| In recent years ,the research of microseism has made some progress in civil engineering realm .but because of the complexity of creation mechanism and the influence factor ,now the research of microseism commonly limit to some idiographic project application such as confirming vibration frequency and partition of field sort .but it hardly refer to mechanism of microseism ,thereby it restricts to application of microseism .This paper emphasize in study of mechanism of vibration frequency for free site in microseism testing.First theories foundation of microseism is introduced ,it is pointed out that microseism wave is made of P waveform and rayleigh wave .The conception of predominate cycle that is tested with microseism is emphasized .through microseism's testing and analyzing date .the vibration frequency of free cite can be required through frequency spectrum of microseism's signal in some area.Second separately free cite's vibration model of single layer and double layer are established , the vibration frequency of single layer is calculated with three different models , vibration frequency of free cite is required , the vibration frequency of free cite calculated by three different models is compared, the veracity of model is validated.Afterward the model is expand to vibration model of multilayer . The formula of vibration frequency is deducted accordingly .The difference between vibration model of multilayer and vibration model of single layer and double layer is analyzed with example .which is prepared for rough estimate of vibration frequency of free cite for the future.At last through microseism testing of some project in Hangzhou and date of report of geological reconnaissance, the vibration frequency of free cite with microseism's testing is compared with vibration frequency which is calculated by establishing vibration model of free cite .possible factor that is compared with factual vibration frequency of free cite is analyzed .the their relation is discovered.
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