Determining the slip rate on the Gowk fault using POST-IR method

 






^*نگارنده رابط:             تلفن: 61118253-021         دورنگار: 88630479-021                              mfattahi@ut.ac.ir:E-mail

 





Iran is one of the most tectonically active regions on the Alpian-Himalian earthquake belt (Figure 1). The Gowk fault located in Kerman Province, eastern Iran, is a part of Sabzevaran-Gowk-Nayband system of strike-slip faults that accommodate north-south right-lateral shear along the western margin of Dasht-e-Lut. Its length is more than 150 km. collectively; the northern part of the fault has been ruptured by five destructive earthquakes between 1981 and 1998. No activity has been seen in the southern segment of the fault. So the southern segment which is our site of study will remain a potential for further earthquakes.
Estimating the slip rate of a single fault is one of the most important parameters to assess the hazard of that fault. In order to determine the slip rate, two parameters, displacement and the duration of the displacement, are needed. In the south Golbaf basin the fault is composed of three main strike slip segments arranged in a right-stepping pattern (Figure 3). Field investigations illustrated around 30 m right-lateral displacement on the fault. We used two main approaches for dating, radiocarbon and optically stimulated luminescence. Two ¹⁴C samples and three OSL samples were collected at a 3 m-high exposure of the lakebed on the eastern side of the fault at 29:47:30 N  57:46:28 E (Figure 4, Figure 5). The first quantitative estimate of the Holocene slip rate on the Gowk fault was provided using ¹⁴C dating result of the two wood fragments which were taken with OSL samples (Walker et al., 2010). 
In this study, we have tried to determine the slip rate of Gowk fault by means of luminescence dating. Luminescence dating is a chronological method that has been used extensively in the earth science. In this method, the event being dated is the last exposure of the sample to daylight. Hence the determined age is the time of the sedimentation which has covered the older sediments. As fault has displaced the rivers in the Golbaf Lake, and the rivers have cut the existing lake bed sediments, therefore last activity of the Gowk fault has occurred after the last sedimentation in the lake. If we date the age of last sedimentation of the Golbaf Lake, we will be able to calculate the fault slip rate, using the relevant age and displacement.
Luminescence dating is based on the emission of light (natural luminescence signal) by commonly occurring minerals, principally quartz and feldspar. These minerals act as a dosimeter in nature, recording the amount of radiation to which they have been exposed according to the decay of radioactive isotopes such as uranium (U), thorium (Th) and potassium (K).
To date a sample using one of the luminescence dating methods, two parameters, the equivalent dose and dose rate, are needed. Single aliquot regeneration (SAR) protocol was used to determine the equivalent dose (De). Ideally, after chemical preparation, we have a sample of just quartz grains. However, this is not always the case. We sometimes face feldspar contamination which means all the feldspar grains have not been removed. Underestimating the age would be a consequence since we are dealing with anomalous fading. This means that the size of the observed luminescence signal decreases as the sample is stored in nature or the laboratory. To identify the purity of quartz in the aliquot, we usually introduce a simple Post IR measurement in the end of SAR experiments. This is a problem if the infrared signal (emitted from feldspar) is more than 10% of the blue signal (emitted from quartz). To sort out this problem we reject the result of that aliquot. However, if the majority of aliquots show this problem, no reliable De can be calculated. For quartz samples that demonstrate such problem after sufficient time of HF etching, the alternative way would be to use POST-IR method. As Golbaf samples suffered from this problem we applied POST-IR method to find the De for these samples. However, some factors, such as the ability of SAR to correct the sensitivity change and recovering the given lab dose were checked in order to insure us that the age results achieved by SAR protocol are trustworthy. Equivalent doses were calculated by analyzing the data with Analyst software. The results are shown in table1. By considering the equivalent dose calculated from the Histogram method and using the following formula, the age of the collected samples was determined:
Age (ka) = equivalent dose (Gy) / dose rate (Gy/ka)                                                   (1)
The results of the dose rates and ages for the three samples (GB1, GB2 and GB3) are shown in table2.
By considering the ages calculated for the three samples and their depths, and extrapolation diagram, we could find the age to be 2800-5400 yrs at the surface (diagram1). So assuming the time of faulting to be close to the age of the lake surface, the slip rate of the Gowk fault would be 5.5-10.7 mm/yr. It should be mentioned that this age (2800-5400 yrs) is less than what can be predicted from 14C and there is a possibility of the effect of fading, and as a result the slip rate is more than the estimated slip rate by Walker et al (2010). We suggest dating these samples using potassium feldspar grains, to enable us for comparison between dating result.
Because of the complexity of the fault zone in depth, estimating the average return period for the Gowk fault is difficult. However, by assuming a 3-meter slip in every earthquake according to the 1998 Fandogha earthquake, and considering the calculated slip rate, the maximum return period will be 280-540 years. According to this short return period and the fact that the southern part of the fault has not recently generated a destructive earthquake, it will remain a potential for making a destructive earthquake in the region.