The observed directivity effects of ground motion have extensive implications for both earthquake source physics and earthquake-hazard analyses on active fault zones. In general, it is still difficult to determine the rupture directivity in detail for smaller earthquakes because of limited azimuthal coverage. Four Lushan aftershocks that occurred on 20 and 21 April 2013 (events I, II, III, and IV) were well recorded by the National Strong-Motion Observation Network System of China. An interesting phenomenon was clearly observed: the distributions of the triggered stations for the four aftershocks, which had adjacent hypocenters and similar magnitudes, differed significantly from each other. The analyses of ground-motion parameters, including the peak ground-motion parameters, response spectra, and durations, indicate that the directivity of the ground motion was significant for events II and III. The Fourier spectral ratio method was used to imprint the rupture directivity. We calculated the rupture direction and rupture velocity by inverting the peak parameters. Random realizations in many times were utilized to consider the uncertainty in the results of the inversion. The results indicate that the four events shared the similar northeast–southwest-trending fault plane and had similar rupture velocities of approximately 2.1km/s. Event I was more likely to be a perfectly symmetric bilateral rupture, though there was large uncertainty in the results of the inversion. The rupture for events II and III propagated predominantly toward the southwest. However, event IV was observed to have an approximately symmetric bilateral propagation, slightly inclining to the northeast. We also confirmed the period-dependent directivity effect by inverting the response spectra at different periods.

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