Rupture directivity of moderate to large earthquakes in the slow deforming Iranian plateau

SUMMARY Rupture directivity significantly increases horizontal peak ground acceleration, elongates aftershock clouds and enlarges meizoseismal areas beyond the fault end in front of the direction of rupture propagation. In this study, we examine the directivity of 25 moderate to large earthquakes (Mw ≥ 6) from 1968 to 2017 in the Iranian plateau by employing relocated earthquake clusters, mapped surface ruptures, focal mechanisms of earthquakes, slip distribution models, spatial distribution of peak ground acceleration amplitudes and macroseismic effects. The methodology overcomes the lack of dense seismic networks required to study directivity using methods based on the azimuthal variation of the spectrum of seismic waves. We show that 16 out of the 25 (i.e. 64 per cent) of the earthquakes investigated have mostly unidirectional rupture. This implies that unidirectional ruptures in a slow deforming continental collision zone such as the Iranian plateau is only slightly less common than those observed globally. With the understanding that unidirectional rupture increases the probability of ground shaking off the termination of the causative faults, our findings highlight the importance of considering the directivity effect in earthquake hazard assessment in Iran and also in other slow deforming continental regions.