Application of the Spatial-autocorrelation Microtremor-array Method for Characterizing S-wave Velocity in the Upper 300 m of Salt Lake Valley, Utah

Spatial-autocorrelation (SPAC) microtremor-array data acquired at 14 sites in Salt Lake Valley, Utah, characterize S-wave velocities to depths as great as 300 m. Three data sets acquired at each site were analyzed simultaneously using equilateral triangular arrays with sensors deployed at 33.3-m, 100-m, and 300-m separation. Of the 14 sites, eight were within 1.2 km of active-source (vibroseis) body- and surface-wave acquisition sites, and two were within 0.7 km of boreholes logged for S-wave velocity (V-S) to at least 50-m depth. A comparison to these existing active-source and borehole models indicates that these SPAC V-S results typically differ by less than 10% on average to 100-m depth. At a majority of the investigation sites, SPAC modeling results can be interpreted confidently to more than 150-m depth. Linear ground-motion amplification spectra derived from these profiles of V-S versus depth suggest amplification factors of more than three can occur at frequencies in the band of 0.5 to 4 Hz from the base of unconsolidated sediments in the upper 300 m.