Optical Dating Research Of Rapidly Deposited Sediments Related To Earthquakes

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.