Delineation of spatial variation of shear wave velocity with high-frequency Rayleigh waves in Anchorage, Alaska

High-frequency Rayleigh waves were generated in the frequency range of abou t 1-100 Hz by an electromagnetic vibrator in co-operation with the Vibratio n Instrument Company, Japan, and the Ensol Corporation, North Carolina, at 36 sites in Anchorage. From phase delay times between two sensors, the phas e velocity of the fundamental mode at each site was computed. These data we re inverted in terms of shear wave velocity structure by a stochastic inver sion scheme. Of the 36 sites, values of shear wave velocity (beta) as a fun ction of depth were available at seven sites from downhole measurements. At these sites, the comparison of the results obtained by others between surf ace and downhole measurements showed reasonable agreement. These results we re therefore combined with those of the remaining 29 sites obtained in this study. The beta structure of the 36 sites could be subdivided into four gr oups. On the basis of NEHRP provision and from the time-averaged beta struc tures for the uppermost 30 m corresponding to the four groups, it has been possible to identify the lateral extent of soil classes C and D in the Anch orage area. From statistical analysis of the time-averaged beta data, it is shown that the lithology of the area in soil class C is distinct from that in D. The same is true for the subunits, namely, glaciofluvial deposits in areas of C and D. Moreover, the areas in soil class D along the Knik Arm i n west Anchorage with relatively low beta -values coincide with the areas o f high ground failure susceptibility identified by others from observations following the Prince William Sound earthquake (M-w= 9.2) of 1964.