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Open Access Publications from the University of California

Development of geologic site classes for seismic site amplification for central and eastern North America

Creative Commons 'BY' version 4.0 license

The time-averaged shear wave velocity in the upper 30 m of a site (Vs30) is the most common site parameter used in ground motion prediction equations for the evaluation of seismic site response. It is often the case that Vs30 is not available at sites with earthquake recordings; for example in the NGA-East site database only 45 of 1149 sites have measured values of Vs30. Accordingly, estimates of Vs30 are often made on the basis of available proxies that are widely available such as ground slope, geomorphic terrain categories, and surface geology. We compile a database of 1930 measured and inferred Vs30 values in Central and Eastern North America (CENA) to test slope and geomorphology-based proxy methods. The results indicate that these existing proxy methods are biased for sites with Vs30greater than 400 m/s. Based on a careful review of geological conditions in the CENA, we propose nineteen geologic classes based on setting (i.e., glaciated or non-glaciated), age, and depositional environmental that can form the basis for geology-based proxy estimates of Vs30 as well as for simplified stratigraphic columns.

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