1-D SEDIMENT VELOCITY STRUCTURE IN THE MISSISSIPPI EMBAYMENT: CONSTRAINT FROM AMBIENT NOISE ANALYSIS ON A DENSE ARRAY

We applied ambient noise analysis on a small tremor array and one adjacent station in the Mississippi embayment. The array was deployed from the November of 2009 to December of 2011, and was composed of 19 stations with a distance of approximately 700 meters. One-month ambient noise from 18 broadband stations was used to cross-correlate with each other (internal-array) and with noise from one adjacent station (station-array). 351 pairs of cross-correlations have been produced. Fundamental-mode Rayleigh and Love waves were observed in internal-array cross-correlations between 1 and 6 Hz, and station-array cross-correlations between 0.5 and 6 Hz. Multiple arrivals were observed in the station-array cross-correlations, and we further applied frequency-wavenumber (FK) analysis to separate body waves and surface waves. 2-D FK analysis of station-array cross-correlations showed the slowness of Rayleigh wave increased from 1 s/km in the 0.6-0.7 Hz frequency band to 2.47 s/km in the 1.2-1.3 Hz. Synthetic dispersion analysis for internal-array cross-correlations showed better dispersion curve match for Rayleigh wave than Love wave, and also indicated the Love wave is slower than Rayleigh wave for the same frequency band. The group velocity of Rayleigh wave ranged from 0.25 km/s to 0.3 km/s in the frequency band of 1-6 Hz, while the group velocity of Love wave is about 0.2 km/s in the same frequency band. Love-Rayleigh discrepancies may indicate radial anisotropy, and the vertical structure like cracks or dikes other than horizontal layers plays an important role on surface wave propagation in the sediments of the embayment. 1-D joint Rayleigh and Love wave inversion show bad matching dispersion curve. Love wave inversion presented the shear wave velocity slowly increases from 0.3 km/s near the surface to 0.6 km/s around 700 meters, and the velocity suddenly increases to 1 km/s below 700 meters. More sophisticated other than 1-D isotropic velocity model should be used for seismic hazard study in the embayment region.