【作者】 付俊东；

【导师】 任金卫；

【作者基本信息】 中国地震局地震预测研究所 ， 构造地质学， 2012， 硕士

【摘要】 古地震是指保存在地质记录中的史前和历史上没有明确记录的地震（Wallace R E,1981），对于地质学家而言，地质记录永远是第一位的（林景星，2008）。古地震研究可以延长和补充短暂仪器记录，以及不长的历史地震记录，有助于在更长的时间范围内研究强震重复规律，建立强震重复模型，评价未来地震危险性，尤其对地震重复周期长的大陆内部地区有更重要意义（邓起东，2008），也是活动断裂研究的重要组成部分，对于活动断裂分段、活动强度对比、动力学研究具有重要意义（闵伟，1998）。塔藏断裂位于东昆仑断裂带东段，属于“玛曲空区”范围（闻学泽等，2007），汶川Ms8.0级地震对该区域造成了应力的加载（邵志刚等，2010），区域GPS数据结果显示该区域可能处在应变积累阶段（任金卫等，2005）。但是本区尚未开展古地震方面研究，地震危险性认识不足。通过近三个月的野外地质调查工作，本文从构造地貌和古地震探槽方面对塔藏断裂的古地震活动进行了探索，取得如下几点认识：1.构造地貌通过野外调查发现沿塔藏断裂，尤其是罗叉段，分布大量较新的断错地貌，如：错动冲沟、错动水系、错动阶地、断塞塘、坡中谷、断层陡坎、线性植物异常带等，形成长约50km的形变带。位错量分布区间为：（5.5⁶）m、（18²7）m、（68⁷5）m、（200²20）m，其中（5.5⁶）m对应的地貌分布较广泛，年龄较新。探槽一、探槽二及探槽三等3处探槽附近的DGPS测量结果揭示：探槽一附近水平位错量为22m、71m，探槽二及探槽三附近水平位错量为25m、27m、72m；探槽附近的位错量分布特征与构造地貌位错量分布特征较相近，位错量范围存在两个分布区间：（22²7）m、（71⁷2）m，推测由多次地震造成。2.古地震研究本文合计开挖5处古地震探槽，共得到33个14C测年结果。5处探槽剖面地层之间存在明显差异。由于地层记录的不完整性、古地震事件时间的不确定性、地震破裂具有分级性等三个方面影响，古地震在地层记录中的保存具有很大的不确定性，因此利用古地震探槽剖面分析古地震事件时，须进行探槽间地层对比。通过对沉积特征、沉积环境、构造信息等的分析，对比地层差异，降低古地震事件的不确定性。地震楔体内部14C样品来源具有一定不确定性，利用其年龄限定古地震事件时间时，需结合楔体上覆地层年龄进一步约束。基于地层切盖关系、构造楔体、测年结果等信息，经地层对比和逐次限定后得得到2次古地震事件，时间分别为（320±30）^500±30aBP、（2730±30）^2840±30aBP；最近一次古地震离逝时间为（320±30）^500±30aBP；实际重复间隔约为2300a。结合玛曲段的速度衰减梯度、滑动速率及玛曲与罗叉之间的距离，得到塔藏断裂罗叉段的估算滑动速率为3.01mm/a。结合估算滑动速率及最大同震位移，得到重复间隔为2000年。结合青藏高原地区地表破裂段落长度L、同震最大位移量D和震级Ms关系式，可估算本段的古地震震级为：Ms7.5和Mw7.4。3.重复间隔探讨由古地震探槽得到的重复间隔与由滑动速率得到的重复间隔较为接近，说明该段的古地震事件具有较高的完整性。结合东昆仑断裂带古地震方面现有资料，2800a以来，东昆仑断裂各段落重复间隔由西向东表现出递增趋势，与东昆仑断裂滑动速率由西向东表现出递减的趋势相对应。更多还原

【Abstract】 Paleoearthquake includes the prehistoric earthquakes stored in the geologicalstrata and the earthquakes without clear historical record（Wallace R E,1981）. Forgeologists, the geological record is primary（Lin Jingxing,2008）. Paleoseismologycan improve and extend instrumental records with a relatively short period andhistorical earthquakes, which can help to research the characteristic of the recurrenceof strong earthquakes in a longer period and set up the model of recurrence, to analyzethe seismic risk, and it is significant for the continental interior with a longerrecurrence interval（Deng Qidong,2008）. In addition, paleoseismology is one of thekey components for the research of active fault, which is important in areas likesegmentation of active fault, active intensity contrast, dynamics study（Min Wei,1998）.Tazang Fault is situated in the east segment of the East Kunlun Fault, inside therange of the Maqu seismic gap （Wen Xueze et al,2007）. The coulomb stress onTazang fault was increased caused by the Wenchuan Ms8.0earthquake（Shao Zhiganget al,2010）, and the result from the regional GPS shows the strain field ischaracterized by an increase（Ren Jinwei et al,2005）. However, there is littlequake-proof research work about paleoearthquakes and the seismic risk of the regionhas not been profoundly recognized. Through3months’ field investigation, this paperexplores paleoearthquakes in the aspects of history earthquake-related landforms andtrenches near Tazang Fault, and obtained several viewpoints as follows:1. Earthquake-related landformsBased on field investigation, there are plenty of tectonic landforms distributingalong Tazang fault, especially near Luocha segment, such as: offset gullies, offsetrivers, offset terraces, sag-pond, slop-hanging-ditch, fault-scarps, linear abnormalvegetation, et al, forming a deformation belt with an approximate length of50km. Thedistribution ranges of offsets are:（5.5⁶）m,（18²7）m,（68⁷5）m and（200²20）m, and （5.5⁶）m corresponding with the landforms are most common,relatively new. According to the statistics of DGPS data around the three trenches, the left-slipdisplacement are22m,71m around trench1, and25m,27m,72m near trench2andtrench3, and the distribution characteristic of displacements near the three geomorphicsurfaces is near to the and trenches’. All above displacements are mainly distributedin two ranges:（22²7）m and（71⁷2）m, which might be caused by severalpaleoearthquakes.2. Paleoearthquakes by trenchesWe have excavated5trenches and got33dating results of samples of14C in all.The strata of the5trenches have a significant difference. Because of thenondeterminacy coming from stratigraphic record, the occurrence-time of earthquakesand grading phenomena of earthquakes, there is much nondeterminacy in stratigraphicrecord related paleoearthquakes. Therefore, it is better to carry out the stratigraphiccorrelation in the analysis of earthquakes using trenches to lessen the nondeterminacy,from plural perspectives: the characteristic of sediment, sedimentary environment andtectonic information. Taking the indeterminacy of the source of14C samples inearthquake-wedges into account, the data from upper covered strata must beconsidered to constrain the time of paleoearthquakes.Synthesizing materials of the relation between cut and cover of strata,earthquake-wedges, and dating results, after the stratigraphic correlation and multipletrench constraining method, we can conclude that the occurrence time of the2constrained events are respectively as follows:（320±30）^500±30aBP and（2730±30）^2840±30aBP, and the departure time of last earthquake is（320±30）^500±30aBP. The recurrence interval is about2300a.Taking the attenuation gradient of velocity、velocity and the distance from Maquto Luocha，the estimated velocity of Luocha segment can be gained as3.01mm/a. Bythe estimated velocity and the maximum co-seismic displacement, the recurrenceinterval can be gathered as2000a.Ground on the relationship of rupture length, maximum co-seismic displacementand magnitude, the calculated magnitude can be gathered as: Ms7.5and Mw7.4. 3. Discussion about the recurrence intervalsThe recurrence interval obtained by trenches is close to the interval obtained byvelocity, which may favor relatively complete earthquake sequence.Combining with the paleoearthquake data of the East Kunlun Fault, It can be foundout that, since2800a, the recurrence intervals of all segments from west to east showan rising tendency, which is corresponding with the speed tendency from west to east.更多还原