Spatio-temporal Evolution Of Minor Earthquakes Around Fault Instability

The relationship between fault slip and spatio-temporal evolution of minor earthquakes has always been an important research topic in seismology.Numerous studies have been done,ranging from the laboratory experiments,numerical simulations to field observations.These studies suggest that studying spatial-temporal evolution of the minor seismic activities before and after major earthquakes provides an effective method for understanding mechanisms of earthquake nucleation and triggering.This thesis focuses on spatial-temporal evolution of minor earthquakes at two different scales.The first is laboratory earthquakes(also known as acoustic emissions or AEs)associated with a meter-scale granite fault and the second is natural minor earthquakes associated with the 2008Mw7.9 Wenchuan and 2017 Ms7.0 Jiuzhaigou earthquakes in Sichuan,China.The first chapter deals with research problem identification and research status,known problems in the current observation,and structure of this thesis.Chapter 2 mainly introduce the analysis procedure,including detection and location of both minor earthquake and AEs.In laboratory experiments,AE catalog is generally incomplete,due to triggered recording or low detection capability by traditional methods.Chapter 3 introduces a new procedure to detect AEs based on a matched filter technique(MFT),also known as template matching method.The template catalog is first constructed by detecting,locating and estimating magnitude of AE events from continuous but weak signals.Next,we apply the MFT to the continuous AE record to detect new AE events,based on the waveforms of the template catalog.The results show that the MFT can detect 5 times more AE events than in the template catalog.This provides a new complete catalog for subsequent work at the laboratory scale.Chapter 4 introduces a follow-up study with the MFT applied to newly optimized continuous recordings in laboratory experiment.By combining the improved AE catalog and fault displacement data,different stick-slip events and the associated spatio-temporal evolutions of AE events can be further studied.The laboratory results suggest that: 1.stick-slip event generally occurs in the area with small amount of existing fault slip.In addition,an increasing AE activity is found around the rupture initiation point of the stick-slip event.A strong correlation betweentemporal evolution of fault slip velocity and the AE rate is observed,following a power-law relationship.These results suggest that the stress change caused by fault pre-slip(i.e.,subtle slip before the major stick-slip event)is responsible for triggering the unstable fault slip and accompanying AE events.2.after each stick-slip event,a logarithmic expansion of early AE events along the fault strike is observed as a function of time,and most AE events occurs in the area with more slip.Moreover,the temporal consistency between the cumulative number of later-stage AE events and the amount of fault slip indicates that stress change caused by continuous slip on fault plays an important role in driving AE events near or on the fault.The MFT is also applied to improve aftershock catalogs following the 2008 Mw7.9Wenchuan and 2017 Ms7.0 Jiuzhaigou earthquakes.Both aftershocks show the characteristic migration along the fault strike and depth with logarithmic time since the mainshock,indicating that afterslip plays an important role in triggering aftershocks following both mainshocks.Due to its fault complexities and numerous studies about post-seismic deformation associated with the Wenchuan earthquake,chapter 5 mostly focuses on the relationship between aftershocks and post-seismic deformation,the fault geometry and the main factors affecting the b value in G-R law and the p value in Omori's law in the Wenchuan earthquake region.The obtained results show that most aftershocks occur in areas without significant coseismic slip and the number of aftershocks in different regions correlates well with the post-seismic deformation,consistent with them being triggered by postseismic deformation.Based on spatio-temporal evolutions of early aftershocks and surface ruptures,several subsurface fault geometries(e.g.,Yinxiu-Beichuan fault,Guanxian-Jiangyou fault,Xiaoyudong fault and Wenchuan-Maoxian fault)can be inferred.Lower b-value in the Qingchuan and Lixian may be related to increasing stress resulting from the afterslip.The b-value also decreases with depth,which may result from either an increase of differential stress or a decrease of inhomogeneity with depth.The distribution of p-value correlates well with the aftershocks number and post-seismic deformation,indicating that the p-value may be related to the post-seismic deformation or temperature resulting from afterslip.The p-value also increases with depth,which may be related to the increasing temperature with depth.