Late Pliestocene-Holocene Paleoseismicity, Temporal Clustering, Probabilities of Future Large (M>7) Earthquake on the Dehshir Fault, Central Iran

The Dehshir fault system (DFS) including six fault segments forms the western border of NS-striking active dextral strike-slip fault cutting the Sanandaj-Sirjan, Uromieh-Dokhtar magmatic arc, and Central Iran. This active fault system right-laterally offset Eocene volcanic rocks and Quaternary alluvial fans. Geomorphic evidence implies the activity of the DFS in the Late Quaternary, and paleoseismic investigations revealed seismic movements along the Marvast fault segment during the Late Pleistocene and Holocene timescales. In order to unravel the seismic history of the DFS over the intermediate geologic (103 105- years) time scale, and based on the morphotectonics and sedimentarystratigraphic properties, three paleoseismic sites have been selected along the 35-km-long stretch of the Marvast fault segment. The southern site (Harabarjan) shows steep fault branches of N140±10º strike with sub-horizontal striations in dextral component. Another trench has been excavated at the North Marvast site whitin the Late Pleistocene-Holocene alluvial and colluvial deposits.Optically stimulated luminescence (OSL) analysis of loose, quartz-rich deposits yielded an age of ~72 ka for the oldest exposed sediments. Paleoseismic stduies along the Marvast fault segment provide evidence for the occurrence of several large seismic events associated with surface ruptures along the DFS. The chronology of the paleoearthquakes on the Marvast segment indicates that at least 7 large (≈Mw > 7) earthquakes occurred in the last 43 ka with an average recurrence time of 3650±150 years. The most recent earthquake, event I, occurred ~2200 years ago, which associated with 2-4 m of dextral slip and >40 km surface rupture along the Marvast fault segment. These investigations are compatible with the lack of destruction in the Marvast historical (~1300 years) castle, located <10 km east of the DFS. Seismic pattern of the Marvast fault segment could be classified in three temporal clusters and two deformation spans: 1) with significant seismic activities; and 2) with small seismic activities or aseismic. Assuming such a seismic pattern remains similar until the next seismic cycle, the Marvast fault segment is experiencing the last seismic cluster. Considering the shortest time interval between two successive earthquakes (2000 years) may indicate the immennet threat of another destructive earthquake along the Marvast fault segment.