【作者】 李伟；

【导师】 刘斌； 倪四道；

【作者基本信息】 中国科学技术大学 ， 地球物理学， 2023， 博士

【摘要】 南北地震带位于青藏高原东部,是我国地形地貌、地质构造和地球物理特征的东西分界带。复杂的构造活动致使南北地震带内断层分布复杂、地震活动性强,严重破坏性地震时有发生。相较于大地震,南北地震带内中强走滑型地震分布广、频次高,给人类生命财产安全和社会稳定造成很大的威胁,然而其中部分走滑型地震的发震断层不明。查明活动断裂的分布和运动特征关系到地震灾害的评估与预防、区域运动学模型的建立以及动力学机制的分析。深入研究这些地震的震源参数和发震断层参数不仅对于理解地震发震机理、指导灾后救援和评估地震风险性等具有重要意义,而且对于认识南北地震带现今断层活动特征、探究构造变形机制具有深远意义。本文重点以近些年发生在南北地震带南段西部的2014年5月盈江双震、中段东南部的2019年6月长宁地震和北段西部的2022年德令哈地震序列为例,利用地震波形资料和大地测量学资料研究它们的震源参数和破裂方向性参数,以揭示这些地震的震源特征和发震机制,并期望以此加深对这些地区现今构造变形机制的认识。2014年5月M5.6和M6.1盈江地震位于南北地震带南段的滇缅活动地块北部。滇缅活动地块内广泛发育着西南-北东走向和西北-南东走向的走滑断裂。前人提出多种形变模型来解释该地区的断层变形机制,但其中还存在较多争议。2014年5月盈江双震为进一步认识该地区的构造活动特征提供了契机。本文基于地震波形资料研究了其震源参数和破裂方向性参数。结果表明两次地震均为浅源走滑型地震。M5.6地震主要发生在西北-南东走向的右旋走滑隐伏断层上,破裂向南扩展,破裂长度约2-3km,质心深度约9km;M6.1地震主要沿着近南北走向的右旋走滑隐伏断层向北破裂,破裂长度约5 km,质心深度约6 km。结合两次地震发震断层信息与现有的构造变形机制,本文认为剪切带模型可能更适用于解释此次地震发震断层的形成。2019年6月M6.0长宁地震发生在南北地震带中段东南部的长宁地区,震中位于页岩气开采区和盐矿开采区附近,发震成因受到广泛关注。本文中,我们基于地震波形资料分析研究了其起始破裂深度、震源参数和破裂方向性参数。结果表明本次地震的起始破裂深度约4 km,浅于周边构造型地震的成核深度。点源震源参数研究结果表明长宁地震是一次伴有显著逆冲成分的走滑型地震,矩震级约Mw5.8,质心深度约4km,震源持续时间约为6s;最佳双力偶节面解为NP1:125°/46°/28°和NP2:14°/70°/132°。破裂方向性分析结果表明走向角为125°的节面是长宁地震的发震断层面,破裂沿着该断层面向西北方向近水平破裂,同震破裂长度大约14km。结合长宁地震震中和起始破裂点深度等参数,我们推测本次地震可能是盐矿开采区长期注水诱发的地震,但还需要更多的跨学科研究来验证这一假设,如流体运移的模拟、背景应力的调查以及震中地区岩石力学参数的分析等。2022年德令哈地震序列(包括M5.8、M6.0和M5.4三次M5+地震)发生在南北地震带北段的祁连山逆冲褶皱带内,震中位于祁连-海原断裂带西端的地震活动性较低区域。此次地震序列为研究该地区的断层活动特性、探讨祁连山褶皱带内断层变形机制提供了难得的机会。本文基于近震宽频带波形数据分析了其震源参数、余震重定位分布和破裂方向性参数。点源震源参数测定结果表明三次主震的震源深度分布在6～8 km之间,均为浅源走滑型地震,震源机制解两个节面的走向分别为近南北向和近东西向。余震重定位结果显示余震区主要呈现近南北向分布特征。破裂方向性分析结果表明M5.8事件沿着近南北的节面向南破裂,同震破裂长度约4km;M6.0事件也是沿着近南北向的节面向南破裂,同震破裂长度约3km。本文还利用InSAR观测资料提取了 M5.8和M6.0事件的同震形变场资料。结果显示两次地震造成的地表最大变形量分别约为2 cm和3 cm。据此,我们进一步约束了地震的震源深度。研究结果表明此次地震序列发生在一个近南北向、近乎直立的右旋走滑未知断层上。三次主震引起的累计库伦应力变化表明该断裂北部未来地震风险性可能较高。最后本文讨论了此次发震断层可能的形成模式,认为此次地震序列发震断层的形成受到了祁连-海原断裂的影响,整体受控于祁连山内部长期的近南北向的右旋剪切作用。为进一步提高对南北地震带内断层活动特征的认识,本文还研究了 2009-2020年发生在南段和中段六次中强走滑型地震的发震断层参数。以上所有地震的震源特征研究结果表明:南北地震带南段的滇缅块体内部存在右旋走滑剪切作用,这与该地区已知的左旋剪切活动形成反差,进一步支持了剪切带模型的可能性。在中段东南部的长宁地区,我们不能忽视工业活动对地震活动性的影响。在北段的西部地区,这里不仅能孕育逆冲型和左旋走滑型中强地震,而且还能孕育右旋走滑中强地震,表明右旋走滑活动也在调节着该地区的变形作用。此外,本文测定的破裂方向性结果表明中强破坏性地震的发震断层可能是隐伏断层,强调了研究活动断层的分布和运动特征是地震风险评估中的重要任务。南北地震带各段是构造活动十分复杂的区域,详细的地质学、地震学和大地测量学研究有助于进一步了解这些地区运动学模式和动力学机制。更多还原

【Abstract】 The North-South Seismic Belt is located in the eastern part of the Tibetan Plateau,serving as the boundary zone between the terrain,geological structure,and geophysical features of the eastern and western regions in China.Complex tectonic activities have resulted in the complex distribution of faults and strong seismic activity in the NorthSouth Seismic Belt,with the occurrence of severe destructive earthquakes.Compared to large earthquakes,there is a higher frequency and wider distribution of moderate strike-slip earthquakes in the North-South Seismic Belt,posing great threats to human life,property,and social stability.However,the rupturing faults of some strike-slip earthquakes are unknown.Identifying the distribution and kinematic characteristics of active faults is crucial for evaluating and preventing seismic disasters,establishing regional kinematic models,and analyzing dynamic mechanisms.Therefore,studying the source parameters and causative faults of moderate earthquakes are not only important for understanding the seismogenic mechanism,mitigating earthquake disasters,and evaluating the future risk of earthquakes but also for understanding the characteristics of the present-day fault activity in these areas and exploring the tectonic deformation mechanism.In this study,the 2014 Yingjiang doublet earthquake,the 2019 Changning earthquake,and the 2022 Delingha earthquake sequence,which occurred in the southern section,middle section,and northern section of the North-South Seismic Belt,are selected as examples to reveal the source characteristics and seismogenic mechanisms and expect to deepen the understanding of the present-day tectonic deformation mechanisms in these areas.The May 2014 M5.6 and M6.1 Yingjiang earthquakes occurred in the northern part of the Yunnan-Myanmar active block,located in the southern section of the North-South Seismic Belt.A series of NE-SW trending and NW-SE trending strike-slip faults are widely developed within the Yunnan-Myanmar block.Various dynamic mechanisms have been proposed to explain the fault deformation in this region,but there still exist many debates.The 2014 Yingjiang doublet earthquake provides an opportunity to further understand the tectonic activity of this region.We determined the source parameters and rupture directivity using seismic waveform data.The result reveals that the centroid depth of M5.6 and M6.0 earthquakes are about 9 km and 6 km.The M5.6 event mainly ruptured along an NW-SE trending right-lateral strike-slip unmapped fault,and the rupture length is about 2～3 km;the M6.1 earthquake mainly ruptured northward along a nearly N-S trending right-lateral strike-slip buried fault,with the rupture length of about 5 km.Comparing the ruptured faults of this doublet earthquake with the existing dynamic models,we think that the shear zone model is more suitable to explain the deformation of the seismogenic faults of this earthquake.The June 2019 M6.0 Changning earthquake occurred in the southeastern part of the middle section of the North-South Seismic Belt.The earthquake close to the shale gas and salt mining region,and its seismogenesis have attracted extensive attention.We analyzed the hypocentral depth,point source parameters,and rupture directivity via modeling local seismic waveforms.The result reveals that its hypocentral depth is about 4 km,much shallower than the most tectonic earthquake in surrounding regions.The moment tensor inversion results show that the mainshock is a dominant strike-slip event with a significant thrust component.Its magnitude is Mw5.8,and the source duration is 6s.The focal plane solutions are 125°/46°/28°(strike/dip/rake)(NP1)and 14°/70°/132°(NP2)for an optimal depth of 4 km.The rupture directivity analysis results indicate that the causative fault of the mainshock is NP1,and the horizontal rupture along the fault to the northwest,with a co-seismic rupture length of about 14 km.Based on its shallow hypocentral depth,and proximity to nearby mining regions,we hypothesize that the Changning earthquake might be an induced event due to long-term fluid injection for salt mining.However,interdisciplinary studies are needed to test the hypothesis,including simulation of fluid migration and investigation of background stress as well as resolving mechanical parameters of the rocks in the epicentral regions.The 2022 Delingha earthquake sequence(M5.8,M6.0,and M5.4)occurred in the Qilian Mountain thrust-fold belt,located in the northern section of the North-South Seismic Belt.The epicenter is located in a low seismicity region at the westernmost end of the Qilian-Haiyuan fault system.These earthquakes provide a precious opportunity to analyze seismogenic settings,fault activity,and seismic hazards within the Qilian Shan fold-thrust belt.We use the local seismic waveforms to analyze its source parameters,aftershock distribution,and rupture directivity.The results show that these mainshocks are shallow strike-slip events with a centroid depth range from 6～8 km.The focal mechanism solutions show that the orientation of the two nodal planes of those earthquakes is nearly N-S and E-W directions.The relocated aftershocks are mainly distributed along the N-S direction.Both the M5.8 and M6.0 events ruptured along the N-S nodal plane and expanded toward the south for～4 km and～3 km.Furthermore,we extracted the co-seismic deformation data of M5.8 and M6.0 events using InSAR observation data.The maximum surface deformation caused by the two earthquakes is about 2 cm and 3 cm,respectively.Therefore,we further constrained the source depth of the earthquakes.Based on those results,we thought that this earthquake sequence ruptured along an unmapped,nearly vertical dextral fault.The accumulated Coulomb stress changes caused by the mainshocks show that the northwest has a higher risk of future earthquakes.Finally,we discuss the possible deformation mechanism for this right-lateral strike-slip faulting and thought that the deformation of this fault was associated with the Qilian-Haiyuan fault,and controlled by long-term protracted nearly N-S trending right-lateral simple shear in the Qilian Shan fold-thrust belt.To further improve the understanding of the characteristics of fault activity in the North-South Seismic Belt,we also studied the focal mechanism parameters of six moderate strike-slip earthquakes that occurred in the southern and middle sections of the North-South Seismic Belt from 2009 to 2020.The analysis results of all earthquakes mentioned above show that:there is right-lateral strike-slip shear inside the YunnanMyanmar block in the southern section of the North-South Seismic Belt,which contrasts with the left-lateral strike-slip shear in this region,further supporting the possibility of the shear zone model.In the Changning area in the southeastern part of the middle section,the influence of industrial activity on seismic activity cannot be ignored.In the western region of the northern section,there are not only thrust and leftlateral strike-slip moderate earthquakes but also right-lateral strike-slip moderate earthquakes,indicating that right-lateral strike-slip activity is also accommodating the deformation in this region.In addition,the rupture directivity results determined in this study show that moderate earthquakes may occur on unmapped faults,emphasizing that studying the distribution and kinematic characteristics of active faults is an important task in seismic risk assessment.Each section of the North-South Seismic Belt is tectonically complex region,and detailed geological,seismological,and geodetic studies can help to further understand the kinematic patterns and dynamical mechanisms in these regions.更多还原