1985年新疆乌恰Ms7.4级地震断层相关沉积物光释光测年探索研究

Paleo-earthquake dating can provide important evidence for reconstruction of earthquake history and prediction of earthquakes in the future. However, some problems remain to be solved in technology for its application to deposits related with earthquakes, especially in semiarid and arid mountainous regions. Because in these areas the sediments associated with earthquakes are mostly near-source mixed deposits that were rapidly settled with heterogeneous lithology. So the luminescence signal reset zero degree and the dose response sensitivity of the quartz in these sediments is relatively complex. It is difficult to evaluate the age of this kind sediments accurately. To solve these problems, this work makes a study on optically stimulated luminescence dating of the modern sediments related to the 1985 year Wuqia earthquake and it’s paleo-earthquakes. The main research contents include: The light bleaching degree and inadequate nature of the quartz related to mixed deposits; The luminescence sensitivity and dose-response characteristics of the different surface structure quartz; How to determine the residual dose of the different grain size quartz by variety of statistical analysis methods (such as MAM, MAMUN, CAM, IEU et al.,) and evaluate the residual dose effect on the age results. The final aim is to explore effective experimental methods and technical processes and provide a reference for past earthquakes luminescence dating and improve the accuracy and measurement efficiency and estimate the time frame and paleo-earthquakes recurrence period of the 1985 Wuqia Ms7.4 earthquake seismogenic fault.

地震相关沉积物年代的准确测定是重建地震活动历史、预测未来地震趋势的重要依据。地震相关沉积物多数为近源快速混杂堆积，岩性不均一，矿物的释光性质和埋藏前释光信号的回零程度都相对复杂，准确测定其年龄是一挑战。本研究拟选取不同类型1985年乌恰地震断层相关沉积物，包括地震前沉积物、地震崩积楔与推覆楔不同位置堆积、不同构造部位近源与远源地震断塞塘沉积物等，对其石英矿物光释光信号光晒退程度进行检验，对不同粒径石英颗粒释光灵敏度和剂量响应范围进行观测，对比分析不同大小测片粗颗粒、单颗粒光释光测量技术以及各种统计分析方法的异同、优劣，对比不同类型地震相关沉积物残留剂量值，评估其测年下限、分辨率及不确定性。探寻地震相关沉积物释光测年有效的实验方法和技术流程，试途厘定出1985年乌恰Ms7.4地震发震断层的古地震发生年代框架和复发周期，为此地区的防震减灾工作提供重要参考。

地震相关沉积物年代的准确测定是重建地震活动历史、预测未来地震趋势的重要依据。地震相关沉积物多数为近源快速混杂堆积，岩性不均一，矿物的释光性质和埋藏前释光信号的回零程度都相对复杂，准确测定其年龄是一挑战。本项目在实验室单颗粒机载放射源性能参数研究的基础上，选取地震断层相关沉积物建立了粗颗粒石英小测片（SA-SGC）测试流程，并对新疆乌恰地震相关物质进行释光信号残、测试流程及数据分析方法研究，主要获得以下几点认识：.1）.在完成单颗粒释光测量仪机载Beta放射源剂量率的标定、仪器重复性能实验、放射源表面一致性检验和单颗粒石英有效颗粒数分析的基础上，建立了粗颗粒石英SA-SGC测试流程(small aliquot standardized growth curve，简称SA-SGC方法)，可以节省测量时间，提高测量效率；.2）.新疆乌恰地震地表破裂带WQWT6探槽中12个样品不同光释光测量方法的对比研究表明：探槽样品中碎屑石英颗粒具有石英矿物典型的TL峰，其光释光信号以快速组分为主，回授对等效剂量影响可以忽略不计，适合采用SAR法光释光测年；样品中石英颗粒特别是细颗粒在埋藏前存在严重的曝光不彻底现象，能用来光释光测年的单颗粒石英矿物颗粒比例仅为1.5-3.6%，不同颗粒的释光灵敏度差异较大，单颗粒石英测定值的离散度（测量仪器和石英本身释光特性差异所引入的离散度）从4%变化到20%；细颗粒石英大测片、粗颗粒石英大测片的SAR光释光方法都得到高估的年龄结果，不适合此类地震相关的近源快速堆积物的光释光测年；探槽剖面揭露出包括1985年Ms7.4级地震在内的4次强震事件，根据初步的单颗粒石英释光测年结果，前三次事件发生时间分别为事件E1发生在5.2 ka之前，事件E2发生在3.8 ka和4.2 ka之间，事件E3发生在2.8 ka之前。前三次古地震的复发周期约为1.0 ka左右。古地震事件E3之后发生了强烈剥蚀作用，致使该探槽可能丢失了2.8 ka至1985年期间的古地震记录。.3）.地震相关沉积物释光测年时，首先要判断样品沉积环境，初步确定矿物在埋藏前的曝光程度，然后选取合适释光测年技术来测量等效剂量和环境剂量率及数据分析模型，具体流程见报告中图5和图6。