Paleoseismological Analysis Along the Astara Fault System (Talesh Mountain, North Iran)

Abstract

AbstractThe 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.