Makran is an active subduction zone with a lower activity rate in comparison with other similar subduction zones. In the extent of Makran seismotectonic province, from coastline up to 70 Km distance onshore, the earthquakes are shallow whilst further inland, focal depths of the earthquakes are deeper. Considering the low seismic activity rate, lack of adequate and reliable seismic data and uncertainty about earthquake hazard contribution ratio of subduction segments and faults, it is intended in this article to determine the most accurate method of seismic source modelling. Estimation of the maximum strong ground motion in this province has been assessed by assuming various combinations of activity rate contribution ratios, i.e. independent activity of either subduction source or faults or simultaneous activity of subduction source and faults. According to the obtained results, the maximum and minimum limits of the strong ground motion acceleration is related to the independent seismic activity models and when subduction source and faults are assumed to be contributing simultaneously to the activity rate of the region, the resultant ground motion acceleration will be between the limits obtained based on independent activity models.
The Astara Fault System (AFS) is located in the northwest Alborz, east of Talesh Mountain (TM) and west of the South Caspian Basin (SCB). The AFS is one of the basement rock faults in Iran that is heavily involved in seismotectonic activity of the Talesh region, and to which subsidence of the SCB is attributed. There is little information available concerning previous AFS seismic activities and its properties. In order to elucidate the seismic behavior and activities of the AFS, we conducted a research study on paleoseismology of the fault. Based on paleoseismic evidence, two scenarios could be taken into consideration, one of which has three and another has four seismic events with magnitudes Mw in the range of 6.7 to 7.2. Evidence of these seismic events is within sedimentary succession as they have occurred during the past 3 ka (this age is determined based on the deposition rate of the region). Six carbon samples were taken for C14 age determination tests, the results of which clearly demonstrated that the EvIV (scenario A) and EvIII (scenario B) had occurred before 27,444 cal BP, while other events occurred in the time period between 27,444 cal BP and 3 ka ago. If we consider the occurrence of three or four seismic events (based on the two scenarios) to be between 27,444 cal BP and 3 ka ago, the average recurrence interval is 7,119 ± 1,017, but evidence for these events has been removed. If we assume EvI to be the youngest event (in both scenarios), the minimum elapsed time is therefore 3 ka.
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