Orientation of maximum spectral demand in the near-fault region

An orientation-dependent parameter, R̄S_a (θ), is used to investigate the orientation of maximum elastic spectral demand for near-fault ground motions. The upper bound on R̄S_a (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 R̄S_a for each pair of ground motions. The maximum value of R̄S_a 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.