【作者】 刘凯；

【导师】 耿江辉； 温扬茂；

【作者基本信息】 武汉大学 ， 大地测量学与测量工程， 2023， 博士

【摘要】 地震的发生过程是一个短暂但释放大量能量的过程,在地震结束后,断层附近并未恢复稳定,而是会发生包括断层滑动、孔隙回弹、粘弹性松弛和余震等一系列震后活动。常规研究大多以天解GNSS为基础,研究分析震后的长期形变特性和构造机制,但是很少有研究注意在地震完成后,存在这样一个长达的数十个小时的观测空区,在此期间会发生显著的快速地表形变。而动态解GNSS则为同震和震后形变监测提供了连续可靠的技术支撑,不仅仅可以捕获地震波和永久同震位移,还能记录震后极早期的低频震后形变信号,从而为研究震后极早期地表形变特性和断层活动提供了一个极佳的研究窗口。而智利俯冲带作为一个狭长、大地震频发的天然地震工厂,则为我们研究大地震同震破裂到稳定震后过程中断层活动属性提供数据支撑。因此,本文基于智利GNSS地震监测网,以2010年以来的3个8级以上大地震为研究目标,通过动态解/高频GNSS细致研究并分析了震后极早期地表形变特性和断层演化过程,论文的主要内容和贡献如下:(1)从动态解GNSS的角度重点分析了震后形变的主要特性。通过对比发现了常规的指数模型和对数模型对于极早期形变的不适用性,提出了适用于极早期震后形变的幂函数震后拟合模型。结合不同时间尺度计算的同震位移,发现基于天解GNSS会轻微高估同震滑动,高估10%左右,但是会显著低估震后形变,最大可达80%;统计结果揭示了震后前2个小时的形变量可达3 cm,累计形变量可达前12小时的40%。综合这些结果,表明震后极早期断层的活跃性,进一步突出了地震极早期形变监测的必要性。(2)以智利俯冲带2010年以来发生的三个8级以上的大地震为载体,围绕GNSS动态解时间序列对比了5分钟和24小时后计算的同震位移场,并在此基础上构建断层模型,反演各个地震的同震滑动分布,并进一步对比两者间的差异。结果表明了震后极早期的形变较为显著,导致反演的滑动分布也存在显著的差异性,并存在一定的空间分布规律,大部分分布在主震滑动区的周围,可能与震后滑动直接相关。快速同震滑动分布和天解同震分布间的差距可以达到实际同震滑动释放地震矩的5%左右,在三个地震中不同分辨率的计算差异分别等价于地震矩为Mw 7.0,Mw 7.5和Mw 7.7级的地震。这样的差异值对于震后研究是及其重要的,是衡量震后断层活动的主要参数和重要证据,合理的区分和量化同震和震后滑动分布对于断层破裂的特性研究和阶段转化具有重要意义。(3)反演分析了震后极早期余滑的时空演化过程,发现余滑是形成震后极早期快速形变的主要驱动力。三个地震的余滑分布结果均表明极早期的余滑与同震滑动分布存在空间互补性,且在同震滑动区的边界区域存在一定的重合现象。同时,我们通过不同时间阶段的划分,呈现了余滑的时空演化。结合余滑时空演化过程,我们可以将震后断层活动分为扩展期、稳固期两个阶段。通过计算地震矩的释放情况,我们量化了不同时期余滑所释放能量的占比情况,结果表明余滑在震后的数小时内非常活跃,并随着时间迅速减弱。震后6小时余滑地震矩可达震后36小时总量的50%以上;三个地震前两小时释放的地震矩均等价于超过Mw 7.0级以上的地震,这说明震后余滑是断层应力调整和释放的一个重要途径。余滑的演变和驱动机制更加集中于震后的前几个小时,因此极早期的形变观测是极其重要的。(4)探讨了极早期余震和余滑在时空演化过程中的空间分布和相关关系。智利俯冲带上的三个8级大地震的结果显示:余震的空间分布与同震破裂区具有一定的联系,但是更多受到余滑发展的影响;同时自由空气重力异常会限制余震的空间分布,大多在重力异常负值区和正值区的低值区内发生,部分俯冲海山会有效的阻碍同震破裂和余震往深部发展的趋势。极早期余滑是主导余震发生的主要因素,特别是余滑增量,余震基本位于余滑增量内,而且存在明显的时空滞后性,只有在余滑发展到一定程度后才会发生聚集性的余震。同时余滑会在重力异常高值区缓慢发生,并伴随着余震的发生,但是这种现象也是被限制在有限的范围内。同震破裂以及震后极早期的这种地震活动特性与地质构造紧密相关,而GNSS则为我们提供了一个视角来分析断层状态变化和地质构造间的关系。更多还原

【Abstract】 The earthquake is a short-lived process that releases a large amount of energy.After the earthquake is ruptured,a series of postseismic activities including fault slip,poroelastic rebound,viscoelastic relaxation and aftershock will occur instead of restoring stability near the fault.Most conventional studies are based on daily GNSS to study and analyze the long-term postseismic deformation characteristics and tectonic mechanisms,but few studies have paid attention to the existence of such a tens of hours-long observation gap after the completion of the earthquake rupture,during which significant and rapid surface deformation occurs.The kinematic solution of GNSS provides a continuous and reliable technical support for coseismic and postseismic deformation monitoring,not only capturing seismic waves and permanent coseismic displacements,but also recording low-frequency postseismic deformation signals in the very early postseismic period,thus providing an excellent window for studying surface deformation properties and faulting activities in this very early period.The Chilean subduction zone,as a narrow,natural seismic factory with frequent large earthquakes,provides data to support our study of fault activity properties during the process from coseismic rupture to postseismic period.Therefore,based on the Chilean GNSS seismic monitoring network,this thesis meticulously investigates and analyzes the very early postseismic surface deformation properties and fault evolution processes by kinematic solution/high-rate GNSS with three large earthquakes of magnitude 8 or higher since 2010 as the research target,and the main contents and contributions of the paper are as follows:(1)The main characteristics of the post-earthquake deformation are focused on from the perspective of kinematic GNSS.The conventional exponential and logarithmic models are found to be inapplicable for very early deformation,and a power function model is proposed to construct a post-earthquake fitting model for very early post-earthquake deformation.Combined with the isoseismic displacements calculated at different time scales,it is shown that the sky-solution GNSS based on the isoseismic sliding will slightly overestimate the isoseismic sliding by about 10%,but will significantly underestimate the postseismic deformation by up to 80%;the statistical results show that the deformation in the first 2 hours after the earthquake can be up to 3 cm,and the cumulative deformation can be up to 40%of the first 12hours.Together,these results indicate that the faults are active in the very early post-earthquake period,further highlighting the need for monitoring the deformation in the very early post-earthquake period.(2)Using three large earthquakes of magnitude 8 or greater that have occurred in the Chilean subduction zone since 2010 as a vehicle,the isoseismic displacement fields calculated after 5 minutes and 24 hours are compared around kinematic GNSS time series,and based on this,a fault model is constructed to invert the isoseismic slip distribution of each earthquake and further compare the differences between them.The results show that the deformation in the very early post-earthquake period is more significant,and the differences in the isoseismic deformation fields are more obvious,and the inverse sliding distributions on this basis also have significant differences,and there is a certain spatial distribution pattern,most of which are distributed around the mainshock sliding area,which may be directly related to the post-earthquake sliding.The difference between the fast isoseismic sliding distribution and the sky solution isoseismic distribution can reach about 5%of the seismic moment released by the fast isoseismic sliding,and the different solution differences of the three earthquakes are equivalent to earthquakes with seismic moments of magnitude Mw 7.0,Mw 7.5and Mw 7.4,respectively.Such difference values are very important for post-earthquake studies and are the main parameters and important evidence for measuring post-earthquake fault activity.Therefore,we need to calculate the isoseismic displacement and post-earthquake deformation accurately,and a reasonable distinction and quantification of the isoseismic and post-earthquake sliding distributions are important for the characterization and phase transformation of fault rupture.(3)The spatial and temporal evolution of afterslip in the very early postseismic period is analyzed in the inversion,and the occurrence of afterslip is the main driving force for the formation of rapid postseismic deformation in the very early postseismic period.The results of the afterslip distribution of all three earthquakes indicate that there is spatial complementarity between the very early afterslip and the distribution of co-seismic sliding,and there is some overlap in the boundary area of the co-seismic sliding zone.At the same time,we present the spatial and temporal evolution of afterslip by dividing it into different temporal stages.Combining the spatio-temporal evolution process,we can divide the sliding characteristics of the post-earthquake fault into two stages:the extensional and the solidification period.By calculating the release of seismic moments,we quantified the percentage of energy released by afterslip in different periods,and the results showed that afterslip was very active in the hours after the earthquake and weakened rapidly with time.The afterslip seismic moment 6 hours after the earthquake can reach more than 50%of the total 36 hours after the earthquake;the seismic moments released two hours before all three earthquakes are equivalent to those exceeding Mw 7.0 magnitude,which indicates that afterslip is an important pathway for fault stress adjustment and release after the earthquake.The evolution and driving mechanism of afterslip is more concentrated in the first few hours after the earthquake,so the observation of very early deformation is extremely important.(4)The spatial distribution and correlation between very early aftershocks and afterslip during the spatial and temporal evolution are explored.The results of three large magnitude 8 earthquakes on the Chilean subduction zone show that the spatial distribution of aftershocks is related to the syncline rupture zone,but more influenced by the development of afterslip;meanwhile,the free-air gravity anomaly will limit the spatial distribution of aftershocks,mostly occurring in the low value zone of the negative and positive gravity anomaly zones,and some subduction seamounts will effectively hinder the trend of syncline rupture and aftershock development to the deep.Very early afterslip is the main factor dominating the occurrence of aftershocks,especially the afterslip increment.Aftershocks are basically located within the afterslip increment,and there is an obvious spatial and temporal lag,and the aggregated aftershocks will occur only after the afterslip has developed to a certain degree.Meanwhile,the afterslip will slightly erode the high value area of gravity anomaly and accompany the aftershock,but this phenomenon is also restricted to a limited extent.This seismic activity characteristic of isoseismic rupture and the very early post-earthquake period is closely related to geological structure,and GNSS provides us a perspective to analyze the relationship between fault state changes and geological structure.更多还原