【作者】 孙长虹；

【导师】 孟小红； 许忠淮；

【作者基本信息】 中国地质大学（北京） ， 地球探测与信息技术， 2006， 博士

【摘要】 2003年4月17日在青海省德令哈市怀头他拉乡西北部高山区发生了ML=6.7的地震,其后发生了一系列余震,本文称其为德令哈地震序列。对该地震序列的研究关系到对青藏高原东北部现今构造运动的认识,同时,对于该地区的地震趋势估计也有重要意义。本研究利用中国地质调查局项目(20010201)布设的流动数字地震观测台网中的8个台站记录的德令哈地震序列近震体波到时和波形资料,并补充国家数字地震台网中28个台站的有关记录,对德令哈地震的时空分布、发震构造和震源破裂特征进行了系统研究,取得了以下几方面的研究成果:1)应用交切法对德令哈地震序列6.7级的主震和截止至2003年10月25日的1.0级以上的117次余震进行了初始定位,并以双差地震定位法对这些地震中的95次地震进行了精确再定位。结果表明,余震震中呈现出NWW-SEE方向的延伸分布,余震的优势深度范围是7～18 km的上地壳,余震在垂直方向上分布比较陡立。2)应用Snoke等(1984)的测定震源机制解的网格尝试法,通过比较理论计算的与实际观测的P波、SV波、SH波的初动方向和振幅比,以选择矛盾符号数最少和振幅比残差最小方式,求得德令哈地震序列60个2.4级以上主要地震的震源机制解。结果表明,主震和大部分余震都是沿NWW-SEE走向的逆断层错动,震源区发生最大压缩的方向为近水平的NNE-SSW方向。3)尽管震源区在主震前几年发生过数次4级多的地震,但主震前40天未记录到震源区的地震活动。德令哈地震序列的时间变化说明,这是一次主震-余震型地震序列。该序列主震后10天内发生的余震是按大森房吉定律正常衰减的,衰减指数p=1.2;以后的余震衰减比较慢,主震后一年左右在震源区又接连发生了6次5级多的晚期强余震。整个余震序列的b值为0.65, b值是较低的。4)对德令哈地震序列余震区的扩展分析结果表明,该地震序列的震源过程是一次不对称的双侧破裂,向NWW的破裂分支长于向SEE的分支。地震断层平均位错约0.39 m,平均地震应力降估算为4.6 MPa,这是一次应力降较大的逆断层活动。5)在综合分析重新定位后的地震序列的空间分布、本研究和哈佛大学测得的主要地震的震源机制解以及震源区主要地质断裂构造的基础上,判定了德令哈地震序列的发震构造是NWW-SEE走向的大柴旦-宗务隆山活动断裂,这是柴达木断陷区与祁连断隆带之间的分区边界断裂,地震序列是该断裂的一次深部活动。6)根据本文对德令哈地震序列空间分布和震源机制解特征的研究结果,结合青藏高原东北缘的新构造活动环境和现今构造应力场特征的分析,可以看出德令哈地震活动是青藏高原东北缘受NNE-SSW向挤压产生隆升活动的显示,这也是青藏高原继续处于隆升活动中的表现。青藏高原东北部这一继承性新构造运动是德令哈地震序列的发震原因。更多还原

【Abstract】 An ML6.7 earthquake occurred on April 17, 2003, in Huaitoutala village (37.56°N,96.52°E), Delingha County, Qinghai Province. The epicentral region is in a mountainarea on northeastern Qingzang plateau. Then followed a series of aftershocks. Theseearthquakes are referred to as the Delingha earthquake sequence. Investigating thisearthquake sequence will be helpful for understanding the present-day tectonic movementof the northeastern Qingzang plateau and for estimating the future earthquake activity inthis region. The author did the research on space-time distribute, seismogenic structureand source fracture characteristics of the Delingha earthquake sequence. The data usedare the digital seismic records from 8 temporary seismic stations installed by the ChinaGeological Survey scientific research project (20010201) and the digital records from 28seismic stations of the China National Seismic Network (CNSN). The main works andresults of this study are as follows.1) The author conducted the preliminary location of the mainshock and 117 ML>1.0aftershocks occurred up to October 25, 2003, using the hypocenter distance trajectorycrossing approach, and then relocated the 117 events with the Double-DifferenceAlgorithm (Waldhauser and E11sworth, 2000). Finally 95 events were relocated. Theresult indicates that the aftershock epicenters distribute in a NWW-SEE elongated region,and the focal depths are dominantly in the range of 7-18 km. Vertically the aftershockregion is a rather steep zone.2) Focal mechanisms of the mainshock and 60 ML≥2.4 aftershocks were determinedusing Snoke’s algorithm (Snoke et al., 1984). The data used are the polarity readings ofthe first P, SH and SV wave, as well as their amplitude ratio observation. The algorithmperforms an efficient, systematic grid search over the variables space of the 3 parametersof a double-couple source model, and finds acceptable solutions based on the selectedcriteria on the inconsistent polarity readings and errors in amplitude ratios. The resultshows that the mainshock and most aftershocks have a reverse faulting mechanism withthe possible fault striking in NWW-SEE and the maximum source compression inNNE-SSW direction.3) Though in a few years before the mainshock there were some middle sizeearthquakes in the source region, no earthquakes have been recorded in the 40 days justbefore the mainshock. The Delingha earthquake series is an earthquake sequence of themainshock-aftershock type. The daily aftershocks within first 10 days after the mainshockdecayed normally following the Omori law, with the decaying index p=1.2. Afterwardsthe aftershocks decayed slowly. More than one year later there successively occurred 6rather strong aftershocks with M beyond 5. The b value of the whole aftershock sequencetakes a low value of 0.65.4) Investigation on aftershock region expanding reveals that the source fractured in aprocess of unsymmetrical bilateral expanding, with NWW branch being longer than theSEE one. Average fault dislocation of the Delingha earthquake sequence is about 0.39 mand the average global stress drop is estimated to be 4.6 MPa, indicating this is a reversefaulting earthquake with high stress drop.5) An integrated analysis on the spatial distribution of aftershocks and the focalmechanisms given by both this study and Harvard university, in combination with theconsideration on relevant major geologic faults, results in the judgment that theseismogenic structure of the Delingha earthquake sequence is the NWW-SEE striking DaQaidam-Zongwulongshan active fault, which is a boundary fault between the Qaidamfaulted depression and the Qilian fold uplift. The earthquake sequence is an exhibition ofthe activity in deep part of the fault.6) Based on the characteristics of the aftershock spatial distribution and focalmechanisms, in reference to the study result of Neo-tectonic movement and present-daytectonic stress in northeastern part of the Qingzang plateau, we can see that theoccurrence of the Delingha earthquake sequence is a manifestation of the present-dayuplifting movement of the Qingzang plateau under the NNE-SSW compression. And thepresent-day uplift movement of the northeastern Qingzang plateau serves as theseismogenic origin of the Delingha earthquake sequence.更多还原