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Frequency-Dependent Asymmetry of Seismic Cross-Correlation Functions Associated with Noise Directionality

Department of Mathematics, Marist College, Poughkeepsie, New York 12601

Aaron N. Ferris and Michelle Johnson

Weston Geophysical Corporation, Lexington, Massachusetts 02420

^* Present address: Department of Earth and Atmospheric Sciences, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964.

Cross correlation of ambient noise recorded by broadband seismometers has become a popular data analysis technique. Because time-averaged cross-correlation functions provide estimates of the Green’s function between two stations, they provide arrival information for waves propagating in opposite directions along a line connecting the stations. Under certain circumstances, amplitude asymmetry in these functions can be associated with preferred directionality of the noise field surrounding the stations. An interesting example of asymmetric cross-correlation functions comes from the site of the 1992 Pakistan Himalayas passive source broadband seismic experiment (PAKH). Here, the comparison of causal and acausal signal-to-noise ratios in several period bands gives an indication of the noise field directivity. The analysis also reveals frequency-dependent asymmetry of the cross-correlation functions, suggesting multiple sources of the ambient noise. At PAKH, some portion of the 6–10 sec noise field band may be attributed to distant ocean wave activity, as seen in other studies, but other period bands show noise propagation from the opposite direction. Our directivity analysis suggests that the regional tectonics of the Indo–Eurasian collision and/or its associated microseismicity may be contributing strongly to the apparent noise field, either by acting as a dominant scatterer or as a noise source.