Document Type : Research
Tehran university-geophysics institute-Tehran Iran
Institute of Geophysics, University of Tehran
3Department of Physics, Technical University of Denmark, Risø Campus, DK 4000 Roskilde, Denmark
International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran
Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran
Soil Conservation and Watershed Management Research Institute, Tehran, Iran
Department of Geoscience, Aarhus University, Risø Campus, Roskilde, Denmark
Over the last decade, extensive studies have been done to date rock surfaces using optical luminescence signals, and recently a model has been proposed that shows the rock surfaces using infrared-stimulated luminescence signal have been successfully dated. This method is based on the resetting of luminescence signal with depth into rock surfaces. When a rock surface is first exposed to sunlight, the luminescence signal that has been stored over time in its constituent minerals (particularly quartz and feldspar) starts to decrease. The longer the rock is exposed to sunlight, the depth of light penetration into the rock also increases and the luminescence signal in the rock decreases, however, the rate of luminescence resetting reduces with depth because of the attenuation of daylight into the rock surface. This differential change in bleaching rate with depth leads to the development of a sigmoidal shape luminescence-depth profile. Such profile provides an internal check on an inadequate daylight exposure, and therefore an incomplete resetting of the luminescence signal and allow us to identify the sample that are most likely to provide reliable OSL age. In this study, we investigated the potential of this method to date debris rocks of Fatalak landslide which were induced by Rudbar-Manjil earthquake in north of Iran in 1990. Cores of ~10 cm long and 1 cm diameter were extracted from the buried and exposed sides of the rock samples using a water-cooled, diamond-tipped drill. The cores were then cut into ~1.5 mm thick slices. The slices were gently broken into small chips and mounted in 10-mm diameter stainless steel cups for natural luminescence signal and dose response measurements. All sub-samples from each slice were stimulated by infrared radiation and the blue and ultraviolet luminescence signals were measured. To determine whether the luminescence signals at the buried surface of the rock were sufficiently bleached before the earthquake event, we measured the natural sensitivity-corrected IR50 and pIRIR225 signals (Ln/Tn) with depth into the core and the luminescence-depth profiles were plotted. Unexpectedly, weak or no IR50 and pIRIR225 signals and no suitable luminescence-depth profiles were observed. According to the experience of the second author, almost all sediment samples taken from Iran have generated IRSL signal, so it is necessary to investigate the cause of the lack of a suitable IRSL signal for rock samples in Fatalak. Due to the fact that with increasing depth, the bleaching rate decreases and the luminescence signal intensity increases and also the luminescence signal is generated by a small percentage (approximately 10%) of the grains of the dosimeter grains (mainly quartz and feldspar), it is possible to produce signals (response to the same dose) with different intensities and properties for different slices. Therefore, the potential of all slices to produce the signal and finally to prepare the luminescence-depth profile were investigated. Unfortunately, this profile did not match the profiles provided by previous studies.
In order to analyze whether this observation is due to the nature of the samples taken from Iran or there was a defect in the luminescence signal measuring device or in the experiment process, we performed similar tests for a rock surface which was taken from another site. The same process was then carried out for two rock art paintings from Spain, which showed acceptable signals and the IR50 depth profile showed a sigmoidal shape where the luminescence signal is almost reset at the surface slice but increases with depth until it reaches saturation, as expected from the model. Then, the luminescence-depth profiles from Fatalak and Spain sites were compared with two previous successful studies in Italy and Denmark. The IRSL luminescence-depth profile for rock art sample in Spain was in a good agreement with that of the two burial samples from Italy and Denmark. However, no such correlation was observed between the profiles of the Fatalak sample and the profiles of the two Italian and Danish samples. As the profiles derived for Fatalak sample were not consistent with the model and none of the previous studies, we could not determine the time of the landslide event in the conventional method.