Orientation of maximum spectral demand in the near-fault region

An orientation-dependent parameter, R(macron)Sa(θ), is used to investigate the orientation of maximum elastic spectral demand for near-fault ground motions. The upper bound on R(macron)Sa(θ), equal to 1, occurs when the spectral demands for an orientation θ are equal to the maximum demands at all periods considered. One hundred forty-seven pairs of near-fault ground motions were used to identify the orientation associated with the maximum value of R(macron)Sa for each pair of ground motions. The maximum value of R(macron)Sa is greater than 0.9 for over 80% of the records in the dataset, meaning that there is generally an axis where the spectral demands are equal or very close to the maximum spectral demands for a wide range of period. The strike-normal direction should not be used as a surrogate for the direction of maximum shaking for a) closest site-to-source distance less than 5 km and periods less than 1 second, and b) closest site-to-source distance greater than 5 km.