Résumé
The North Tehran Fault is located at the southernmost piedmont of Central Alborz. It stands out as a major active fault menacing directly the city of Tehran, a 12 millions people metropolis, and would have been the source of several major historical earthquakes in the past. The fault zone extends up to 110 km and corresponds mainly to a reverse fault crossing the northern suburbs of the Tehran metropolis. We carried out a morphological and paleoseismological study of the fault zone in order to determine whether the fault was activated during the Holocene and to define the characteristics of its activity in terms of kinematics, magnitude and recurrence intervals. We found a approximately 3 m fault scarp affecting Upper-Pleistocene and Holocene deposits in the central part of the fault. This scarp is the result of repeated events along a N115 degrees E trending 30 degrees N dipping fault plane with mainly reverse movement and only a small left-lateral component. Our paleoseismological investigations, based on observations from two trenches dug across this fault scarp reveal evidence for a maximum of 8 surface-rupturing events within the past approximately 30000 yr (based on optically stimulated luminescence ages). The two most recent events produced 2.35 m of total reverse displacement during the past approximately 8000 yrs (0.77 m for the latest event and 1.58 m for the penultimate). According to the empirical relationships among average displacement per event and Moment magnitude (Wells and Coppersmith, 1994), the 8 events have magnitudes Mw > 6.5 and the mean return period is comprised between 3200 and 4100 yrs. The average slip rate along the North Tehran fault is 0.3+ or - 0.05 mm/yr.