Relationships among various definitions of horizontal spectral accelerations in central and eastern North America


A single ground-motion intensity measure, typically spectral acceleration (SA), is required as the main input in deriving empirical ground-motion prediction equations (GMPEs). Traditionally, a single horizontal orientation has been used in calculating SA for all periods. The spectrum changes with orientation, and using a single orientation to represent 2D ground motions leads to loss of useful information regarding the variation of SA with orientation. Different techniques have been proposed in the literature to combine the two horizontal components of ground motions into a scalar horizontal definition. The ratios between different definitions of the horizontal component of ground motions have been studied because of the urgent need to use multiple GMPEs combined in a logic-tree framework in the preliminary stages of performing probabilistic seismic hazard analysis, or at further stages in which the uniform hazard response spectrum is required to be converted to another spectrum for a different horizontal definition. Although the most recent studies produced similar results using different subsets of the Next Generation Attenuation-West2 Project (NGA-West2) database, it is possible that such directionality results may differ for other earthquake datasets and is region specific. The purpose of this study is to derive ratios between median values and the associated standard deviations for different definitions of the horizontal component of ground motions in central and eastern North America using a subset of the NGA-East database. The computed median ratios are similar to the ratios provided in recent studies for other regions with a shift in some period ranges with noticeable differences between the standard deviations. The results of this study fulfill the engineering requirements of considering the maximum direction elastic response spectrum for design of structures.

Publication Title

Bulletin of the Seismological Society of America