نوع مقاله : مقاله تحقیقی (پژوهشی)
1 مؤسسه ژئوفیزیک دانشگاه تهران، ایران
2 پژوهشکده سازمان زمینشناسی و اکتشافات معدنی کشور، تهران، ایران
عنوان مقاله [English]
Having a left lateral strike slip, the Doruneh fault is located in the northeast of Iran. It is thesecond longest fault system (~700 km), after the Main Zagros Recent Fault system, in Iran. This fault is the response of the convergence between the Arabian and Eurasian plates in the regional tectonic map. Despite high potential seismicity, only two historical earthquakes with magnitudes greater than 7 more than, and no instrumental earthquakes with greater than 6, have been recorded for this fault. Therefore, 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, and 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. Optically Stimulated Luminescence (OSL) is one of the best methods for determining sediment age in arid and semi-arid zones such as Iran.
Displacements of quaternary geological evidence (e.g., river beds) by the Doruneh fault also suggest this fault is active. In this study, the Shesh-Taraz area was selected because it is near populated areas, including Kashmar, Bardaskan, Khalilabd, Ali mardankhan, and Chenar. Fattahi et al. (2006) collected Optically Stimulated Luminescence (OSL) samples from the Shesh-Taraz and employed the weighted mean Infrared Stimulated Luminescence (IRSL) from the feldspar for slip rate determination. This investigation used similar samples but calculated the ages of both quartz and feldspar fragments using a different OSL method.
The samples were processed under subdued red light. A portion of the sample was taken by dry and wet sieving. The quartz and feldspar fragments were purified and separated by chemical treatment. The separated quartz and feldspar were mounted as monolayers on the 10 mm diameter Al discs for the single aliquot analysis using a silicon spray as an adhesive. The Equivalent Doses (De) of quartz and feldspar of each sample were calculated using the Minimum Age and Histogram Methods for which the Analyst program was used. By dividing the calculated Equivalent Dose by the Total Dose Rate, the ages of these samples were calculated.
The estimated ages for T1, T1a and T2, from the quartz samples using the Minimum Age Method were 3.49±0.75, 7.67±0.95, and 4.57±1.34 ka, respectively, and employing the Histogram Method were 3.88±3.15, 10.99±10.5, and 7.99±4.45 ka, respectively. The estimated ages for the feldspar samples with the Minimum Age Method were 6.23±0.48, 7.40±2.23, and 4.96±0.25 ka, respectively, and with the Histogram Method were 7.61±1.22, 9.40±1.79, and 8.07±2.43 thousand years, respectively.
The displacement of geomorphologic evidence was measured by satellite images (Landsat and Quick Bird Image). Because of high accuracy of the field-measured displacement, in comparison to the satellite image displacement measurements, we calculated the slip rate by dividing the values calculated using the former method of displacement by OSL ages into the newly estimated ages.
The estimated slip rates for T1, T1a and T2, using quartz samples with the Minimum Age Method were 6.30±1.39, 3.26±0.43, and 1.75 ±0.56 mm.yr-1, respectively, and with the Histogram Method were 5.67 ±4.61, 2.28±2.18, and 1±0.57mm.yr-1, respectively. The estimated slip rates for T1, T1a and T2, using the feldspar samples with the Minimum Age Method were 3.53±0.31, 3.38±1.03, and 1.61±0.22 mm.yr-1, respectively, and with the Histogram Method were 2.89 ±0.48, 2.66 ±0.52, and 0.99 ±0.32 mm.yr-1, respectively.
Comparative studies of the luminescence signals of quartz and feldspar showed that the results of feldspar are more reliable. The range of the slip rates calculated in this study includes the range of slip rates determined by previous studies. Therefore, additional studies in this region are required.