بررسی پتانسیل لومینسانس تحریک شده با اشعه مادون قرمز جهت سن‌یابی سنگ‌های واریزه‌ای زمین‌لغزش فتلک

نوع مقاله : پژوهشی

نویسندگان

1 دانشکده ژیوفیزیک دانشگاه تهران-تهران -ایران

2 هیئت علمی موسسه ژئوفیزیک

3 دانشکده فیزیک، دانشگاه فنی دانمارک، آزمایشگاه ریزو، رزکیلده، دانمارک

4 مدیر گروه زمین‌شناسی مهندسی (پژوهشکده مهندسی ژئوتکنیک) و مدیر امور پژوهشی/ پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله

5 پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله ژئوتکنیک، تهران ، ایران

6 موسسه تحقیقات حفاظت خاک و آبخیزداری، آموزش و ترویج کشاورزی، تهران، ایران

7 دانشکده علوم زمین، دانشگاه آرهوس، آزمایشگاه سن‌یابی لومینسانس نوردیک، رزکیلده، دانمارک

چکیده

طی دهه‌ی گذشته روش نوینی جهت سن‌یابی سطوح سنگی با استفاده از سیگنال لومینسانس تحریک شده با اشعه مادون قرمز ارائه شد. این روش بر پایه‌ی نمودار لومینسانس-عمق است که در آن با افزایش مدت زمانی که سنگ در معرض نور خورشید قرار می‌گیرد عمق نفوذ نور به داخل سنگ افزایش و سیگنال لومینسانس موجود در سنگ کاهش می‌یابد. با استفاده از این رویکرد، سایت‌های مختلفی از جمله در ایتالیا و دانمارک با موفقیت سن‌یابی شده‌اند. لذا در این پژوهش امکان سن‌یابی نمونه‌های سنگی واریزه‌ای ناشی از زمین‌لغزش فتلک به کمک این روش مورد بررسی قرار گرفت. اما متاسفانه نتایج حاصل از نمونه‌ی فتلک منطبق با مدل ارائه شده نبود و تعیین سن انجام نشد. از اینرو نمونه‌های سنگی از سایتی در اسپانیا برداشت شد تا با همان روش‌های آزمایشگاهی که نمونه‌ی فتلک مطالعه شد مورد سن‌یابی قرار گیرند که نتایج با مدل همخوانی داشت. اطلاعات حاصل از این پژوهش نشان داد که مراحل نمونه‌برداری و انجام آزمایشات به طور صحیح انجام شده است و روش به درستی عمل می‌کند اما مشکل اصلی در ذات و حساسیت نمونه‌های برداشت شده از منطقه فتلک جهت تولید لومینسانس است.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Investigating the Potential of Infrared Stimulated Luminescence for Dating the Debris rocks of Fatalak Landslide

نویسندگان [English]

  • Nasrin Karimi Moayed 1
  • Morteza Fattahi 2
  • Reza Sohbati 3
  • Ebrahim Haghshenas 4
  • Vahid Tajik 5
  • Zieaoddin Shoaei 6
  • Andrew Murray 7
1 Tehran university-geophysics institute-Tehran Iran
2 Institute of Geophysics, University of Tehran
3 3Department of Physics, Technical University of Denmark, Risø Campus, DK 4000 Roskilde, Denmark
4 International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran
5 Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran
6 Soil Conservation and Watershed Management Research Institute, Tehran, Iran
7 Department of Geoscience, Aarhus University, Risø Campus, Roskilde, Denmark
چکیده [English]

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.

کلیدواژه‌ها [English]

  • Rock surface dating
  • IRSL
  • Luminescence-depth profile
  • Rudbar-Manjl earthquake
  • Fatalak landslide