New evidence for large earthquakes on the Central Iran plateau: palaeoseismology of the Anar fault

SUMMARY The Central Iran plateau appears aseismic during the last few millenniums based on instrumental and historical seismic records. Nevertheless, it is sliced by several strike‐slip faults that are hundreds of kilometres long. These faults display along‐strike, horizontal offsets of intermittent gullies that suggest the occurrence of earthquakes in the Holocene. Establishing this is crucial for accurately assessing the regional seismic hazard. The first palaeoseismic study performed on the 200‐km long, NS striking Anar fault shows that this right‐lateral fault hosted three large ( M w ≈ 7) earthquakes during the Holocene or possibly Uppermost Pleistocene for the older one. These three seismic events are recorded within a sedimentary succession, which is not older than 15 ka, suggesting an average recurrence of at most 5 ka. The six optically stimulated luminescence ages available provide additional constraints and allow estimating that the three earthquakes have occurred within the following time intervals: 4.4 ± 0.8, 6.8 ± 1 and 9.8 ± 2 ka. The preferred age of the more recent event, ranging between 3600 and 5200 yr, suggests that the fault is approaching the end of its seismic cycle and the city of Anar could be under the threat of a destructive earthquake in the near future. In addition, our results confirm a previous minimum slip rate estimate of 0.8 ± 0.1 mm yr −1 for the Anar fault indicating that the westernmost prominent right‐lateral faults of the Central Iran plateau are characterized by slip rates close to 1 mm yr −1 . These faults, which have repeatedly produced destructive earthquakes with large magnitudes and long recurrence interval of several thousands of years during the Holocene, show that the Central Iran plateau does not behave totally as a rigid block and that its moderate internal deformation is nonetheless responsible for a significant seismic hazard.