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 (VS) horizon. These depths provide a first-order representation of basin geometry. Common basin depth terms are z1.0 and z2.5, which are depths to the VS= 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, VS30). 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 VS30. This requires the use of VS30-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 z2.5 depth parameter, which has more physical meaning than z1.0 for this region. We assign z2.5 values for Pacific Northwest basins using the USGS 3D velocity model for locations of VS30 measurements and strong motion recording stations. Preliminary results indicate that overall there is no particular VS30-z2.5 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 z2.5 conditional on certain basin structural characteristics and VS30. These models are being used in some of the NGA-Subduction GMMs to develop basin-related site amplification models.
Presenting Author: Sean K. Ahdi
Authors
Sean K Ahdi seanahdi@gmail.com University of California, Los Angeles, Los Angeles, California, United States Presenting Author
Corresponding Author
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Timothy D Ancheta tim.ancheta@rms.com Risk Management Solutions, Newark, California, United States |
Jonathan P Stewart jstewart@seas.ucla.edu University of California, Los Angeles, Los Angeles, California, United States |
Development of Basin Depths and VS30-Depth Centering Models for the Pacific Northwest
Category
Earthquake Ground Motions and Structural Response in Subduction Zones: A Focus on Cascadia