise of both types shows seasonal variation. We also observed strong near-zero arrivals with apparent velocity about 30 km/s, the possible mechanics of this precursors are discussed.

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