【作者】 杨会丽；

【导师】 陈杰；

【作者基本信息】 中国地震局地质研究所 ， 构造地质学， 2013， 博士

【摘要】 古地震的年代确定是重建强震活动历史、预测未来地震趋势的重要依据，对于地震学研究具有重要的理论价值和实践意义。目前用于测定古地震事件年龄的方法和技术多达十余种，释光方法是其中常用的一种（Noller et al.,2000）。释光测年方法经过二十多年的发展，已形成了多种适用于几十年至十万年的沉积物年龄测定技术，通常可获得与独立年龄相吻合的结果（Murry and Olley,2002）。然而，这些技术在地震相关堆积物中的应用，尤其在干旱-半干旱山区与古地震相关的快速混杂堆积物年龄测定的应用，仍然存在一些急需解决的问题。在干旱-半干旱山区地震伴生的沉积物多数为近源快速混杂堆积，岩性不均一，在堆积过程中石英、长石等矿物颗粒的释光信号的回零程度和对剂量响应的灵敏度都相对复杂。此外，这类快速混合堆积物的环境剂量率也比较难于准确测定。针对上述问题，我们选择汶川地震和新疆帕米尔北缘乌恰地震破裂带，开展了现代和古地震混杂堆积物的光释光年龄测定的研究，主要内容包括：（1）地震混杂堆积物中石英、长石颗粒光晒退程度，即光释光信号回零程度及其不均匀性和不充分性的观测；（2）混杂堆积物中单颗粒石英等效剂量（De）和样品等效剂量（Db）测定；（3）应用石英颗粒小测片SAR（单测片再生剂量）技术和狭义“SGC”（狭义的“综合生长曲线，Standardised Growth Curve”，文中被称作小测片SGC技术）技术测定混杂堆积物的光释光等效剂量；（4）进行混杂堆积物石英颗粒光释光等效剂量（De）的各种测定技术（单颗粒技术、小测片技术、常规大测片SAR和SMAR（简单多测片剂量再生技术））可行性和可靠性的对比研究；（5）比较不同技术（如高纯锗Gamma谱仪、便携式Gamma谱仪、就地埋藏剂量片、厚源Apha计数仪、以及ICP-MS放射性核素含量测定等）测定混杂堆积物环境剂量率的可行性和可靠性。取得了以下初步研究成果：1)现代地震相关各种沉积物释光信号回零程度的检验对2008年汶川地震相关的堰塞湖堆积物、地震前古地面沉积物样品中不同矿物、同一矿物不同粒径颗粒、不同测量方法所获得残留等效剂量值的对比研究结果表明：a)细颗粒（4-11μm）石英测片光释光信号足够强能满足释光测年要求，而粗颗粒（90-250μm）石英仅有¹.3%颗粒数发光信号达到释光测年要求，暗的粗颗粒石英导致测定的De的不确定性较大；b)震前古地面沉积物样品无论是碎屑石英还是碎屑长石，其释光信号的残留等效剂量（De）仅为0.2Gy，对于百年或者千年尺度以上的地震相关沉积物测年的影响几乎可以忽略不计，此类样品适合于古地震释光测年研究；c)现代地震相关的堰塞湖沉积物样品释光信号残留值与所测碎屑矿物种类、颗粒大小、所用的测量方法有关。细颗粒长石残留De值高达44¹03Gy，细颗粒石英残留De值为1.8¹1.2Gy，说明长石的光释光信号比石英的更难被晒退。单颗粒石英残留De值0.1⁰.8Gy，低于细颗粒石英。因此，当进行百年或者千年尺度的古地震释光测年时，一定要慎重选择测量方法和流程，尽量通过单颗粒光释光De测量，分辩出埋藏前光晒退比较彻底的颗粒组分。2)单颗粒仪器实验参数检验与小测片标准生长曲线（SGC）方法的建立在对本实验室Ris-2释光单颗粒测量仪机载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，应用石英颗粒大测片SAR技术建立各个样品的标准生长曲线（SGC），再用石英颗粒小测片SAR技术获得De值（小测片SGC方法）。结果表明：a)本实验室Risoe-2释光测量仪机载beta放射源粗颗粒石英辐照剂量率是细颗粒石英的92%，粗颗粒石英大测片的辐照剂量率比小测片的低10%以上，单颗粒石英辐照剂量率为0.1104Gy/s。使用释光测量仪之前，需要对每台仪器不同粒径，以及大、小测片的辐照剂量率进行准确的标定。b)本实验室Ris-2单颗粒测量仪重复测量的不确定性约为9%，由机载Beta放射源表面不完全均匀导致等效剂量的不确定性为11.2%，在进行单颗粒数据处理前需要对放射源的不均一性进行校正。c)对埋藏前光晒退不均匀样品进行单颗粒测量时，至少测量40个有效颗粒进行统计分析，所计算的样品De值才具有代表性。d)对于各个样品采用完全光晒退的粗颗粒石英大测片，应用SAR技术建立标准生长曲线（SGC），根据1-2个测片单颗粒测量确定粗颗粒石英小测片的大小，尽量每个小测片的释光信号仅来自1-2个发光石英颗粒，结合小测片SAR技术获得多个“单颗粒”的De值，可以大大节省测量时间，提高测量工作效率。3)沉积物不同环境剂量率测量方法的对比对新购置和安装的高纯锗Gamma谱仪、便携式NaI Gamma谱仪和AL₂O₃:C剂量片进行了标定，对比同一个样品不同方法的环境剂量率的测量结果，获得以下初步认识：a)本实验室ORTEC GEM70P4-95P型高纯锗Gamma谱仪的本底极低，当测量时间达到8个小时后，核素含量不再变化，即可满足测量要求。选取的片麻岩、片岩、花岗岩及第四纪沉积物共12个样品，采用高纯锗Gamma谱仪测定的U、Th和K含量与澳大利亚ICP-MS方法测定结果对比显示两种方法测定的U和Th含量在10%的误差范围内一致，K含量在6%的误差内一致，说明本实验室高纯锗Gamma谱仪标定结果是可靠的。对2个封盒的第四纪松散沉积物样品放置不同时间后测量，结果在1σ误差范围内一致，说明仪器具有较好的稳定性和可重复性。b)高纯锗Gamma谱仪、便携式Gamma谱仪、就地埋藏剂量片、厚源Apha计数、以及ICP-MS放射性核素含量测定等多种方法对比测量结果显示，对于U/Th放射性核素衰变处于平衡体系的、周围30cm内岩性相对均一的样品，建议应用厚源Apha技术技术和K含量测定。对于样品周围30cm内岩性比较混杂的样品，优先选择高纯锗Gamma谱仪与AL₂O₃剂量片或者便携式NaI（TI）Gamma谱仪相结合的测量方法。高纯锗gamma谱仪可以检测长周期放射性核素衰变链的放射性平衡性，测量用的样品量较多更具有代表性，可同时测量多种核素、测量效率高。AL₂O₃剂量片或者便携式NaI（TI）Gamma谱仪野外就地测量可以克服周围30cm岩性不均一导致γ贡献的不确定性。4)新疆1985年乌恰地震地表破裂带古地震发生时间：通过对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：a)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；b)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%；c)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；d)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5.2ka之前，事件E2发生在3.8ka和4.2ka之间，事件E3发生在2.8ka之前。前三次古地震的复发周期约为1.0ka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2.8ka至1985年期间的古地震记录。5)对地震相关沉积物释光测年方法的建议通过上述研究，对于应用释光测年技术进行地震相关沉积物测年提出如图A和图B的流程建议。如果样品为快速混杂堆积，周围30cm内岩性不够均一，应该采用室内高精度的高纯锗Gamma谱仪和便携式gamma谱仪就地测量相结合的方法来测量环境剂量率。更多还原

【Abstract】 Paleoearthquake dating can provide important evidence for reconstruction of earthquakehistory and prediction of earthquakes in the future. At present, there are more than ten datingmethods and techniques to determine the ages of paleoearthquake events, among which theluminescence method is one of the most commonly used dating methods.With development in the last two decades or more, luminescence dating has become atechnological system including a variety of methods suitable for dating sediments of ages fromseveral tens to100thousands years, usually yielding age results consistent with the independentage. However, some problems remain to be solved in technology for its application to depositsrelated with earthquakes, especially in semiarid and arid mountainous regions. Because in theseareas the sediments associated with earthquakes are mostly near-source mixed deposits thatwere rapidly settled with heterogeneous lithologiy. So the luminescence signal zero degree andthe dose response sensitivity of the quartz and feldspar in these sediments are relativelycomplex. In addition, the environmental dose rate of the rapid mixed deposition is difficult toaccurately determine. To solve these problems, this work makes a study on optically stimulatedluminescence dating of sediments related to the2008year Wenchuan earthquake and Wuqiaearthquake rupture on the northern margin of the Pamirs in Xinjiang. The main contents include:（1） The light bleaching degree and inadequate nature of the quartz and feldspar related to mixeddeposits;（2） How to determine the equivalent dose of the quartz single grain and the bureddose of the samples,（3） application of SAR （single-aliquot regenerative-dose） of quartztechnology and narrow sense "SGC"（in the narrow sense " Standardised Growth Curve"）technique to determine the equivalent dose of the mixed sediment;（4） Whether comparativestudy of De determination in single grain OSL technology, small aliquot and conventional largealuquot SAR and SMAR methods are feasible and reliable for rapidly and mixed sediment;（5）Comparison of the feasibility and reliability of the variety methods （such as ORTECGEM70P4-95P type Ge gamma ray spectrometer system, NaI gamma system, thick sourceApha counter, AL₂O₃:C chips buried, ICP-MS determination the radionuclides content and soon） to determine the environmental dose rate of the mixed sediment. The primary results are summarized below.1) Luminescence signal reset to zero degree for sediments related modern earthquakes:This work makes a contrastive study of different minerals, different grain sizes of the samemineral and measured protocols for the sediments related with the2008Wenchuan great event,and comes to the following conclusions:a) Fine quartz （4-11μm） grains have enough luminescence signals and suitable forluminescence dating. But only¹.3%of the coarse quartz（90-250μm） grains have enoughluminescence sensitivity, while dim grains can lead to a greater uncertainty of single De.b) The earth’s surface deposits before the earthquake has minimum equivalent dose（0.2Gy）both for fine grain quartz and fine grain feldspar, bleached relatively fully before buried, so theearth’s surface deposits can be used to constrain the time of seismic events.c) The residual dose of remaining samples is found to vary with mineralogy, grain size,and the methods used for the De determination, suggesting that they are both poorly andheterogeneously bleached. The residual dose of the fine feldspar is as high as44¹03Gy, butthe fine quartz is about1.8¹1.2Gy. It shows that the fine feldspar is more difficult to bebleached than fine quartz. The results of the single grain quartz are about0.1-11.2Gy, smallerthan fine grain quartz. Using the single grain quartz luminescence dating method can separatecompletely bleached quartz from unbleached grains. It is recommended to take caution wheninterpreting ages derived from optical dating for paleoearthquakes that are hundreds tothousands of years old.2) Parameter test of single grain apparatus and establishment of small aliquot SGCa) The beta radioactive source of coarse quartz irradiation dose rate is92%of the finequartz, and coarse-grained large aliquot quartz irradiation dose rate is10%lower than the smallaliquot quartz. Before using luminescence measuring instrument, it needs to calibrate theirradiation dose rate of the different aliquote size and grain size.b) The uncertainty of the Risoe-2single grain instrument reproducibility is about9%.c) The⁹⁰Sr/⁹⁰Y beta sources used to irradiate all100grains simultaneously on a sampledisc do not always produce uniform irradiation to all grains. The uncertainty of the De is11.2%due to the nonuniform source surface. The irradiation source dose should be corrected beforecalculating the equivalent dose. d) When using the single grain measured the equivalent dose of the uneven light bleachingsamples, it is only representative at least40Des.e) The method can use large quartz aliquot to establish standard growth curve （SGC）, andsmall quartz aliquot to measure the De can save time and improve work efficiency.3) Measure methods of the environmental dose rate:By calibrating the Ge gamma ray spectrometer system, NaI gamma system and AL₂O₃:Cchips dose rate, and comparing different methods of the same samples, this work attains someconclusions:a) The ORTEC GEM70P4-95P type Ge gamma ray spectrometer system takes on the lowbackground, and when measuring the time reaches8hours, radionuclide content does not change,which can meet the measurement requirement. The sample measure results of the calibrated Gegamma ray spectrometer system show that the calibration is exact and this system has goodstability and reduplication. The Ge gamma ray spectrometer system can measure more nuclidesat the same time. It can save measure time and test whether radioactive decay is in the balancesystem.b) AL₂O₃:C chips that the lab bought have enough sensitivity, but dose responsecharacteristics are different. The AL₂O₃:C dose rate of the Risoe-2instrument is11.19±1.37μGy/s.c) Comparing results of the different environment dose rate measured methods indicatesthat they are consistent and suitable for the relatively samples with uniform lithology, and thiswork suggests to use the thick source Apha technology and K content measure method. For themixed sediments, it is better to choose the Ge gamma ray spectrometer system and AL₂O₃:C orNaI（TI）gamma system to avoid lithologic heterogeneity that can lead to around30cm γcontribution to the uncertainty.4) The ages of the paleoearthquakes on the1985Wiqia earthquake surface rupture zoneBy researching the WQWT6trench samples related to the1985Wuqia earthquake, thiswork attains such conclusion:a) The quartz of the trench samples have typical TL peak, the fast component of OSLsignals and ignored recuperation. So they are suited to use SAR or SMAR protocol.b) The quartz of the samples are not completely bleached. The residual doses are very high. The proportion of grains in a sample having a detectable OSL signal varies from one sample tothe other. Only1.5-3.6%of the single quartz grains from the trench samples can be used forOSL dating. Within a single sample there may be a large variation in the brightness of the OSLsignal. The uncertainty in the measurement of De is from4%to20%termed ‘intrinsic’. Theyare related to experimental and instrumental causes of scatters in the De.c) The large aliquots of the fine grain quartz and coarse-grained quartz OSL ages areoverestimated. This kind large aliquots is not suitable for the rapidly accumulated deposisrelated with earthquakes.d) This work obtains ten good single grain ages to analyze occurrence times of the events.The four events exposed in the trench might have happened at～5.2ka,3.8～4.2ka,～2.8ka,1985year, respectively. The paleoearthquake recurrence interval is about1.0ka.5) Some suggestions for the luminescence dating method to earthquake related sediments:Through the above research, this thesis proposes some suggestions for the luminescencedating procedures to the earthquake related sediments as shown in figure A and figure B. If thesample is from rapidly settled and mixed deposits, the lithology of the sample in a30cm rangeis not uniform, it is better to use the Ge gamma ray spectrometer system or NaI（TI）gammasystem to measure environmental dose rates.更多还原