Shallow Shear Wave Velocities for Downtown Salt Lake City: Relationships With Surface Topography, Basin Depth and Earthquake Site Amplifications

Date: 4/26/2019

Time: 10:45 AM

Room: Pine

As part of an earthquake hazards assessment, we acquired more than 10,000 shear wave velocity (Vs) measurements along the city streets of Salt Lake City (SLC), Utah. From these data, we produce Vs and near-surface site amplification maps. We compare time-averaged measurements for the upper 30 m (Vs30) from this ~15 sq km area with: 1) mapped geology and active fault locations; 2) lidar-derived surface elevation and slope; 3) seismic reflection-derived structure and stratigraphy; and 4) Bouguer gravity measurements and modeled layer boundary depths. We show the slowest Vs30 values (site class D1 soils) at the lowest elevations, with velocities consistent with modern marine silts, clays and sands. We observe a linear increase of 4.6 m/s increase per meter of elevation gain within latest Pleistocene and Holocene lacustrine deposits. Where fan alluvium was mapped, we measure Vs30 between 230 to 420 m/s (site class D2 to C1 soils) with a poor correlation with surface elevation. We compare slope to Vs30 and observe a linear relationship in log-log space, with slower Vs30 values relative to slope when compared to global averages. We attribute slower Vs30 values in SLC to the latest Pleistocene and Holocene lacustrine sediments that fill the valley. We show a high correlation between Vs30 and Bouguer gravity values, suggesting that Vs30 is a reasonable proxy for basin depth for the SLC area. When comparing Vs30 to Vs20 or shallower measurements, we identify shallow hard boundary regions that may produce local changes in site response. We estimate site amplification relative to bedrock sites using our Vs30 measurements and physical property estimates from previous studies. We suggest that slow Vs Holocene lacustrine deposits mapped to the south and west of downtown may produce increased site amplifications when compared to sites that host higher Vs fan alluvium.

Presenting Author: Lee M. Liberty

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