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

Abstract

High-frequency Rayleigh waves were generated in the frequency range of about 1–100 Hz by an electromagnetic vibrator in co-operation with the Vibration Instrument Company, Japan, and the Ensol Corporation, North Carolina, at 36 sites in Anchorage. From phase delay times between two sensors, the phase velocity of the fundamental mode at each site was computed. These data were inverted in terms of shear wave velocity structure by a stochastic inversion scheme. Of the 36 sites, values of shear wave velocity (β) as a function of depth were available at seven sites from downhole measurements. At these sites, the comparison of the results obtained by others between surface and downhole measurements showed reasonable agreement. These results were therefore combined with those of the remaining 29 sites obtained in this study. The β structure of the 36 sites could be subdivided into four groups. On the basis of NEHRP provision and from the time-averaged β structures 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 Anchorage area. From statistical analysis of the time-averaged β 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 in west Anchorage with relatively low β-values coincide with the areas of high ground failure susceptibility identified by others from observations following the Prince William Sound earthquake (Mw = 9.2) of 1964.