On the correlation of seismic microtremors

[1] We present results of the analysis of microtremor measurements using a small array. Our analysis is based on the computation of cross-correlation functions between stations in both frequency and time domains. We obtain similar results in both domains and link those results to the application of Aki's spatial autocorrelation method of using a single station pair and to recent studies that have shown that the Green's function between two stations can be retrieved from the temporal cross correlation of seismic noise. We show that the same simple subsoil structure allows interpretation of our correlation results in time and frequency. We observe both Love and Rayleigh waves; however, Love waves dominate the records in our lower-frequency range (between 3.6 and 6 Hz), while Rayleigh waves are prevalent in the records in our higher-frequency band (from 6 to 20 Hz). Frequency domain cross correlation yields better results in the lower-frequency range, while time domain cross correlation shows very clear results in the higher-frequency band. Thus both analyses are complementary. Our results show that ambient vibration recorded at the free surface includes different types of waves but that the correlation between any two stations is governed by the more stable propagation mode between them, surface waves in the case of a layered medium. Our results shed some light on the nature of microtremors and on the reasons why the spatial autocorrelation method or the horizontal-to-vertical spectral ratios are useful in geophysical and site response studies.

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