Using IRSL to determine the slip rate of the Doruneh fault in Khalilabad

Introduction   The left lateral Doruneh Fault System (DFS), is one of the longest, and most prominent, strike-slip faults in Iran, extending from the eastern border of Iran to the central Dasht-e-Kavir with a curved geometry. DFS is the response of the convergence between the Arabian and Eurasian plates in the regional tectonic map. Regarding to scaling relationships this fault with length of >600 km has the potential to produce earthquakes with magnitudes higher than 7.5. M. It performs an important role in the regional tectonics and is certified to be active by its well-preserved geomorphological features all along its trace. However, despite clear Quaternary activity expressed in the geomorphology, has no record of historical and instrumental earthquakes of magnitudes M ≥ 6, which is in contrast to the neighbouring Dasht-e Bayaz region, which has suffered from many earthquakes recorded both instrumentally and historically. Determining whether the Doruneh fault is capable of generating large-magnitude earthquakes similar to the Dasht-e Bayaz is important due to its risk to close cities and towns. In other word, it is necessary to determine the hazard of this fault in the area. One way to conduct the Earthquake Hazard Assessment is to calculate the slip rate. The slip rate of a seismogenic fault is a crucial parameter for establishing the contribution of the fault to the seismic hazard. one method for calculating the slip rate is measuring the displacement by the fault and dividing it by the period during which this displacement has occurred. Clear scarps in alluvial fans and river terraces can be observed both in satelite imagery and in the field along the entire Doruneh fault length. The geomorphology of the fault contains numerous indications of cumulative left-lateral slip over various scales. By dating and finding the age of these offset geologic features, we can determine the average slip rate. We describe one site where Late Quaternary landforms are displaced by the fault. Two generations of alluvial fan are present on the north of Khalilabad village. There are two parallel east-west fault traces on the fan surfaces at this site, with the two traces separated from each other.     Methods To determine the slip rate two parameters are required: the displacement and the period that has taken that the displacement occur. Optically Stimulated Luminescence (OSL) is one of the best methods for determining sediment age in arid and semi-arid zones such as Iran. OSL enable evaluation of the time that has elapsed since quartz and feldspar, were last exposed to daylight. During exposure to light the luminescence signal within the grains is optically bleached until the signal is zeroed. Once the grains are buried and sealed from daylight, the luminescence signal being induced by naturally occurring radiation and being accumulates again. The time passed since the last daylight exposure is calculated by dividing the dose accumulated in the sample since its last exposure to light by the dose rate. The study site in this research is the eastern alluvial fan near Khalilabad, which is located in the west of Kashmar. Google earth was employed to observe and measure the apparent offset of a riser between eastern and the western fan surface, which appears to be relatively straight both upstream and downstream of the fault. The estimate is that Doruneh fault has displaced the eastern alluvial fan by about 40-60 meters. Two sediment samples were collected from this alluvial fan for luminescence dating. Potassium feldspar was then separated from them. To determine the age of potassium feldspar, two parameters of equivalent dose and annual dose are required. SAR method was used to measure the equivalent dose and analyst program was used to analyze the data. The dose rate was calculated using present-day moisture content, radioisotope concentrations and burial depths.   Results The last sedimentation period was determined to be 11000 – 16000 years ago. The slip rate of this part of the fault was estimated to be 2.5-5.5 mm per year by dividing the displacement rate over the displacement period.