Development of Basin Depths and VS30-Depth Centering Models for the Pacific Northwest

Date: 4/24/2019

Time: 02:30 PM

Room: Pine

Basin depth terms in ground motion models (GMMs) are the vertical distances from the ground surface to a particular shear-wave velocity (V_S) horizon. These depths provide a first-order representation of basin geometry. Common basin depth terms are z_1.0 and z_2.5, which are depths to the V_S= 1.0 and 2.5 km/s horizons, respectively.

To some extent, the effects on ground motion of 3D site amplification in basins are reflected in site terms within GMMs that are conditioned on parameters other than depth (i.e., time-averaged velocity in the upper 30 m, V_S30). As a result, it is desirable to center basin amplification models in GMMs (i.e. predict no amplification) for the case of an average depth conditional on V_S30. This requires the use of V_S30-depth correlation relationships, such as those developed for Japan and California in NGA-West2. Basin amplification or de-amplification in such models is then predicted for depths larger or smaller, respectively, than the centered value.

For Cascadia, we focus on the z_2.5 depth parameter, which has more physical meaning than z_1.0 for this region. We assign z_2.5 values for Pacific Northwest basins using the USGS 3D velocity model for locations of V_S30 measurements and strong motion recording stations. Preliminary results indicate that overall there is no particular V_S30-z_2.5 relationship across the entire PNW, but there are strong differences in depths within specific basin structures. The Seattle basin has the largest depths; the Portland and Tualatin basins have consistent distribution of depths between about 1.5–2.0 km; and the Everett basin is much shallower than the Seattle basin. The reliability of these depths is based on the accuracy of the 3D velocity model, but it is important to represent these differences among basins for NGA-Subduction GMM development for Cascadia.

We provide mean relationships for z_2.5 conditional on certain basin structural characteristics and V_S30. These models are being used in some of the NGA-Subduction GMMs to develop basin-related site amplification models.

Presenting Author: Sean K. Ahdi

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