Comparing Observed and Predicted Directivity in Near-Source Ground Motion

Accurately characterizing near-source ground motion is an important consideration for dam safety in California. Near-source ground motion can contain velocity pulses that are amplified by directivity, which is the constructive interference of S waves radiated by a propagating rupture front. Accordingly, Somerville et. al. (1997) developed an empirical model for predicting fault-normal (maximum) and fault-parallel (minimum) spectral acceleration for periods>0.5 sec. We compiled near-source ground motion records representing significant directivity and rotated them to the component with maximum overall spectral acceleration for common periods of directivity amplification (and importance to dam stability analyses, 0.5≤T≤3.0 sec), which we call SA_MAX. As expected, SA_MAX correlates with the orientation of a strong velocity pulse in the directivity record. Comparing the amplitude and orientation of SA_MAX to Somerville's predictions, we find that strong velocity pulses produced by strike-slip faulting are reasonably aligned with the fault normal, and their corresponding SA_MAX is satisfactorily predicted by Somerville's model as modified by Abrahamson. However, the orientations of strong velocity pulses in reverse-faulting records can depart significantly from fault normal, and their corresponding SA_MAX can exceed Somerville's predictions appreciably.