An investigation into the activity of the North Neyshabour fault, eastern Iran



Iran is one the most tectonically active parts of the world and regularly experiences earthquakes of both low and high magnitude. Therefore, earthquake hazard assessment before any kind of building construction and for already built and populated area is essential. A vital first step in this type of study is to identify, map, and determine the activity of faults within a given region. Investigating fault activity requires estimation of the average fault slip-rate, the recurrence interval between earthquakes, and the time of the last earthquake produced by each individual fault. Neyshabour is one of the most important cities in NE Iran. The city has been destroyed four times by major historical earthquakes. Three large faults exist in the region (the North Neyshabour, Binalud and Neyshabour faults). The North Neyshabour and Binalud faults lie at the foot of the Binalud range north of Neyshabour. The North Neyshabour fault has a relatively sinuous surface trace, typical of a thrust fault, and does not show any clear strike-slip component.
The North Neyshabur thrust fault is exposed in a river section, at 36o180N 58o500E. The fault dips 60o north, and forms ~8-9 m high fault scarp at the surface which vertically offsets a Quaternary terrace. Within the river section, a yellow sandstone unit is offset by 9 m. A 60 by 40 cm sample of this unit was collected from the river exposure for optically stimulated luminescence (OSL) dating (location: 36o18.3090N 58o50.2700E). The sample was dated in the Oxford luminescence lab using a Riso (Model TL/OSL-DA-15) automated TL/OSL system under subdued red light (for details of the method see Fattahi et al., 2006, 2007; Fattahi and Walker, 2007). Eighteen subsamples of sample N5 demonstrated a wide paleodose distribution. This suggests that the sediment may not have been completely reset upon deposition (i.e. not all ‘trapped’ electrons from an earlier burial period were reset during sediment transport). This causes the mean age determination using weighted mean, 42000-68000 year, to overestimate the real deposition age. One solution to this problem is to assume the date of the youngest grains represent the time of deposition, giving a lower age of 22200-26000 year. However, we decided to use both age estimates for slip rate determination. As the top Quaternary terrace, has been displaced ~ 8-9 m at the surface, we calculated two slip rate using both average and minimum ages for calculating the slip-rate on the North Neyshabur fault (~0.1–0.2 and ~0.3-0.4mm/yr), respectively.