Determining the long-term slip rate along the Mosha Fault, Central Alborz, Iran. Implications in terms of seismic activity

The Mosha fault is one of the major active fault in Central Alborz as shown by its strong historical seismicity and its obvious morphological signature. Situated at the vicinity of Tehran city, this ~150 km long ~N100°E trending fault represents an important potential seismic source that threatens the Iranian metropolis. In the framework of an Iranian-French joint research program devoted to seismic hazard assessment in the Tehran region, we undertook a morphotectonic (determination of the cumulative displacements and the ages of offset morphologic markers) and paleoseismic (determination of the ages and magnitudes of ancient events) study along the Mosha fault. Our objectives are the estimation of the long-term slip rate (Upper Pleistocene-Holocene) and the mean recurrence interval of earthquakes along the different segments of the fault.Our investigations within the Tar Lake valley, along the eastern part of the fault – potentially the site of the 1665 (VII, 6.5) historical earthquake -allows us to calculate a preliminary 2 ± 0.1mm/yr minimum left lateral slip rate. If we assume a characteristic coseismic average displacement comprised between 0.35 m (Mw 6.5) and 1.2 m (Mw 7.1) – calculated from Wells & Coppersmith’s functions (1994) and taking the moment magnitudes attributed to the 1665 and 1830 earthquakes (e.g. Berberian & Yeats, 2001) – the mean maximum recurrence intervals along this segment of the Mosha fault are comprised between 160 and 620 yrs